From: Alexey Dobriyan <adobriyan@gmail.com>
Date: Thu, 7 May 2020 23:07:04 +0300
Subject: [PATCH] nvme-pci: fix "slimmer CQ head update"
Git-commit: a8de6639169b90e3dc4f27e752a3c5abac5e90da
Patch-mainline: v5.7-rc5
References: jsc#SLE-15608
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>
Acked-by: Hannes Reinecke <hare@suse.com>
---
drivers/nvme/host/pci.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
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;
}
}
--
2.16.4