Serious problem with ticket spinlocks on ia64

[Petr Tesarik]

Hi everybody,

SGI has recently experienced failures with the new ticket spinlock
implementation. Hedi Berriche sent me a simple test case that can
trigger the failure on the siglock. To debug the issue, I wrote a
small module that watches writes to current->sighand->siglock and
records the values.

I observed that the __ticket_spin_lock() primitive fails when the
tail wraps around to zero. I reconstructed the following:

CPU 7 holds the spinlock
CPU 5 wants to acquire the spinlock
Spinlock value is 0xfffcffff
	 (now serving 0x7fffe, next ticket 0x7ffff)

CPU 7 executes st2.rel to release the spinlock.
At the same time CPU 5 executes a fetchadd4.acq.
The resulting lock value is 0xfffe0000 (correct), and CPU 5 has
recorded its ticket number (0x7fff).

Consequently, the first spinlock loop iteration succeeds, and CPU 5
now holds the spinlock.

Next, CPU 5 releases the spinlock with st2.rel, changing the lock
value to 0x0 (correct).

SO FAR SO GOOD.

Now, CPU 4, CPU 5 and CPU 7 all want to acquire the lock again.
Interestingly, CPU 5 and CPU 7 are both granted the same ticket,
and the spinlock value (as seen from the debug fault handler) is
0x0 after single-stepping over the fetchadd4.acq, in both cases.
CPU 4 correctly sets the spinlock value to 0x1.

I don't know if the simultaneos acquire attempt and release are
necessary to trigger the bug, but I noted it here.

I've only seen this happen when the spinlock wraps around to zero,
but I don't know whether it cannot happen otherwise.

In any case, there seems to be a serious problem with memory
ordering, and I'm not an expert to tell exactly what it is.

Any ideas?

Petr Tesarik
L3 International
Novell, Inc.
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