Lustre Coding Style Guidelines

Beautiful Code
A note from Eric Barton, a Lustre pioneer:

More important than the physical layout of code (which is covered in detail below) is the idea that the code should be beautiful to read.

What makes code beautiful to me? Fundamentally, it's readability and obviousness. The code must not have secrets but should flow easily, pleasurably and accurately off the page and into the mind of the reader.

How do I think beautiful code is written? Like this...


 * The author must be confident and knowledgeable and proud of her work. She must understand what the code should do, the environment it must work in, all the combinations of inputs, all the valid outputs, all the possible races and all the reachable states. She must grok it.


 * Names must be well chosen. The meaning a human reader attaches to a name can be orthogonal to what the compiler does with it, so it's just as easy to mislead as it is to inform. "Does exactly what it says on the tin" is a popular UK English expression describing something that does exactly what it tells you it's going to do, no more and no less. For example, if I open a tin labeled "soap", I expect the contents to help me wash and maybe even smell nice. If it's no good at removing dirt, I'll be disappointed. If it removes the dirt but burns off a layer of skin with it, I'll be positively upset. The name of a procedure, a variable or a structure member should tell you something informative about the entity without misleading - just "what it says on the tin".


 * Names must be well chosen. Local, temporary variables can almost always remain relatively short and anonymous, while names in global scope must be unique. In general, the wider the context you expect to use the name in, the more unique and informative the name should be. Don't be scared of long names if they help to, but   either - we don't write COBOL. Related names should be obvious, unambiguous and avoid naming conflicts with other unrelated names, e.g. by using a consistent prefix. This applies to all API procedures (if not all procedures period) within a given subsystem. Similarly, unique member names for global structures, using a prefix to identify the parent structure type, helps readability.


 * Names must be well chosen. Don't choose names that are easily confused - especially not if the compiler can't even tell the difference when you make a spelling mistake.  and   aren't the worst example -   and   are much worse (and taken from our own code!!!).  "Generic" variable names like   and   are easily mistaken between different parts of the code and should be avoided.


 * Names must be well chosen. I can't emphasize this issue enough - I hope you get the point.


 * Assertions must be used intelligently. They combine the roles of active comment and software fuse. As an active comment they tell you something about the program that you can trust more than a comment. And as a software fuse, they provide fault isolation between subsystems by letting you know when and where invariant assumptions are violated. Overuse must be avoided - it hurts performance without helping readability - and any other use is just plain wrong. For example, assertions must never be used to validate data read from disk or the network. Network and disk hardware does fail and Lustre has to handle that - it can't just crash. The same goes for user input. Checking data copied in from userspace with assertions just opens the door for a denial of service attack.


 * Formatting and indentation rules should be followed intelligently. The visual layout of the code on the page should lend itself to being read easily and accurately - it just looks clean and good.
 * Separate "ideas" should be separated clearly in the code layout using blank lines that group related statements and separate unrelated statements.
 * Procedures should not ramble on. You must be able to take in the meaning of a procedure without scrolling past page after page of code or parsing deeply nested conditionals and loops. The 80-column rule is there for a reason.
 * Declarations are easier to refer to while scanning the code if placed in a block locally to, but visually separate from, the code that uses them. Readability is further enhanced by limiting variable declarations to one per line and avoiding complex initializations in the declaration that may be missed.
 * Parameters in multi-line procedure calls should be aligned so that they are visually contained by their brackets.
 * Brackets should be used in complex expressions to make operator precedence clear, but not excessively.
 * Formatting and indentation rules should not be followed slavishly. If you're faced with either breaking the 80-chars-per-line rule or the parameter indentation rule or creating an obscure helper function, then the 80-chars-per-line rule might have to suffer. The overriding consideration is how the code reads.

I could go on, but I hope you get the idea. Just think about the poor reader when you're writing, and whether your code will convey its meaning naturally, quickly and accurately, without room for misinterpretation.

I didn't mention clever as a feature of beautiful code because it's only one step from clever to tricky - consider...

t = a; a = b; b = t; /* dumb swap */ a ^= b; b ^= a; a ^= b; /* clever swap */

You could feel quite pleased that the clever swap avoids the need for a local temporary variable - but is that such a big deal compared with how quickly, easily and accurately the reader will read it? This is a very minor example which can almost be excused because the "cleverness" is confined to a tiny part of the code. But when clever code gets spread out, it becomes much harder to modify without adding defects. You can only work on code without screwing up if you understand the code and the environment it works in completely. Or to put it more succinctly...

''Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.'' - Brian W. Kernighan

IMHO, beautiful code helps code quality because it improves communication between the code author and the code reader. Since everyone maintaining and developing the code is a code reader as well as a code author, the quality of this communication can lead either to a virtuous circle of improving quality, or a vicious circle of degrading quality. You, dear reader, will determine which.

Style and Formatting Guidelines
All of our rules for formatting, wrapping, parenthesis, brace placement, etc., are derived from the Linux kernel rules, which are basically K&R style. Some of these rules are automatically verified at commit time by the  script (formerly  ) included with newer versions of the Lustre code, but many depend on the good judgment of the coder and inspector. Note that there is also Lustre Script Coding Style that describes the formatting for shell scripts used for userspace utilities and testing.

Whitespace and Comments
Whitespace gets its own section because it is critical to helping the reader understand the logic of the code, and if there are inconsistencies between the whitespace used by different coders it can lead to confusion and hidden defects. Please ensure that you comply with the guidelines in this section to avoid these issues. We've included default formatting rules for emacs and vim to help make it easier.

Use Tabs Instead of Spaces

 * Tabs should be used in all    and   files, including test scripts. This matches the upstream Linux kernel coding style, and is the default method of code indentation.


 * NOTE The use of tabs for indentation is required since May 2012 (Lustre 2.3 and later). This is being done in order to facilitate code integration with the Linux kernel. All new patches should be submitted using tabs for ALL modified lines in the patch ( will complain if not). If there are 6 or fewer lines using spaces for indentation between two lines changed by a patch, or between modified lines and the start or end of the function/structure or a nearby tab-indented line, then *all* of the in-between lines should also have the indentation changed to use tabs. Similarly, if there are only a handful of lines (6-8) remaining in a modified function or test that are still using spaces for indentation, convert *all* of the lines in that function or test to use tabs. In this manner, we can migrate the remaining chunks of code over to tabs without having huge patches breaking the commit history of every line of code, and also avoid breaking code that is in existing branches/patches that still need to merge. The conversion to tabs is over 90% complete for the 2.13.0 release, so the preference is to convert more lines to tabs rather than just the minimum, including entire modified functions when practical.

right: void func_helper(...) {       struct foobar_counter *foo; unsigned long last; int rc = 0; do_sth2_1; if (cond3) do_sth_which_needs_a_very_long_line(and, lots, of, arguments); do_sth2_2; } void func(...) {       long first; int i;       if (!cond1) return; do_sth1_1; if (cond 2) func_helper(...) do_sth1_2; } wrong: void func(...) {       int rc; struct foobar_counter *foo; unsigned long last; if (cond1) { do_sth1_1; if (cond2) { do_sth2_1; if (cond3) { do_sth_which_needs_a_very_long(and, lots, of, arguments); }                       do_sth2_2; }               do_sth1_2; } }
 * Tabs are preferred to align variable names in structure and enum declarations. This may temporarily misalign them with other variable names in the structure that are using spaces for indentation, or consider fixing up the whole struct in the same patch if there aren't too many of them. In some cases (long variable names, long comments, nested unions) then it isn't practical to use full tabs to align the fields and spaces can be used for partial indentation.
 * Only a single space should separate the variable type and variable name for local function declarations. This is to match upstream Linux kernel coding style.
 * All lines should wrap at 80 characters. This is to avoid the need to have extra-wide terminal windows for the few lines of code that exceed 80 columns, and avoids nesting functions and conditionals too deeply. Exceptions to this rule in the upstream kernel include long text strings.  If it's getting too hard to wrap at 80 characters, you probably need to rearrange conditional order or break it up into more functions.
 * Do not include spaces or tabs on blank lines or at the end of lines. Please ensure you remove all instances of these in any patches you submit to Gerrit. You can find them with grep or in vim using the following regexps  or use git  . Alternatively, if you use vim, you can put this line in your   file, which will highlight whitespace at the end of lines and spaces followed by tabs in indentation (only works for C/C++ files)  .  Or you can use this command, which will make tabs and whitespace at the end of lines visible for all files (but a bit more discretely)  .  In emacs, you can use   or   depending on the version. You could also consider using.

