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From:   Dexuan Cui <decui@microsoft.com>
To:     kimbrownkd <kimbrownkd@gmail.com>,
        Michael Kelley <mikelley@microsoft.com>
CC:     Sasha Levin <sashal@kernel.org>, Long Li <longli@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
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        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] Drivers: hv: vmbus: Add mutex lock to channel show
 functions
Thread-Topic: [PATCH] Drivers: hv: vmbus: Add mutex lock to channel show
 functions
Thread-Index: AQHUsfdSwFQKguS0PEqWQ+jwS7iPOKW6o/swgAAzdACAAAXaYIAKRoiAgAAQ2xCABIFoHYABmzrw
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> From: Kimberly Brown <kimbrownkd@gmail.com>
> Sent: Thursday, January 31, 2019 9:47 AM
> ...
> 2) Prevent a deadlock that can occur between the proposed mutex_lock()
> call in the vmbus_chan_attr_show() function and the sysfs/kernfs function=
s.
Hi Kim,
Can you please share more details about the deadlock?=20
It's unclear to me how the deadlock happens.

> I've identified two possible solutions to the deadlock:
>=20
> 1) Add a new mutex to the vmbus_channel struct which protects changes to
> "channel->state". Use this new mutex in vmbus_chan_attr_show() instead of
> "vmbus_connection.channel_mutex".
>=20
> 2) Use "vmbus_connection.channel_mutex" in vmbus_chan_attr_show() as
> originally proposed, and acquire it with mutex_trylock(). If the mutex
> cannot be acquired, return -EINVAL.
It looks more like a workaround. IMO we should try to find a real fix. :-)
=20
> I'm leaning towards (2), using mutex_trylock().
> "vmbus_connection.channel_mutex"
> appears to be used only when a channel is being opened or closed, so
> vmbus_chan_attr_show() should be able to acquire the mutex when the
> channel is in use.
>=20
> If anyone has suggestions, please let me know.
>=20
> Thanks,
> Kim

Thanks,
-- Dexuan