netdev - Re: [patch net] mlxsw: core: Fix possible deadlock

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Date:   Wed, 18 Oct 2017 12:21:13 +0100 (WEST)
From:   David Miller <davem@...emloft.net>
To:     jiri@...nulli.us
Cc:     netdev@...r.kernel.org, idosch@...lanox.com, mlxsw@...lanox.com
Subject: Re: [patch net] mlxsw: core: Fix possible deadlock

From: Jiri Pirko <jiri@...nulli.us>
Date: Mon, 16 Oct 2017 16:28:28 +0200

> From: Ido Schimmel <idosch@...lanox.com>
> 
> When an EMAD is transmitted, a timeout work item is scheduled with a
> delay of 200ms, so that another EMAD will be retried until a maximum of
> five retries.
> 
> In certain situations, it's possible for the function waiting on the
> EMAD to be associated with a work item that is queued on the same
> workqueue (`mlxsw_core`) as the timeout work item. This results in
> flushing a work item on the same workqueue.
> 
> According to commit e159489baa71 ("workqueue: relax lockdep annotation
> on flush_work()") the above may lead to a deadlock in case the workqueue
> has only one worker active or if the system in under memory pressure and
> the rescue worker is in use. The latter explains the very rare and
> random nature of the lockdep splats we have been seeing:
 ...
> Fix this by creating another workqueue for EMAD timeouts, thereby
> preventing the situation of a work item trying to flush a work item
> queued on the same workqueue.
> 
> Fixes: caf7297e7ab5f ("mlxsw: core: Introduce support for asynchronous EMAD register access")
> Signed-off-by: Ido Schimmel <idosch@...lanox.com>
> Reported-by: Jiri Pirko <jiri@...lanox.com>
> Signed-off-by: Jiri Pirko <jiri@...lanox.com>

Applied.