Function comment blocks
right: /** * Initialize or update CLIO structures for the regular file \a inode * when new meta-data arrives from the server. * * \param[in] inode regular file inode * \param[in] md   new file metadata from MDS * - the \a md->body must have a valid FID (valid & OBD_MD_FLID) * - allocates cl_object if necessary, * - updated layout, if object was already here. * * \retval 0 if the inode was initialized/updated properly * \retval negative errno if there was a problem */ int cl_file_inode_init(struct inode *inode, struct lustre_md *md) { right: /** * Implements cl_page_operations::cpo_make_ready method for Linux. * * This is called to yank a page referred to by \a slice from the transfer * cache and to send it out as a part of transfer. This function try-locks * the page. If try-lock failed, page is owned by some concurrent IO, and * should be skipped (this is bad, but hopefully rare situation, as it usually * results in transfer being shorter than possible). * * \param[in] env    lu environment for large temporary stack variables * \param[in] slice  per-layer page structure being prepared * * \retval   0       success, page can be placed into transfer * \retval -EAGAIN   page either used by concurrent IO or was truncated. Skip it. */ static int vvp_page_make_ready(const struct lu_env *env,                              const struct cl_page_slice *slice) { wrong: /* finish inode */ void cl_inode_fini(struct inode *inode) {
 * Functions that are more than a few lines long should be declared with a leading comment block that describes what the function does, any assumptions for the caller (locks already held, locks that should not be held, etc), and return values. Lustre uses the DOxygen markup style for formatting the code comments, as shown in the example here.  can be used in the descriptive text to name a function argument.   should be used to define each of the function parameters, and can be followed by ,  , or   to indicate whether each parameter is used for input, output, or both.   should be used to define the function return values.

Organizing #include files
See Lustre Style Guide Includes for details on  file ordering.

Avoid inline Functions

 * Don't use  unless you're doing something so performance critical that the function call overhead will make a difference -- in other words - almost never. It makes debugging harder and overuse can actually hurt performance by causing instruction cache thrashing or crashes due to stack overflow.

Do Not Use typedef

 * Do not use  for new declarations, as this hides the type of a field for very little benefit, and is against Linux kernel coding stye. Never typedef pointers, as the   makes C pointer declarations obvious. Hiding it inside a typedef just obfuscates the code.

No Assignment in Expression
right: ptr = malloc(size); if (ptr != NULL) { ... wrong: if ((ptr = malloc(size)) != NULL) { ...
 * Do not embed assignments inside Boolean expressions. Although this can make the code one line shorter, it doesn't make it more understandable and you increase the risk of confusing  with   or getting operator precedence wrong if you skimp on brackets. It's even easier to make mistakes when reading the code, so it's much safer simply to avoid it altogether.

Conditional Expressions
right: void *pointer = NULL; bool writing = false; int ref_count = 0; if (!writing &&    /* not writing? */            pointer != NULL && /* valid pointer? */            reval == 0)     /* no error is returned? */               do_this; wrong: if (writing == 0 && /* not writing? */           pointer &&      /* valid pointer or not? */            !retval)        /* return value is good or bad? */              not_clear_if_we_do_this_or_not; right: if (a->a_field == 3 &&           ((bar & BAR_MASK_1) || (bar & BAR_MASK_2))) do this; wrong: if (((a == 1) && (b == 2) && ((c == 3) && (d = 4)))) maybe_do_this wrong: if (a->a_field == 3 || b->b_field & BITMASK1 && c->c_field & BITMASK2) maybe_do_this wrong: if ((((a->a_field) == 3) || (((b->b_field) & (BITMASK1)) &&          ((c->c_field) & (BITMASK2))))) maybe_do_this
 * Conditional expressions read more clearly if only Boolean expressions are implicit (i.e., non-Boolean and pointer expressions compare explicitly with  and   respectively.)
 * Use parentheses to help readability and reduce the chance of operator precedence errors, but not so heavily that it is difficult to determine which parentheses are a matched pair or accidentally hide bugs (e.g. the aforementioned assignments inside Boolean expressions).

Avoid Mixing NULL and ERR_PTR
right: struct foo *foo_init {       struct foo *foo; int rc; OBD_ALLOC_PTR(foo); if (foo == NULL) RETURN(ERR_PTR(-ENOMEM)); rc = init_foo(foo); if (rc != 0) { OBD_FREE_PTR(foo); RETURN(ERR_PTR(rc)); }       RETURN(foo); } wrong: struct foo *foo_init {       struct foo *foo; int rc; OBD_ALLOC_PTR(foo); if (foo == NULL) RETURN(NULL); rc = init_foo(foo); if (rc != 0) RETURN(ERR_PTR(rc)); RETURN(foo); }
 * Function return values should not mix  and   values. This avoids complexity and bugs in each of the callers of that function, since   is false (i.e.   does not consider   an error).

