Re: [PATCH] btrfs: Relax memory barrier in btrfs_tree_unlock

[Nikolay Borisov <nborisov@suse.com>]

On 24.02.2018 02:14, David Sterba wrote:
> 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
> btrfs_tree_unlock appears.

Admittedly I haven't :) But I'd say "every little bit helps"

> 
>> 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.
>