From bbf5410bc69e131c82ad970ce7ee28b5906a6cc5 Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Thu, 20 Oct 2022 17:35:40 +0200 Subject: [PATCH] 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 Reviewed-by: Sagi Grimberg Signed-off-by: Christoph Hellwig --- drivers/nvme/target/core.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/drivers/nvme/target/core.c b/drivers/nvme/target/core.c index aecb5853f8da..560b6bbb6c44 100644 --- a/drivers/nvme/target/core.c +++ b/drivers/nvme/target/core.c @@ -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); } -- 2.11.0