Layout

 * Code can be much more readable and efficient if the simple or common actions are taken first in a set of tests. Re-ordering conditions like this also eliminates excessive nesting and helps avoid overflowing the 80-column limit.

No else After return
right: list_for_each_entry(...) { if (!condition1) { do_sth1; continue; }               do_sth2_1; do_sth2_2; ...               do_sth2_N; if (!condition2) break; do_sth3_1; do_sth3_2; if (!condition3) break; ...               do_sth3_N; } wrong: list_for_each_entry(...) { if (condition1) { do_sth2_1; do_sth2_2; ...                       do_sth2_N; if (condition2) { do_sth3_1; do_sth3_2; if (condition3) { ...                                       do_sth3_N; continue; }                       }                        break; } else { do_sth1; }       }
 * Do not place  blocks after terminal statements like ,  ,  , or   in the   block.

Use Common Cleanup and Return for Error Handling
right: struct bar *bar_init(...) {       foo = alloc_foo; if (!foo) RETURN(-ENOMEM); rc = setup_foo(foo); if (rc) GOTO(free_foo, rc); bar = init_bar(foo); if (IS_ERR(bar)) GOTO(cleanup_foo, rc = PTR_ERR(bar)); :       :        RETURN(bar); cleanup_foo: cleanup_foo(foo); free_foo: free_foo(foo); return ERR_PTR(rc); } wrong struct bar *bad_func(...) {       foo = alloc_foo; if (foo == NULL) RETURN(NULL); rc = setup_foo(foo); if (rc != 0) { free_foo(foo); RETURN(ERR_PTR(rc)); bar = init_bar(foo); if (IS_ERR(bar)) { cleanup_foo(foo); free_foo(foo); RETURN(PTR_ERR(bar)); }       :        :        RETURN(bar); }
 * Function bodies that have complex state to clean up in case of an error should do so only once at the end of the function with a single, and use   to jump there in case of an error:

About Variable Declarations
right: struct inode *dir_inode; int count; int len; len = path_name(mnt, dir_inode); wrong: int max, count, flag; len = path_name(mnt, dir_inode); struct inode *dir_inode; right: int len; if (len > 0) { struct inode *inode; int count;
 * Variable should be declared one per line, type and name, even if there are multiple variables of the same type. There should be one space between the variable type and the variable name to match upstream kernel coding guidelines (this is new since 2015-06 due to upstream kernel requirements). For maximum readability, longer or more important declarations should be at the top. There should always be an empty line after the variable declarations, before the start of the code.  There shouldn't be complex variable initializers (e.g. function calls) in the declaration block, since this may be easily missed by the reader and make it confusing to debug.
 * Variable declarations should be kept to the most internal scope, if practical and reasonable, to simplify understanding of where these variables are accessed and modified, and to avoid errors of using/reusing variables that only have meaning if certain branches are used:

inode = iget(foo); count = inode->i_size; if (count > 32) { int left = count; :               }        } wrong: int len = path_length(bar), count, ret; struct inode *inode = iget(foo); if (len > 0) { count = inode->i_size; if (count > 42) ret = 0; } else if (len == 0) { ret = -ENODATA; } else { CERROR("count is bad: %d\n", count); }

return ret;

Wrapping Long Lines
right: if (foo) bar; wrong: if (foo) bar; right: variable = do_something_complicated(long_argument, longer_argument,                                           longest_argument(sub_argument, foo_argument),                                           last_argument); if (some_long_condition(arg1, arg2, arg3) < some_long_value &&           another_long_condition(very_long_argument_name, another_long_argument_name) >           second_long_value) { do_something; ... wrong: variable = do_something_complicated(long_argument, longer_argument,               longest_argument(sub_argument, foo_argument),                last_argument); if (some_long_condition(arg1, arg2, arg3) < some_long_value &&               another_long_condition(very_long_argument_name, another_long_argument_name) >               second_long_value) { do_something; ...       off = le32_to_cpu(fsd->fsd_client_start) + cl_idx * le16_to_cpu(fsd->fsd_client_size);
 * Even for short conditionals, the operation should be on a separate line:
 * When you wrap a line containing parenthesis, start the continued line after the parenthesis so that the expression or argument is visually bracketed.
 * If you're wrapping an expression, put the operator at the end of the line. If there are no parentheses to which to align the start of the next line, just indent one more tab stop.

right: a++; b |= c;       d = (f > g) ? 0 : 1;
 * Binary and ternary (but not unary) operators should be separated from their arguments by one space.

