2010/8/4 <m...@freebsd.org>: > On Fri, Jul 30, 2010 at 2:31 PM, John Baldwin <j...@freebsd.org> wrote: >> On Friday, July 30, 2010 10:08:22 am John Baldwin wrote: >>> On Thursday, July 29, 2010 7:39:02 pm m...@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? > > 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.
Are the 2 values just different or one of the 2 is NULL? Thanks, Attilio -- Peace can only be achieved by understanding - A. Einstein _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"