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call_function_many: fix list delete vs add race 

Peter pointed out there was nothing preventing the list_del_rcu in
smp_call_function_interrupt from running before the list_add_rcu in
smp_call_function_many.

Fix this by not setting refs until we have gotten the lock for the list.
Take advantage of the wmb in list_add_rcu to save an explicit additional
one.

I tried to force this race with a udelay before the lock & list_add and
by mixing all 64 online cpus with just 3 random cpus in the mask, but
was unsuccessful.  Still, inspection shows a valid race, and the fix is
a extension of the existing protection window in the current code.

Cc: stable@kernel.org (v2.6.32 and later)
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Milton Miller 2011-03-15 13:27:16 -06:00 committed by Linus Torvalds
parent ef2b4b95a6
commit e6cd1e07a1
1 changed files with 13 additions and 7 deletions
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20
kernel/smp.c
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@ -491,14 +491,15 @@ void smp_call_function_many(const struct cpumask *mask,
cpumask_clear_cpu(this_cpu, data->cpumask);
/*
* To ensure the interrupt handler gets an complete view
* we order the cpumask and refs writes and order the read
* of them in the interrupt handler. In addition we may
* only clear our own cpu bit from the mask.
* We reuse the call function data without waiting for any grace
* period after some other cpu removes it from the global queue.
* This means a cpu might find our data block as it is writen.
* The interrupt handler waits until it sees refs filled out
* while its cpu mask bit is set; here we may only clear our
* own cpu mask bit, and must wait to set refs until we are sure
* previous writes are complete and we have obtained the lock to
* add the element to the queue.
*/
smp_wmb();
atomic_set(&data->refs, cpumask_weight(data->cpumask));
raw_spin_lock_irqsave(&call_function.lock, flags);
/*
@ -507,6 +508,11 @@ void smp_call_function_many(const struct cpumask *mask,
* will not miss any other list entries:
*/
list_add_rcu(&data->csd.list, &call_function.queue);
/*
* We rely on the wmb() in list_add_rcu to order the writes
* to func, data, and cpumask before this write to refs.
*/
atomic_set(&data->refs, cpumask_weight(data->cpumask));
raw_spin_unlock_irqrestore(&call_function.lock, flags);
/*