linux-kernel - RE: [PATCH 5.8 298/464] PCI: hv: Fix a timing issue which causes kdump to fail occasionally

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Date:   Mon, 17 Aug 2020 23:27:49 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
CC:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Wei Hu <weh@...rosoft.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: RE: [PATCH 5.8 298/464] PCI: hv: Fix a timing issue which causes
 kdump to fail occasionally

From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>  Sent: Monday, August 17, 2020 8:14 AM
> 
> From: Wei Hu <weh@...rosoft.com>
> 
> [ Upstream commit d6af2ed29c7c1c311b96dac989dcb991e90ee195 ]
> 
> Kdump could fail sometime on Hyper-V guest because the retry in
> hv_pci_enter_d0() releases child device structures in hv_pci_bus_exit().
> 
> Although there is a second asynchronous device relations message sending
> from the host, if this message arrives to the guest after
> hv_send_resource_allocated() is called, the retry would fail.
> 
> Fix the problem by moving retry to hv_pci_probe() and start the retry
> from hv_pci_query_relations() call.  This will cause a device relations
> message to arrive to the guest synchronously; the guest would then be
> able to rebuild the child device structures before calling
> hv_send_resource_allocated().
> 
> Link:
> https://lore.kernel.org/linux-hyperv/20200727071731.18516-1-weh@microsoft.com/
> Fixes: c81992e7f4aa ("PCI: hv: Retry PCI bus D0 entry on invalid device state")
> Signed-off-by: Wei Hu <weh@...rosoft.com>
> [lorenzo.pieralisi@....com: fixed a comment and commit log]
> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Reviewed-by: Michael Kelley <mikelley@...rosoft.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  drivers/pci/controller/pci-hyperv.c | 71 +++++++++++++++--------------
>  1 file changed, 37 insertions(+), 34 deletions(-)
> 

Greg --

Don't backport this patch to 5.8 and earlier.  It doesn't break anything,
but it doesn't fully accomplish what was intended either.  As such it will
probably need a revision in 5.9.  Wei Hu is unavailable for a few days
for personal reasons, so I'm jumping in here on his behalf.

Michael