Function Calls and Spaces
right: do_foo(bar, baz); wrong: do_foo ( bar,baz );
 * Function calls should be nestled against the parentheses, the parentheses should crowd the arguments, and one space should appear after commas:

right: for (a = 0; a < b; a++) if (a < b || a == c)       while (1) wrong: for( a=0; a<b; a++ ) if( a<b || a==c ) while( 1 ) int foo(void) {       if (bar) { this; that; } else if (baz) { stuff; } else { other_stuff; }       do { cow; } while (condition); }
 * Put a space between if, for, while, etc. and the following parenthesis. Put a space after each semicolon in a for statement.
 * Opening braces should be on the same line as the line that introduces the block, except for function calls. Bare closing braces (i.e. not else or while in do/while) get their own line.

Consistent if and else Blocks
right: if (foo) { bar; baz; } else { salmon; } wrong: if (foo) { bar; baz; } else moose;
 * If one part of a compound if block has braces, all should.

Safe Preprocessor Usage
right: do {                                                   \ int b = (a) + MAGIC;                           \ do_other_stuff(b);                             \ } while (0) wrong: do { \ int b = a + MAGIC; \ do_other_stuff(b); \ } while (0); right: static inline int invalid_dentry(struct dentry *d) {       return d->d_flags & DCACHE_LUSTRE_INVALID; return d_unhashed(d); }
 * When you define a macro, protect callers by placing parentheses round every parameter reference in the body.
 * Line up the backslashes of multi-line macros one tabstop from the end of the line to help readability.
 * Use a do/while (0) block with no trailing semicolon to ensure multi-statement macros are syntactically equivalent to procedure calls.
 * 1) define DO_STUFF(a)                                    \
 * 1) define DO_STUFF(a) \
 * If you write conditionally compiled code in a procedure body, make sure you do not create unbalanced braces, quotes, etc. This really confuses editors that navigate expressions or use fonts to highlight language features. It can often be much cleaner to put the conditionally compiled code in its own helper function which, by good choice of name, documents itself too.
 * 1) ifdef DCACHE_LUSTRE_INVALID
 * 1) else
 * 1) endif

int do_stuff(struct dentry *parent) {       if (invalid_dentry(parent)) { ... wrong: int do_stuff(struct dentry *parent            struct foo_extra *foo) ) {       if (parent->d_flags & DCACHE_LUSTRE_INVALID) {        if (d_unhashed(parent)) {                ...        } }
 * 1) ifdef HAVE_FOO_FEATURE
 * 1) else
 * 1) endif
 * 1) ifdef DCACHE_LUSTRE_INVALID
 * 1) else
 * 1) endif

/* lots of  stuff */ /* more * stuff */ /* This is a short comment */ /* * This is a multi-line comment. I wish the line would wrap already, * as I don't have much to write about. */
 * If you nest preprocessor commands, use spaces to visually delineate:
 * 1) ifdef __KERNEL__
 * 2) include
 * 3) define MOOSE steak
 * 4) else /* !__KERNEL__ \*/
 * 5) include
 * 6) define MOOSE prancing
 * 7) endif
 * For long or nested  blocks, include the conditional as a comment with each   and   to make it clear which block is being terminated:
 * 1) ifdef __KERNEL__
 * 2) if HAVE_SOME_KERNEL_API
 * 1) endif /* HAVE_SOME_KERNEL_API */
 * 2) else /* !__KERNEL__ */
 * 3) if HAVE_ANOTHER_FEATURE
 * 1) endif /* HAVE_ANOTHER_FEATURE */
 * 2) endif /* __KERNEL__ */
 * Single-line comments should have the leading  and trailing   on the same line as the comment.  Multi-line comments should have the leading   and trailing   on their own lines, to match the upstream kernel coding style. Intermediate lines should start with a   aligned with the   on the first line:

