Date: Fri, 30 Jul 2010 10:08:22 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: mdf@freebsd.org Subject: Re: sched_pin() versus PCPU_GET Message-ID: <201007301008.22501.jhb@freebsd.org> In-Reply-To: <AANLkTikY20TxyeyqO5zP3zC-azb748kV-MdevPfm%2B8cq@mail.gmail.com> References: <AANLkTikY20TxyeyqO5zP3zC-azb748kV-MdevPfm%2B8cq@mail.gmail.com>
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On Thursday, July 29, 2010 7:39:02 pm mdf@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:
>=20
> thread_lock(td);
> td->td_flags &=3D ~TDF_SELECT;
> thread_unlock(td);
>=20
> 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:
>=20
> 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 =3D PCPU_GET(spinlocks);
> ffffffff802db21f: 65 48 8b 04 25 48 00 mov %gs:0x48,%rax
> ffffffff802db226: 00 00
> if (lock_list !=3D NULL && lock_list->ll_count !=3D 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 =3D 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 !=3D NULL && lock_list->ll_count !=3D 0) {
> ffffffff802db239: 0f 84 ff 00 00 00 je ffffffff802db33e
> <witness_warn+0x30e>
> ffffffff802db23f: 44 8b 60 50 mov 0x50(%rax),%r12d
>=20
> is it possible for the hardware to do any re-ordering here?
>=20
> 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.
>=20
> 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.
>=20
> 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:
=E2=80=A2 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=20
could reorder as well. Since sched_pin/unpin are frequently used to provid=
e=20
this sort of synchronization, we could use memory barriers in pin/unpin
like so:
sched_pin()
{
td->td_pinned =3D 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 t=
hey=20
are slightly more heavyweight, though it would be more clear what is happen=
ing=20
I think.
=2D-=20
John Baldwin
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