From: Juergen Gross <jgross@suse.com>
To: Ross Lagerwall <ross.lagerwall@citrix.com>,
xen-devel@lists.xenproject.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [Xen-devel] [PATCH v2] xenbus: Avoid deadlock during suspend due to open transactions
Date: Mon, 27 May 2019 07:11:29 +0200 [thread overview]
Message-ID: <60d4d497-184f-c2ae-857a-46c906f05e22@suse.com> (raw)
Message-ID: <20190527051129.oOdZuTxop1UsLnNtV9VfSc7C1_g02CglmcUzqPyKLf0@z> (raw)
In-Reply-To: <20190513135635.22406-1-ross.lagerwall@citrix.com>
On 13/05/2019 15:56, Ross Lagerwall wrote:
> During a suspend/resume, the xenwatch thread waits for all outstanding
> xenstore requests and transactions to complete. This does not work
> correctly for transactions started by userspace because it waits for
> them to complete after freezing userspace threads which means the
> transactions have no way of completing, resulting in a deadlock. This is
> trivial to reproduce by running this script and then suspending the VM:
>
> import pyxs, time
> c = pyxs.client.Client(xen_bus_path="/dev/xen/xenbus")
> c.connect()
> c.transaction()
> time.sleep(3600)
>
> Even if this deadlock were resolved, misbehaving userspace should not
> prevent a VM from being migrated. So, instead of waiting for these
> transactions to complete before suspending, store the current generation
> id for each transaction when it is started. The global generation id is
> incremented during resume. If the caller commits the transaction and the
> generation id does not match the current generation id, return EAGAIN so
> that they try again. If the transaction was instead discarded, return OK
> since no changes were made anyway.
>
> This only affects users of the xenbus file interface. In-kernel users of
> xenbus are assumed to be well-behaved and complete all transactions
> before freezing.
>
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Juergen
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next prev parent reply other threads:[~2019-05-27 5:11 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-13 13:56 [PATCH v2] xenbus: Avoid deadlock during suspend due to open transactions Ross Lagerwall
2019-05-13 13:56 ` [Xen-devel] " Ross Lagerwall
2019-05-22 15:25 ` Ross Lagerwall
2019-05-22 15:25 ` [Xen-devel] " Ross Lagerwall
2019-05-27 5:11 ` Juergen Gross [this message]
2019-05-27 5:11 ` Juergen Gross
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