| From | Andi Kleen <> | Subject | [PATCH] [212/275] call_function_many: fix list delete vs add race | Date | Wed, 30 Mar 2011 14:07:36 -0700 (PDT) |
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2.6.35-longterm review patch. If anyone has any objections, please let me know.
------------------ From: Milton Miller <miltonm@bga.com>
commit e6cd1e07a185d5f9b0aa75e020df02d3c1c44940 upstream.
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.
Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Milton Miller <miltonm@bga.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> Signed-off-by: Andi Kleen <ak@linux.intel.com>
--- kernel/smp.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-)
Index: linux-2.6.35.y/kernel/smp.c =================================================================== --- linux-2.6.35.y.orig/kernel/smp.c 2011-03-29 23:02:59.591310628 -0700 +++ linux-2.6.35.y/kernel/smp.c 2011-03-29 23:54:07.475811139 -0700 @@ -468,14 +468,15 @@ 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); /* @@ -484,6 +485,11 @@ * 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); /*
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