On Tue, Dec 03, 2013 at 09:52:33AM +0100, Ingo Molnar wrote:
> Indeed: this comes from mutex->count being separate from
> mutex->wait_lock, and this should affect every architecture that has a
> mutex->count fast-path implemented (essentially every architecture
> that matters).
>
> Such bugs should also magically go away with mutex debugging enabled.
Confirmed: I ran the reproducer with CONFIG_DEBUG_MUTEXES for a few
hours, and never got a single poison overwritten notice.
> I'd expect such bugs to be more prominent with unlucky object
> size/alignment: if mutex->count lies on a separate cache line from
> mutex->wait_lock.
>
> Side note: this might be a valid light weight debugging technique, we
> could add padding between the two fields to force them into separate
> cache lines, without slowing it down.
>
> Simon, would you be willing to try the fairly trivial patch below?
> Please enable CONFIG_DEBUG_MUTEX_FASTPATH=y. Does your kernel fail
> faster that way?
I didn't see much of a change other than the incremented poison byte is
now further in due to the padding, and it shows up in kmalloc-256.
I also tried with Linus' udelay() suggestion, below. With this, there
were many occurrences per second.
Simon-
diff --git a/kernel/mutex.c b/kernel/mutex.c
index d24105b..f65e735 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -25,6 +25,7 @@
#include <linux/spinlock.h>
#include <linux/interrupt.h>
#include <linux/debug_locks.h>
+#include <linux/delay.h>
/*
* In the DEBUG case we are using the "NULL fastpath" for mutexes,
@@ -740,6 +741,11 @@ __mutex_unlock_common_slowpath(atomic_t *lock_count, int
nested)
wake_up_process(waiter->task);
}
+ /* udelay a bit if the spinlock isn't contended */
+ if (lock->wait_lock.rlock.raw_lock.tickets.head + 1 ==
+ lock->wait_lock.rlock.raw_lock.tickets.tail)
+ udelay(1);
+
spin_unlock_mutex(&lock->wait_lock, flags);
}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
