On 02/21/2014 06:18 PM, Tejun Heo wrote:
On Fri, Feb 21, 2014 at 06:01:29PM -0500, Peter Hurley wrote:smp_mb__after_unlock_lock() is only for ordering memory operations between two spin-locked sections on either the same lock or by the same task/cpu. Like: i = 1 spin_unlock(lock1) spin_lock(lock2) smp_mb__after_unlock_lock() j = 1 This guarantees that the store to j happens after the store to i. Without it, a cpu can spin_lock(lock2) j = 1 i = 1 spin_unlock(lock1); Hmmm? I'm pretty sure that's a full barrier. Local processor is always in order (w.r.t. the compiler).
It's a long story but the short version is that Documentation/memory-barriers.txt recently was overhauled to reflect what cpus actually do and what the different archs actually deliver. Turns out that unlock + lock is not guaranteed by all archs to be a full barrier. Thus the smb_mb__after_unlock_lock(). This is now all spelled out in memory-barriers.txt under the sub-heading "IMPLICIT KERNEL MEMORY BARRIERS". Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
