sched_pin() versus PCPU_GET

[mdf at FreeBSD.org]

On Thu, Jul 29, 2010 at 4:39 PM,  <mdf at freebsd.org> wrote:
> We've seen a few instances at work where witness_warn() in ast()
> indicates the sched lock is still held, but the place it claims it was
> held by is in fact sometimes not possible to keep the lock, like:
>
>        thread_lock(td);
>        td->td_flags &= ~TDF_SELECT;
>        thread_unlock(td);
>
> What I was wondering is, even though the assembly I see in objdump -S
> for witness_warn has the increment of td_pinned before the PCPU_GET:
>
> ffffffff802db210:       65 48 8b 1c 25 00 00    mov    %gs:0x0,%rbx
> ffffffff802db217:       00 00
> ffffffff802db219:       ff 83 04 01 00 00       incl   0x104(%rbx)
>         * Pin the thread in order to avoid problems with thread migration.
>         * Once that all verifies are passed about spinlocks ownership,
>         * the thread is in a safe path and it can be unpinned.
>         */
>        sched_pin();
>        lock_list = PCPU_GET(spinlocks);
> ffffffff802db21f:       65 48 8b 04 25 48 00    mov    %gs:0x48,%rax
> ffffffff802db226:       00 00
>        if (lock_list != NULL && lock_list->ll_count != 0) {
> ffffffff802db228:       48 85 c0                test   %rax,%rax
>         * Pin the thread in order to avoid problems with thread migration.
>         * Once that all verifies are passed about spinlocks ownership,
>         * the thread is in a safe path and it can be unpinned.
>         */
>        sched_pin();
>        lock_list = PCPU_GET(spinlocks);
> ffffffff802db22b:       48 89 85 f0 fe ff ff    mov    %rax,-0x110(%rbp)
> ffffffff802db232:       48 89 85 f8 fe ff ff    mov    %rax,-0x108(%rbp)
>        if (lock_list != NULL && lock_list->ll_count != 0) {
> ffffffff802db239:       0f 84 ff 00 00 00       je     ffffffff802db33e
> <witness_warn+0x30e>
> ffffffff802db23f:       44 8b 60 50             mov    0x50(%rax),%r12d
>
> is it possible for the hardware to do any re-ordering here?
>
> The reason I'm suspicious is not just that the code doesn't have a
> lock leak at the indicated point, but in one instance I can see in the
> dump that the lock_list local from witness_warn is from the pcpu
> structure for CPU 0 (and I was warned about sched lock 0), but the
> thread id in panic_cpu is 2.  So clearly the thread was being migrated
> right around panic time.
>
> This is the amd64 kernel on stable/7.  I'm not sure exactly what kind
> of hardware; it's a 4-way Intel chip from about 3 or 4 years ago IIRC.
>
> So... do we need some kind of barrier in the code for sched_pin() for
> it to really do what it claims?  Could the hardware have re-ordered
> the "mov    %gs:0x48,%rax" PCPU_GET to before the sched_pin()
> increment?

So after some research, the answer I'm getting is "maybe".  What I'm
concerned about is whether the h/w reordered the read of PCPU_GET in
front of the previous store to increment td_pinned.  While not an
ultimate authority,
http://en.wikipedia.org/wiki/Memory_ordering#In_SMP_microprocessor_systems
implies that stores can be reordered after loads for both Intel and
amd64 chips, which would I believe account for the behavior seen here.

Thanks,
matthew