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io_uring: remove unnecessary barrier before reading cq head

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The memory operations before reading cq head are unrelated and we
don't care about their order.

Document that the control dependency in combination with READ_ONCE and
WRITE_ONCE forms a barrier we need.

Signed-off-by: Stefan Bühler <source@stbuehler.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Stefan Bühler 2019-04-24 23:54:18 +02:00 committed by Jens Axboe
parent 4f7067c3fb
commit 115e12e58d
1 changed files with 5 additions and 2 deletions
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7
fs/io_uring.c
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@ -431,8 +431,11 @@ static struct io_uring_cqe *io_get_cqring(struct io_ring_ctx *ctx)
unsigned tail;
tail = ctx->cached_cq_tail;
/* See comment at the top of the file */
smp_rmb();
/*
* writes to the cq entry need to come after reading head; the
* control dependency is enough as we're using WRITE_ONCE to
* fill the cq entry
*/
if (tail - READ_ONCE(ring->r.head) == ring->ring_entries)
return NULL;