linux-kernel - Re: hrtimer possible issue

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Date:	Mon, 4 Feb 2013 11:12:56 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Izik Eidus <izik.eidus@...ellosystems.com>
cc:	linux-kernel@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>,
	Leonid Shatz <leonid.shatz@...ellosystems.com>
Subject: Re: hrtimer possible issue

On Sun, 3 Feb 2013, Izik Eidus wrote:

> Hi,
> 
> it seems like hrtimer_enqueue_reprogram contain a race which could result in
> timer.base switch during unlock/lock sequence.
> 
> See the code at __hrtimer_start_range_ns where it calls
> hrtimer_enqueue_reprogram. The later is releasing lock protecting the timer
> base for a short time and timer base switch can occur from a different CPU
> thread. Later when __hrtimer_start_range_ns calls unlock_hrtimer_base, a base
> switch could have happened and this causes the bug
> 
> Try to start the same hrtimer from two different threads in kernel running
> each one on a different CPU. Eventually one of the calls will cause timer base
> switch while another thread is not expecting it.
> 
> This can happen in virtualized environment where one thread can be delayed by
> lower hypervisor, and due to time delay a different CPU is taking care of
> missed timer start and runs the timer start logic on its own.

Nice analysis.
 
> This simple patch (just to give example of a fix) refactor this function to
> get rid of unneeded lock which immediately was followed by the unlock (with
> possible undesired base switch).
> 
> (Both the bug and the fixed were found/patched by Leonid Shatz)

The patch got mangled by your mail client and it is missing the proper
Signed-off-by annotation in the patch description. See
Documentation/SubmittingPatches.

Can you please resend ?

Thanks,

	tglx


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