Function Declarations

 * External function declarations absolutely should *NEVER* go into .c files. The only exception is forward declarations for static functions in the same file that can't otherwise be moved before the caller. Instead, the declaration should go into the most "local" header available (e.g.  for a given subdirectory).  Having external function declarations in .c files can cause very difficult to diagnose runtime bugs, because the compiler takes the local declaration as correct, can not compare it to the actual function declared in a different file, and does not have a declaration in a header to compare it against, but the linker does not check that the number and type of function arguments match.

right: void ldlm_lock_addref_internal(struct ldlm_lock *lock, enum ldlm_lock_mode lock_mode, int refcount, int rc); wrong: void ldlm_lock_addref_internal(struct ldlm_lock *, int, int, int) int foo_stuff(int arg, char *buf) {       int bar; do_stuff; } EXPORT_SYMBOL(foo_stuff);
 * Function declarations in header files should include the variable names for the parameters, so that they are self explanatory in the header without having to look at the code to see what the parameter is:
 * Place  line immediately after the function that is being exported.  Having the   immediately following the function makes it clear to the reader whether there are users of this function outside this module and it can not be declared static.

Declaring Structures

 * Structure fields should have a common prefix derived from the structure name, so that they are easily found in the code and tag-jump works properly. This avoids problems with generic field names like page or count that have dozens or hundreds of potential matches in the code.
 * Structure and constant declarations should not be declared in multiple places. Put the struct into the most "local" header possible.
 * Structures that are passed over the wire need to be declared in, into the   and   files, and needs to be correctly swabbed when the RPC message is unpacked.  On-disk structures should be declared in   if they are not also passed over the network.  All protocol/disk structures should correctly align 64-bit values and supply explicit padding for alignment to avoid compiler-generated holes in the data structures.

struct foo { int    foo_val; int    foo_flag; int    foo_count; void  *foo_ptr; void  *foo_bar_ptr; void  *foo_baz_ptr; }; right: void func(void *data) {       struct foo fooz = { .foo_val = 1, .foo_flag = 0x20, .foo_ptr = data, .foo_baz_ptr = param }; } wrong: void func(void *data) {       /* not sure which field is being initialized, breaks (maybe silently) if structure adds a new field */ struct foo fooz = { 1, 0x20, data, param }; }
 * Structure initialization should be done by field names instead of using positional arguments:

Printing Functions
void _debug_req(struct ptlrpc_request *req, __u32 mask,               struct libcfs_debug_msg_data *data, const char *fmt, ...) __attribute__ ((format (printf, 4, 5)));
 * Functions that take variable parameters for -style argument processing should be declared with   so that the format string can be verified against the argument list by GCC:

Lustre Specific Guidelines
These guidelines are more specific to Lustre and not necessarily specific to the upstream kernel.

Lustre Variable Types
__u64               %llu/%llx/%lld (unsigned, hex, signed) __u32/int           %u/%x/%d (unsigned, hex, signed) (unsigned)long long %llu/%llx/%lld loff_t              %lld after a cast to long long (unfortunately) struct lu_fid       PFID, DFID struct ost_id       POSTID, DOSTID right: char buf[PATH_MAX]; int *array; OBD_ALLOC(array, 10 * sizeof(*array)); rc = strncpy(buf, src, sizeof(buf)); wrong: OBD_ALLOC(array, 40);                  /* This is just a random number, who knows what it means? */       OBD_ALLOC(array, 10 * sizeof(int));     /* silently breaks if array becomes __u64 */ OBD_ALLOC(array, 10 * sizeof(array));  /* This is the pointer size, not array size */ rc = strncpy(buf, src, sizeof(*buf));  /* This is the character size, not array size */
 * The types and /  formats used by Lustre code are:
 * Use  or   instead of   followed by
 * When using  it should be used on the variable itself, rather than specifying the type of the variable, so that if the variable changes type/size then   will remain correct:

Memory Allocation
right: OBD_ALLOC_PTR(mds_body); OBD_FREE_PTR(mds_body); wrong: OBD_ALLOC(mds_body, sizeof(*mds_body)); OBD_FREE(mds_body, sizeof(*mds_body));
 * When allocating/freeing a single struct, use  for clarity:
 * When allocating a large variable (pretty much anything above 8KiB, use  and   in order to avoid allocation failures.  This will first try to use   for the fastest allocation, but if this fails (or is too large for  ) then it will fall back to using   as needed.  This makes it easier to see which allocations may consume a lot of RAM, and avoids errors hit on long-running systems when memory is fragmented that are not seen during short-lived test runs or right after mount.
 * When allocating/freeing an array, use  or   or   and corresponding   function to avoid overflow during calculation of array size.

