netdev - Re: [PATCH net] inet: fix potential deadlock in reqsk_queue

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Date:	Tue, 18 Aug 2015 07:25:16 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] inet: fix potential deadlock in reqsk_queue_unlink()

On Tue, 2015-08-18 at 13:57 +0000, David Laight wrote:
> s the purpose of del_timer_sync()
> 
> I wasn't worried about del_timer_sync().
> The problem is the call to timer_pending().
> 
> If the timer has just expired, and the timeout function is running
> (on another cpu) then timer_pending() will return false.

This case is not a problem. refcount will be properly handled. No leak,
no reuse, no crash.


> So any tidyup path (apart from that called by the timeout function itself)
> will fail to wait for the function to finish.
> 
> So, in effect, you've converted the code back into a call to del_timer().

This is fine.

> 
> timer_pending() should probably never be called without the relevant
> timer lock help - because the result is stale.

Thats absolutely not true. Please read again timer code and linux code,
you'll see lot of timer uses without a lock being held (unserialized use
of timers)

This is why we have mod_timer_pending() for example.


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