blktrace: re-write setting q->blk_trace
This is really about simplifying the double xchg patterns into a single cmpxchg, with the same logic. Other than the immediate cleanup, there are some subtleties this change deals with: (i) While the load of the old bt is fully ordered wrt everything, ie: old_bt = xchg(&q->blk_trace, bt); [barrier] if (old_bt) (void) xchg(&q->blk_trace, old_bt); [barrier] blk_trace could still be changed between the xchg and the old_bt load. Note that this description is merely theoretical and afaict very small, but doing everything in a single context with cmpxchg closes this potential race. (ii) Ordering guarantees are obviously kept with cmpxchg. (iii) Gets rid of the hacky-by-nature (void)xchg pattern. Signed-off-by:Davidlohr Bueso <dbueso@suse.de> eviewed-by:
Jeff Moyer <jmoyer@redhat.com> Signed-off-by:
Jens Axboe <axboe@fb.com>
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