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authorAlexey Dobriyan <adobriyan@gmail.com>2020-05-07 23:07:04 +0300
committerJens Axboe <axboe@kernel.dk>2020-05-09 16:07:58 -0600
commita8de6639169b90e3dc4f27e752a3c5abac5e90da (patch)
tree8d8734baf42b3aaa451e54415fef1f799c8176d9
parent6bd87eec23cbc9ed222bed0f5b5b02bf300e9a8d (diff)
nvme-pci: fix "slimmer CQ head update"
Pre-incrementing ->cq_head can't be done in memory because OOB value can be observed by another context. This devalues space savings compared to original code :-\ $ ./scripts/bloat-o-meter ../vmlinux-000 ../obj/vmlinux add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-32 (-32) Function old new delta nvme_poll_irqdisable 464 456 -8 nvme_poll 455 447 -8 nvme_irq 388 380 -8 nvme_dev_disable 955 947 -8 But the code is minimal now: one read for head, one read for q_depth, one increment, one comparison, single instruction phase bit update and one write for new head. Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> Reported-by: John Garry <john.garry@huawei.com> Tested-by: John Garry <john.garry@huawei.com> Fixes: e2a366a4b0feaeb ("nvme-pci: slimmer CQ head update") Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@kernel.dk>
-rw-r--r--drivers/nvme/host/pci.c6
1 files changed, 5 insertions, 1 deletions
diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 4e79e412b276..e13c370de830 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -973,9 +973,13 @@ static inline void nvme_handle_cqe(struct nvme_queue *nvmeq, u16 idx)
static inline void nvme_update_cq_head(struct nvme_queue *nvmeq)
{
- if (++nvmeq->cq_head == nvmeq->q_depth) {
+ u16 tmp = nvmeq->cq_head + 1;
+
+ if (tmp == nvmeq->q_depth) {
nvmeq->cq_head = 0;
nvmeq->cq_phase ^= 1;
+ } else {
+ nvmeq->cq_head = tmp;
}
}