Messages in this thread Patch in this message | | | Date | Mon, 19 Aug 2013 11:35:33 -0400 | From | Steven Rostedt <> | Subject | [PATCH RT 3/3] swait: Add smp_mb() after setting h->list |
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From: Steven Rostedt <rostedt@goodmis.org>
The raw_spin_unlock() is not a full memory barrier. It only keeps things from leaking past it, but does not prevent leaks from entering the critical section. That is:
p = 1;
raw_spin_lock(); [...] raw_spin_unlock();
y = x
Can turn into:
p = 1;
raw_spin_lock();
load x
store p = 1
raw_spin_unlock();
y = x
This means that the condition check in __swait_event() (and friends) can be seen before the h->list is set.
raw_spin_lock();
load condition;
store h->list;
raw_spin_unlock();
And the other CPU can see h->list as empty, and this CPU see condition as not set, and possibly miss the wake up.
To prevent this from happening, add an mb() after setting the h->list.
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org> --- kernel/wait-simple.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/kernel/wait-simple.c b/kernel/wait-simple.c index 9725a11..2c85626 100644 --- a/kernel/wait-simple.c +++ b/kernel/wait-simple.c @@ -16,6 +16,8 @@ static inline void __swait_enqueue(struct swait_head *head, struct swaiter *w) { list_add(&w->node, &head->list); + /* We can't let the condition leak before the setting of head */ + smp_mb(); } /* Removes w from head->list. Must be called with head->lock locked. */ -- 1.7.10.4
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