Commit bbf5410b authored Oct 20, 2022 by Uros Bizjak Committed by Christoph Hellwig Nov 15, 2022

nvmet: use try_cmpxchg in nvmet_update_sq_head



Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
nvmet_update_sq_head.  x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg (and related move instruction in
front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

Note that the value from *ptr should be read using READ_ONCE to prevent
the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

parent 5626196a

drivers/nvme/target/core.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -695,11 +695,10 @@ static void nvmet_update_sq_head(struct nvmet_req *req)
		if (req->sq->size) {
		u32 old_sqhd, new_sqhd;

		old_sqhd = READ_ONCE(req->sq->sqhd);
		do {
		old_sqhd = req->sq->sqhd;
		new_sqhd = (old_sqhd + 1) % req->sq->size;
		} while (cmpxchg(&req->sq->sqhd, old_sqhd, new_sqhd) !=
		old_sqhd);
		} while (!try_cmpxchg(&req->sq->sqhd, &old_sqhd, new_sqhd));
		}
		req->cqe->sq_head = cpu_to_le16(req->sq->sqhd & 0x0000FFFF);
		}