Commit 115e12e5 authored Apr 24, 2019 by Stefan Bühler Committed by Jens Axboe Apr 30, 2019

io_uring: remove unnecessary barrier before reading cq head



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>

parent 4f7067c3

fs/io_uring.c

+5 −2

Original line number	Diff line number	Diff line
		@@ -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;

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