sched_pin() versus PCPU_GET
John Baldwin
jhb at freebsd.org
Wed Aug 4 18:56:44 UTC 2010
On Wednesday, August 04, 2010 12:20:31 pm mdf at freebsd.org wrote:
> On Wed, Aug 4, 2010 at 2:26 PM, John Baldwin <jhb at freebsd.org> wrote:
> > On Tuesday, August 03, 2010 9:46:16 pm mdf at freebsd.org wrote:
> >> On Fri, Jul 30, 2010 at 2:31 PM, John Baldwin <jhb at freebsd.org> wrote:
> >> > On Friday, July 30, 2010 10:08:22 am John Baldwin wrote:
> >> >> On Thursday, July 29, 2010 7:39:02 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?
> >> >>
> >> >> Hmmm, I think it might be able to because they refer to different locations.
> >> >>
> >> >> Note this rule in section 8.2.2 of Volume 3A:
> >> >>
> >> >> • Reads may be reordered with older writes to different locations but not
> >> >> with older writes to the same location.
> >> >>
> >> >> It is certainly true that sparc64 could reorder with RMO. I believe ia64
> >> >> could reorder as well. Since sched_pin/unpin are frequently used to provide
> >> >> this sort of synchronization, we could use memory barriers in pin/unpin
> >> >> like so:
> >> >>
> >> >> sched_pin()
> >> >> {
> >> >> td->td_pinned = atomic_load_acq_int(&td->td_pinned) + 1;
> >> >> }
> >> >>
> >> >> sched_unpin()
> >> >> {
> >> >> atomic_store_rel_int(&td->td_pinned, td->td_pinned - 1);
> >> >> }
> >> >>
> >> >> We could also just use atomic_add_acq_int() and atomic_sub_rel_int(), but they
> >> >> are slightly more heavyweight, though it would be more clear what is happening
> >> >> I think.
> >> >
> >> > However, to actually get a race you'd have to have an interrupt fire and
> >> > migrate you so that the speculative read was from the other CPU. However, I
> >> > don't think the speculative read would be preserved in that case. The CPU
> >> > has to return to a specific PC when it returns from the interrupt and it has
> >> > no way of storing the state for what speculative reordering it might be
> >> > doing, so presumably it is thrown away? I suppose it is possible that it
> >> > actually retires both instructions (but reordered) and then returns to the PC
> >> > value after the read of listlocks after the interrupt. However, in that case
> >> > the scheduler would not migrate as it would see td_pinned != 0. To get the
> >> > race you have to have the interrupt take effect prior to modifying td_pinned,
> >> > so I think the processor would have to discard the reordered read of
> >> > listlocks so it could safely resume execution at the 'incl' instruction.
> >> >
> >> > The other nit there on x86 at least is that the incl instruction is doing
> >> > both a read and a write and another rule in the section 8.2.2 is this:
> >> >
> >> > • Reads are not reordered with other reads.
> >> >
> >> > That would seem to prevent the read of listlocks from passing the read of
> >> > td_pinned in the incl instruction on x86.
> >>
> >> I wonder how that's interpreted in the microcode, though? I.e. if the
> >> incr instruction decodes to load, add, store, does the h/w allow the
> >> later reads to pass the final store?
> >
> > Well, the architecture is defined in terms of the ISA, not the microcode, per
> > se, so I think it would have to treat the read for the incl as being an earlier
> > read than 'spinlocks'.
> >
> >> I added the following:
> >>
> >> sched_pin();
> >> lock_list = PCPU_GET(spinlocks);
> >> if (lock_list != NULL && lock_list->ll_count != 0) {
> >> + /* XXX debug for bug 67957 */
> >> + mfence();
> >> + lle = PCPU_GET(spinlocks);
> >> + if (lle != lock_list) {
> >> + panic("Bug 67957: had lock list %p, now %p\n",
> >> + lock_list, lle);
> >> + }
> >> + /* XXX end debug */
> >> sched_unpin();
> >>
> >> /*
> >>
> >> ... and the panic triggered. I think it's more likely that some
> >> barrier is needed in sched_pin() than that %gs is getting corrupted
> >> but can always be dereferenced.
> >
> > Actually, I would beg to differ in that case. If PCPU_GET(spinlocks)
> > returns non-NULL, then it means that you hold a spin lock,
>
> ll_count is 0 for the "correct" pc_spinlocks and non-zero for the
> "wrong" one, though. So I think it can be non-NULL but the current
> thread/CPU doesn't hold a spinlock.
Hmm, does the 'lock_list' pointer value in the dump match 'lock_list'
from another CPU?
--
John Baldwin
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