Commit-ID: daf1aab9acfaaded09f53fa91dfe6e4e6926ec39 Gitweb: http://git.kernel.org/tip/daf1aab9acfaaded09f53fa91dfe6e4e6926ec39 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Mon, 2 Feb 2015 08:08:25 -0800 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Thu, 26 Feb 2015 11:57:31 -0800 documentation: Clarify memory-barrier semantics of atomic operations All value-returning atomic read-modify-write operations must provide full memory-barrier semantics on both sides of the operation. This commit clarifies the documentation to make it clear that these memory-barrier semantics are provided by the operations themselves, not by their callers. Reported-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> --- Documentation/atomic_ops.txt | 45 ++++++++++++++++++++++---------------------- 1 file changed, 23 insertions(+), 22 deletions(-) diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt index 183e41b..dab6da3 100644 --- a/Documentation/atomic_ops.txt +++ b/Documentation/atomic_ops.txt @@ -201,11 +201,11 @@ These routines add 1 and subtract 1, respectively, from the given atomic_t and return the new counter value after the operation is performed. -Unlike the above routines, it is required that explicit memory -barriers are performed before and after the operation. It must be -done such that all memory operations before and after the atomic -operation calls are strongly ordered with respect to the atomic -operation itself. +Unlike the above routines, it is required that these primitives +include explicit memory barriers that are performed before and after +the operation. It must be done such that all memory operations before +and after the atomic operation calls are strongly ordered with respect +to the atomic operation itself. For example, it should behave as if a smp_mb() call existed both before and after the atomic operation. @@ -233,21 +233,21 @@ These two routines increment and decrement by 1, respectively, the given atomic counter. They return a boolean indicating whether the resulting counter value was zero or not. -It requires explicit memory barrier semantics around the operation as -above. +Again, these primitives provide explicit memory barrier semantics around +the atomic operation. int atomic_sub_and_test(int i, atomic_t *v); This is identical to atomic_dec_and_test() except that an explicit -decrement is given instead of the implicit "1". It requires explicit -memory barrier semantics around the operation. +decrement is given instead of the implicit "1". This primitive must +provide explicit memory barrier semantics around the operation. int atomic_add_negative(int i, atomic_t *v); -The given increment is added to the given atomic counter value. A -boolean is return which indicates whether the resulting counter value -is negative. It requires explicit memory barrier semantics around the -operation. +The given increment is added to the given atomic counter value. A boolean +is return which indicates whether the resulting counter value is negative. +This primitive must provide explicit memory barrier semantics around +the operation. Then: @@ -257,7 +257,7 @@ This performs an atomic exchange operation on the atomic variable v, setting the given new value. It returns the old value that the atomic variable v had just before the operation. -atomic_xchg requires explicit memory barriers around the operation. +atomic_xchg must provide explicit memory barriers around the operation. int atomic_cmpxchg(atomic_t *v, int old, int new); @@ -266,7 +266,7 @@ with the given old and new values. Like all atomic_xxx operations, atomic_cmpxchg will only satisfy its atomicity semantics as long as all other accesses of *v are performed through atomic_xxx operations. -atomic_cmpxchg requires explicit memory barriers around the operation. +atomic_cmpxchg must provide explicit memory barriers around the operation. The semantics for atomic_cmpxchg are the same as those defined for 'cas' below. @@ -279,8 +279,8 @@ If the atomic value v is not equal to u, this function adds a to v, and returns non zero. If v is equal to u then it returns zero. This is done as an atomic operation. -atomic_add_unless requires explicit memory barriers around the operation -unless it fails (returns 0). +atomic_add_unless must provide explicit memory barriers around the +operation unless it fails (returns 0). atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0) @@ -460,9 +460,9 @@ the return value into an int. There are other places where things like this occur as well. These routines, like the atomic_t counter operations returning values, -require explicit memory barrier semantics around their execution. All -memory operations before the atomic bit operation call must be made -visible globally before the atomic bit operation is made visible. +must provide explicit memory barrier semantics around their execution. +All memory operations before the atomic bit operation call must be +made visible globally before the atomic bit operation is made visible. Likewise, the atomic bit operation must be visible globally before any subsequent memory operation is made visible. For example: @@ -536,8 +536,9 @@ except that two underscores are prefixed to the interface name. These non-atomic variants also do not require any special memory barrier semantics. -The routines xchg() and cmpxchg() need the same exact memory barriers -as the atomic and bit operations returning values. +The routines xchg() and cmpxchg() must provide the same exact +memory-barrier semantics as the atomic and bit operations returning +values. Spinlocks and rwlocks have memory barrier expectations as well. The rule to follow is simple: -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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