netdev - Re: [PATCH] RxRPC: Fix a potential deadlock between the call resend_timer and state

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Message-Id: <20100804.215510.258099641.davem@davemloft.net>
Date:	Wed, 04 Aug 2010 21:55:10 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dhowells@...hat.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] RxRPC: Fix a potential deadlock between the call
 resend_timer and state_lock

From: David Howells <dhowells@...hat.com>
Date: Wed, 04 Aug 2010 13:34:17 +0100

> RxRPC can potentially deadlock as rxrpc_resend_time_expired() wants to get
> call->state_lock so that it can alter the state of an RxRPC call.  However, its
> caller (call_timer_fn()) has an apparent lock on the timer struct.
> 
> The problem is that rxrpc_resend_time_expired() isn't permitted to lock
> call->state_lock as this could cause a deadlock against rxrpc_send_abort() as
> that takes state_lock and then attempts to delete the resend timer by calling
> del_timer_sync().
> 
> The deadlock can occur because del_timer_sync() will sit there forever waiting
> for rxrpc_resend_time_expired() to return, but the latter may then wait for
> call->state_lock, which rxrpc_send_abort() holds around del_timer_sync()...
> 
> This leads to a warning appearing in the kernel log that looks something like
> the attached.
> 
> It should be sufficient to simply dispense with the locks.  It doesn't matter
> if we set the resend timer expired event bit and queue the event processor
> whilst we're changing state to one where the resend timer is irrelevant as the
> event can just be ignored by the processor thereafter.
 ...
> Signed-off-by: David Howells <dhowells@...hat.com>

Applied, thanks!
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