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author | Jens Axboe <jens.axboe@oracle.com> | 2010-04-29 07:36:24 (GMT) |
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committer | Jens Axboe <jens.axboe@oracle.com> | 2010-04-29 07:36:24 (GMT) |
commit | 7407cf355fdf5500430be966dbbde84a27293bad (patch) | |
tree | 922861288ff38558ed721a79653f52b17b13bb95 /Documentation/RCU/lockdep.txt | |
parent | 6a47dc1418682c83d603b491df1d048f73aa973e (diff) | |
parent | 79dba2eaa771c3173957eccfd288e0e0d12e4d3f (diff) | |
download | linux-fsl-qoriq-7407cf355fdf5500430be966dbbde84a27293bad.tar.xz |
Merge branch 'master' into for-2.6.35
Conflicts:
fs/block_dev.c
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Diffstat (limited to 'Documentation/RCU/lockdep.txt')
-rw-r--r-- | Documentation/RCU/lockdep.txt | 28 |
1 files changed, 26 insertions, 2 deletions
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt index fe24b58..d7a49b2 100644 --- a/Documentation/RCU/lockdep.txt +++ b/Documentation/RCU/lockdep.txt @@ -32,9 +32,20 @@ checking of rcu_dereference() primitives: srcu_dereference(p, sp): Check for SRCU read-side critical section. rcu_dereference_check(p, c): - Use explicit check expression "c". + Use explicit check expression "c". This is useful in + code that is invoked by both readers and updaters. rcu_dereference_raw(p) Don't check. (Use sparingly, if at all.) + rcu_dereference_protected(p, c): + Use explicit check expression "c", and omit all barriers + and compiler constraints. This is useful when the data + structure cannot change, for example, in code that is + invoked only by updaters. + rcu_access_pointer(p): + Return the value of the pointer and omit all barriers, + but retain the compiler constraints that prevent duplicating + or coalescsing. This is useful when when testing the + value of the pointer itself, for example, against NULL. The rcu_dereference_check() check expression can be any boolean expression, but would normally include one of the rcu_read_lock_held() @@ -59,7 +70,20 @@ In case (1), the pointer is picked up in an RCU-safe manner for vanilla RCU read-side critical sections, in case (2) the ->file_lock prevents any change from taking place, and finally, in case (3) the current task is the only task accessing the file_struct, again preventing any change -from taking place. +from taking place. If the above statement was invoked only from updater +code, it could instead be written as follows: + + file = rcu_dereference_protected(fdt->fd[fd], + lockdep_is_held(&files->file_lock) || + atomic_read(&files->count) == 1); + +This would verify cases #2 and #3 above, and furthermore lockdep would +complain if this was used in an RCU read-side critical section unless one +of these two cases held. Because rcu_dereference_protected() omits all +barriers and compiler constraints, it generates better code than do the +other flavors of rcu_dereference(). On the other hand, it is illegal +to use rcu_dereference_protected() if either the RCU-protected pointer +or the RCU-protected data that it points to can change concurrently. There are currently only "universal" versions of the rcu_assign_pointer() and RCU list-/tree-traversal primitives, which do not (yet) check for |