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* Christian Borntraeger <borntrae@de.ibm.com> wrote: > From: Christian Borntraeger <borntrae@de.ibm.com> > > This patch adds a barrier() in futex unqueue_me to avoid aliasing of > two pointers. > > On my s390x system I saw the following oops: > So the code becomes more or less: > if (q->lock_ptr != 0) spin_lock(q->lock_ptr) > instead of > if (lock_ptr != 0) spin_lock(lock_ptr) > > Which caused the oops from above. interesting, how is this possible? We do a spin_lock(lock_ptr), and taking a spinlock is an implicit barrier(). So gcc must not delay evaluating lock_ptr to inside the critical section. And as far as i can see the s390 spinlock implementation goes through an 'asm volatile' piece of code, which is a barrier already. So how could this have happened? I have nothing against adding a barrier(), but we should first investigate why the spin_lock() didnt act as a barrier - there might be other, similar bugs hiding. (we rely on spin_lock()s barrier-ness in a fair number of places) > As a general note, this code of unqueue_me seems a bit fishy. The > retry logic of unqueue_me only works if we can guarantee, that the > original value of q->lock_ptr is always a spinlock (Otherwise we > overwrite kernel memory). We know that q->lock_ptr can change. I dont > know what happens with the original spinlock, as I am not an expert > with the futex code. yes, it is always a pointer to a valid spinlock, or NULL. futex_requeue() can change the spinlock from one to another, and wake_futex() can change it to NULL. The futex unqueue_me() fastpath is when a futex waiter was woken - in which case it's NULL. But it can still be non-NULL if we timed out or a signal happened, in which case we may race with a wakeup or a requeue. futex_requeue() changes the spinlock pointer if it holds both the old and the new spinlock. So it's race-free as far as i can see. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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