Messages in this thread Patch in this message | | | From | Uros Bizjak <> | Subject | [PATCH] nvmet: use try_cmpxchg in nvmet_update_sq_head | Date | Thu, 20 Oct 2022 17:35:40 +0200 |
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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.
Cc: Christoph Hellwig <hch@lst.de> Cc: Sagi Grimberg <sagi@grimberg.me> Cc: Chaitanya Kulkarni <kch@nvidia.com> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- 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 14677145bbba..a384a0927aed 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); } -- 2.37.3
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