linux-kernel - Re: [PATCH] af_unix: Revert 'lock_interruptible' in stream receive code

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Message-ID: <56727EE9.5020805@stressinduktion.org>
Date:	Thu, 17 Dec 2015 10:22:49 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] af_unix: Revert 'lock_interruptible' in stream receive
 code

On 16.12.2015 21:09, Rainer Weikusat wrote:
> With b3ca9b02b00704053a38bfe4c31dbbb9c13595d0, the AF_UNIX SOCK_STREAM
> receive code was changed from using mutex_lock(&u->readlock) to
> mutex_lock_interruptible(&u->readlock) to prevent signals from being
> delayed for an indefinite time if a thread sleeping on the mutex
> happened to be selected for handling the signal. But this was never a
> problem with the stream receive code (as opposed to its datagram
> counterpart) as that never went to sleep waiting for new messages with the
> mutex held and thus, wouldn't cause secondary readers to block on the
> mutex waiting for the sleeping primary reader. As the interruptible
> locking makes the code more complicated in exchange for no benefit,
> change it back to using mutex_lock.
> 
> Signed-off-by: Rainer Weikusat <rweikusat@...ileactivedefense.com>
> ---
> 
> Considering that the datagram receive routine also doesn't go the sleep
> with the mutex held anymore, the 37ab4fa7844a044dc21fde45e2a0fc2f3c3b6490
> change to unix_autobind is now similarly purposeless.

I wouldn't do this conversion, yet. There is still a deadlock lingering
around which should be solved earlier:

http://lists.openwall.net/netdev/2015/11/10/4

Unfortunately I haven't found a good way how to solve it, yet.

Thanks,
Hannes

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