Proper use of LASSERT
right: len = strcat(foo, bar); LASSERT(len > 0); wrong: LASSERT(strcat(foo, bar) > 0);
 * Do not embed operations inside assertions. If assertions are disabled for performance reasons this code will not be executed.

Console Error Messages
right: CERROR("%s: error invoking upcall %s %s %s: rc = %d",       CERROR("%s: cannot open/create O: rc = %d\n", obd->obd_name,rc); wrong:         CERROR("err %d on param %s\n", rc, ptr);        CERROR("Can't get index (%d)\n", rc); right:         LCONSOLE_INFO("%s: %s now active, deleting orphans objects from "DFID" to "DFID\n",                      obd->obd_name, obd_uuid2str(uuid), PFID(start_fid), PFID(end_fid));        LCONSOLE_WARN("%s: new disk, initializing\n", obd->obd_name);        CERROR("%s: unsupported incompat filesystem feature(s) %x: rc = %d\n", obd->obd_name, incompat, rc);        CERROR("%s: cannot create root dentry: rc = %d\n", ll_get_fsname(sb, NULL, 0), rc);        CERROR("%s: error initializing 'fid' object: rc = %d\n",               mdd2obd_dev(m)->obd_name, rc);        CERROR("%s: cannot start coordinator thread: rc = %d\n", mdt_obd_name(mdt), rc); wrong: CERROR("Cannot get thandle\n"); CERROR("NULL bitmap!\n"); CERROR("invalid event\n"); CERROR("allocation failed\n");
 * Messages on the console should be written so they provide useful information to the *administrator* and/or support person in case of a *significant event* or *failure condition*.  They should not print "debugging" information in cases that might be hit under normal usage or user-generated race conditions, since verbose console error messages lose the important messages in a flood of noise.  Consider that there may be thousands of servers and tens of thousands of clients hitting some failure at the same time.
 * Console messages should print the Lustre device or filesystem name at the start of the message, since there are usually multiple targets running on a single server or multiple mountpoints on a client.
 * Error messages that print a numeric error value should print it at the end of the line in a consistent format using .  This makes it more clear what error was returned to the client/application, to make it easier to correlate server messages with application errors.  This will also allow automatic replacement of the numeric error numbers with error names/strings.
 * Error messages should also include enough information to make some useful diagnosis of the problem (e.g. FID and/or filename, client NID, actual values that caused failures, etc). Otherwise, there is little value in having printed an error, but then needing to reproduce the problem to diagnose it:

Configure Checks for Kernels
right: AC_DEFUN([LC_BI_DISK], [        LB_CHECK_COMPILE([if 'bi_disk' exist], bi_bdev, [ #include          ],[ ((struct bio *)0)->bi_disk = NULL; ],[                                            AC_DEFINE(HAVE_BI_DISK, 1,                      ['bi_disk' is available]) ])                                      ]) # LC_BI_DISK wrong: AC_DEFUN([LC_BI_BDEV], [        LB_CHECK_COMPILE([if 'bi_bdev' exist], bi_bdev, [ #include          ],[ ((struct bio *)0)->bi_bdev = NULL; ],[                                            AC_DEFINE(HAVE_BI_BDEV, 1,                      ['bi_bdev' is available]) ])                                      ]) # LC_BI_BDEV AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2], [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) or alternately: AC_CACHE_CHECK([for EMX OS/2 environment],              [ac_cv_emxos2],               [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],                                                   [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])])
 * Configure checks should be annotated with the kernel version and Git commit hash (as returned by ) where the change was introduced, so that it is easier to know when the checks are obsolete and can be removed.  It also simplifies efforts to investigate that change again in the future if needed.
 * Configure checks should check for the new feature being added rather than the old feature being removed. This ensures that the conditional code will only use the newly detected feature, and will not break if the old feature has undergone changes between different kernels and the configure check itself might fail on various older kernels (e.g. struct moved between headers that didn't affect operations in the past).
 * 1) LC_BI_DISK
 * 2) 4.14 replaced bi_bdev to bi_disk
 * 3) commit: v4.13-rc2-339-g74d46992e0d9
 * 1) 4.14 replaced bi_bdev to bi_disk
 * 2) commit: v4.13-rc2-339-g74d46992e0d9
 * 1) LC_BI_BDEV
 * 1) LC_BI_BDEV
 * For Autoconf macros, follow the style suggested in the autoconf manual.