linux-kernel - [PATCH] dynticks: don't unlock spinlock twice

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Date:	Fri, 02 Mar 2007 21:52:40 -0500
From:	Andres Salomon <dilinger@...ian.org>
To:	linux-kernel@...r.kernel.org
CC:	Marcelo Tosatti <marcelo@...ck.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>
Subject: [PATCH] dynticks: don't unlock spinlock twice

During boot with dynticks enabled, we would sometimes get:

[   35.271900] Switched to high resolution mode on CPU 0
[   35.304468] BUG: spinlock already unlocked on CPU#0, swapper/1
[   35.338099]  lock: c06428a0, .magic: dead4ead, .owner: <none>/-1,
.owner_cpu:
 -1
[   35.373597]  [<c04d7cf0>] _raw_spin_unlock+0x28/0x67
[   35.406647]  [<c05ba279>] _spin_unlock+0x5/0x23
[   35.439369]  [<c04255f7>] tick_sched_timer+0x4e/0xa7
[   35.472388]  [<c04255a9>] tick_sched_timer+0x0/0xa7
[   35.504833]  [<c0422528>] hrtimer_run_queues+0x199/0x1ec
[   35.537617]  [<c0416b72>] run_timer_softirq+0x12/0x166
[   35.570019]  [<c04144d9>] __do_softirq+0x40/0x85

[   35.601542]  [<c0405494>] do_softirq+0x53/0xa9
...

This appears to be caused by run_hrtimer_queue() (called by
hrtimer_run_queues) calling spin_unlock_irq(&cpu_base->lock) before
calling timer->function(timer).  The callback function
(tick_sched_timer) expects cpu_base->lock to be held when it is called,
and attempts to unlock it.  Since it doesn't seem like anything within
tick_sched_timer really needs to hold the lock (afaict), the attached
patch simply removes the lock handling from tick_sched_timer.  Things
called by tick_sched_timer may grab the base->lock, but that's fine (and
their responsibility).  Let me know if there's some reason why the lock
should be held, and I can rework this.

Signed-off-by: Andres Salomon <dilinger@...ian.org>

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