sched_pin() versus PCPU_GET

[Kostik Belousov]

On Thu, Jul 29, 2010 at 04:57:25PM -0700, mdf at freebsd.org wrote:
> 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.
> 

Am I right that you suggest that in the sequence
	mov	%gs:0x0,%rbx      [1]
	incl	0x104(%rbx)       [2]
	mov	%gs:0x48,%rax     [3]
interrupt and preemption happen between points [2] and [3] ?
And the %rax value after the thread was put back onto the (different) new
CPU and executed [3] was still from the old cpu' pcpu area ?

I do not believe this is possible. CPU is always self-consistent. Context
switch from the thread can only occur on the return from interrupt
handler, in critical_exit() or such. This code is executing on the
same processor, and thus should already see the effect of [2], that
would prevent context switch.

If interrupt happens between [1] and [2], then context saving code
should still see the consistent view of the register file state,
regardless of the processor issuing speculative read of
*%gs:0x48. Return from the interrupt is the serialization point due to
iret, causing read in [3] to be reissued.

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