linux-kernel - Re: [PATCH 1/4] nbd: Remove signal usage

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Message-ID: <20151110044617.GC22011@ZenIV.linux.org.uk>
Date:	Tue, 10 Nov 2015 04:46:18 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Markus Pargmann <mpa@...gutronix.de>
Cc:	nbd-general@...ts.sourceforge.net, Oleg Nesterov <oleg@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] nbd: Remove signal usage

On Thu, Oct 29, 2015 at 04:42:37PM +0100, Markus Pargmann wrote:
>  	del_timer_sync(&nbd->timeout_timer);
> +
> +out:
> +	spin_unlock_irq(&nbd->sock_lock);

... and in its callback we have this:

> @@ -148,17 +155,15 @@ static void nbd_xmit_timeout(unsigned long arg)
>  
>  	nbd->disconnect = true;
>  
> -	spin_lock_irqsave(&nbd->tasks_lock, flags);
> +	spin_lock_irqsave(&nbd->sock_lock, flags);

* CPU 1 enters sock_shutdown() and grabs ->sock_lock.
* on CPU2 the timer hits and we enter the callback, where we spin on that
spinlock.
* in the meanwhile, CPU1 calls del_timer_sync()

Deadlock...
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