APPLIED[C/Unstable]: [SRU][Bionic][PATCH 1/1] nvme-pci: add a memory barrier to nvme_dbbuf_update_and_check_event

Seth Forshee seth.forshee at
Thu Aug 23 19:43:37 UTC 2018

On Fri, Aug 17, 2018 at 02:27:53PM -0400, Khalid Elmously wrote:
> From: Michal Wnukowski <wnukowski at>
> BugLink:
> In many architectures loads may be reordered with older stores to
> different locations.  In the nvme driver the following two operations
> could be reordered:
>  - Write shadow doorbell (dbbuf_db) into memory.
>  - Read EventIdx (dbbuf_ei) from memory.
> This can result in a potential race condition between driver and VM host
> processing requests (if given virtual NVMe controller has a support for
> shadow doorbell).  If that occurs, then the NVMe controller may decide to
> wait for MMIO doorbell from guest operating system, and guest driver may
> decide not to issue MMIO doorbell on any of subsequent commands.
> This issue is purely timing-dependent one, so there is no easy way to
> reproduce it. Currently the easiest known approach is to run "Oracle IO
> Numbers" (orion) that is shipped with Oracle DB:
> orion -run advanced -num_large 0 -size_small 8 -type rand -simulate \
> 	concat -write 40 -duration 120 -matrix row -testname nvme_test
> Where nvme_test is a .lun file that contains a list of NVMe block
> devices to run test against. Limiting number of vCPUs assigned to given
> VM instance seems to increase chances for this bug to occur. On test
> environment with VM that got 4 NVMe drives and 1 vCPU assigned the
> virtual NVMe controller hang could be observed within 10-20 minutes.
> That correspond to about 400-500k IO operations processed (or about
> 100GB of IO read/writes).
> Orion tool was used as a validation and set to run in a loop for 36
> hours (equivalent of pushing 550M IO operations). No issues were
> observed. That suggest that the patch fixes the issue.
> Fixes: f9f38e33389c ("nvme: improve performance for virtual NVMe devices")
> Signed-off-by: Michal Wnukowski <wnukowski at>
> Reviewed-by: Keith Busch <keith.busch at>
> Reviewed-by: Sagi Grimberg <sagi at>
> [hch: updated changelog and comment a bit]
> Signed-off-by: Christoph Hellwig <hch at>
> (cherry-picked from 07e860202d187d35ad24f0100caa0d244d4df2af git:// )
> Signed-off-by: Khalid Elmously <khalid.elmously at>

Applied to cosmic/master-next and unstable/master, thanks!

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