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. > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
