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Date:   Sat, 14 May 2022 11:55:39 -0400
From:   Demi Marie Obenour <demi@invisiblethingslab.com>
To:     Xen developer discussion <xen-devel@lists.xenproject.org>
Cc:     Juergen Gross <jgross@suse.com>,
        Boris Ostrovski <boris.ostrovsky@oracle.com>,
        Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>, linux-kernel@vger.kernel.org
Subject: Hang in 5.17.4+ that appears to be due to Xen
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Date: Sat, 14 May 2022 11:55:39 -0400
From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: Xen developer discussion <xen-devel@lists.xenproject.org>
Cc: Juergen Gross <jgross@suse.com>,
	Boris Ostrovski <boris.ostrovsky@oracle.com>,
	Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= <marmarek@invisiblethingslab.com>,
	linux-kernel@vger.kernel.org
Subject: Hang in 5.17.4+ that appears to be due to Xen

In https://github.com/QubesOS/qubes-issues/issues/7481, a user reported
that Xorg locked up when resizing a VM window.  While I do not have the
same hardware the user does and thus cannot reproduce the bug, the stack
trace seems to indicate a deadlock between xen_gntdev and i915.  It
appears that gnttab_unmap_refs_sync() is waiting for i915 to free the
pages, while i915 is waiting for the MMU notifier that called
gnttab_unmap_refs_sync() to return.  Result: deadlock.

The problem appears to be that a mapped grant in PV mode will stay in
the =E2=80=9Cinvalidating=E2=80=9D state until it is freed.  While MMU noti=
fiers are
allowed to sleep, it appears that they cannot wait for the page to be
freed, as is happening here.  That said, I am not very familiar with
this code, so my diagnosis might be incorrect.
--=20
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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