NACK: [xenial][PATCH] UBUNTU: SAUCE: nvme-pci: fix memory barrier at nvme_ext_write_doorbell

Mon Sep 24 13:12:31 UTC 2018

Hi, Stefan.

I searched for the commit on the web interface and I found it
(although it was pretty slow):

http://git.infradead.org/nvme.git/commit/07e860202d18

Let me know if you want to adjust anything.

-- 
Regards,
Marcelo

On Thu, Sep 13, 2018 at 05:39:23PM +0200, Stefan Bader wrote:
> On 13.09.2018 16:02, Marcelo Henrique Cerri wrote:
> > On Wed, Sep 12, 2018 at 10:26:29AM +0200, Stefan Bader wrote:
> >> On 11.09.2018 21:50, Marcelo Henrique Cerri wrote:
> >>> BugLink: https://bugs.launchpad.net/bugs/1788222
> >>>
> >> --- cut --
> >>> That's a custom backport for Xenial of the commit 07e860202d18
> >>> ("nvme-pci: add a memory barrier to nvme_dbbuf_update_and_check_event"
> >>> from git://git.infradead.org/nvme.git).
> >>>
> >>> Original description:
> >>>
> >> -- cut end --
> >>> 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: 4a4037f212ae ("UBUNTU: SAUCE: nvme: improve performance for virtual NVMe devices")
> >> (backported from commit ??? git://git.infradead.org/nvme.git)
> >> [mhcerry: <rough description of what had to be changed>]
> > 
> > I can add these lines describing the origin and changes.
> > 
> >>> Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>
> >>> ---
> >>
> >> I cannot find the commit you refer to via the git web interface. Also, even
> >> reading through the comments, I am not sure this really is a backport of some
> >> upstream patch (in which case I would expect the commit message to be the same
> >> as the source of the backport.
> > 
> > What commit couldn't you find? That fix was crafted by me because
> > xenial carries a version of the original patchset that is completely
> > different from upstream.
> > 
> > I didn't keep the original title because it refers to the upstream
> > function that doesn't exist in Xenial.
> 
> At the top you wrote:
> >>> That's a custom backport for Xenial of the commit 07e860202d18
> >>> ("nvme-pci: add a memory barrier to nvme_dbbuf_update_and_check_event"
> >>> from git://git.infradead.org/nvme.git).
> 
> So I can only assume that this patch here is somehow related to that and to a
> certain degree adjusted. Like the rest of the commit message seems to come from
> that source.
> 
> As a reviewer I have no history of things. I can looks at the related upstream
> change and maybe make an educated guess whether this looks ok. But I just don't
> know what to do with this as searching the infradead git did find nothing with
> that sha1 or the commit message.
> 
> -Stefan
> 
> > 
> >> But the way it is now, I have no clue where this comes from and what was done to
> >> it before submission.
> >>
> >> -Stefan
> >>
> >>>  drivers/nvme/host/pci.c | 8 +++++++-
> >>>  1 file changed, 7 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> >>> index 5e5f065bf729..3752052ae20a 100644
> >>> --- a/drivers/nvme/host/pci.c
> >>> +++ b/drivers/nvme/host/pci.c
> >>> @@ -375,7 +375,13 @@ static void nvme_ext_write_doorbell(u16 value, u32 __iomem* q_db,
> >>>  	old_value = *db_addr;
> >>>  	*db_addr = value;
> >>>  
> >>> -	rmb();
> >>> +	/*
> >>> +	 * Ensure that the doorbell is updated before reading the event
> >>> +	 * index from memory.  The controller needs to provide similar
> >>> +	 * ordering to ensure the envent index is updated before reading
> >>> +	 * the doorbell.
> >>> +	 */
> >>> +	mb();
> >>>  	if (!nvme_ext_need_event(*event_idx, value, old_value))
> >>>  		goto no_doorbell;
> >>>  
> >>>
> >>
> >>
> > 
> > 
> > --
> > Regards,
> > Marcelo
> > 
> 
> 

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