Two CPUs in critical section under Qemu/ia32/SMP

[Jakub Jermář]

After applying the attached changes on top of changeset:mainline,377, I am observing (under Qemu 0.11.?, 0.12.2+3) what appears to be more than one CPU in a spinlock-protected critical section.

In order to figure out more, I improved the spinlock code to be more sensitive to random lock corruption (which I can thus rule out) and also to be more observable by providing a global ring buffer for recording the locking history. See the attachement to see the diff. I am also going to attach screenshots which illustrate the panics.

Frankly speaking, my suspect number one is actually Qemu (since the HelenOS code looks good to me atm.), but I am logging this ticket anyway just for the case I am wrong. One more thing which makes me think that this is rather a Qemu issue is that with the given ring buffer and the spinlock_lock_debug() code, I would expect the panic to occur in spinlock_lock_debug() on either of the two checks for multiple CPUs in the CS, and not so late in spinlock_unlock(). With this behavior, the simulated CPUs appear to use some very strange memory model (i.e. we observe the effect of the lock_event_record() on both CPUs that manage to "lock" the spinlock, but in most of the cases do not hit the "not alone in critical section" panic).

[Jakub Jermář]

Diff from changeset:head,377.

[Jakub Jermář]

kernel.raw for the sake of running a disassembler on it.

[Jakub Jermář]

Screenshot with a ring buffer showing the lock-lock-unlock-unlock pattern

[Jakub Jermář]

Screenshot which shows panic on one of the CS invariant checks in the modified spinlock_lock_debug()

[Jakub Jermář]

Too old branch of a too old version of HelenOS on a too old version of QEMU. Deferring indefinitely.