On Wed, Feb 14, 2018 at 02:37:26PM +0200, Nikolay Borisov wrote:
> When performing an unlock on an extent buffer we'd like to order the
> decrement of extent_buffer::blocking_writers with waking up any
> waiters. In such situations it's sufficient to use smp_mb__after_atomic
> rather than the heavy smp_mb. On architectures where atomic operations
> are fully ordered (such as x86 or s390) unconditionally executing
> a heavyweight smp_mb instruction causes a severe hit to performance
> while bringin no improvements in terms of correctness.
Have you measured this severe performance hit? There is an impact, but I
doubt you'll ever notice it in the profiles given where the
> The better thing is to use the appropriate smp_mb__after_atomic routine
> which will do the correct thing (invoke a full smp_mb or in the case
> of ordered atomics insert a compiler barrier). Put another way,
> an RMW atomic op + smp_load__after_atomic equals, in terms of
> semantics, to a full smp_mb. This ensures that none of the problems
> described in the accompanying comment of waitqueue_active occur.
> No functional changes.
I tend to agree.
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