On Fri, 17 Jul 2015 14:39:55 +0100
Laszlo Ersek <ler...@redhat.com> wrote:
> On 07/17/15 15:28, Marc Zyngier wrote:
> > On Fri, 17 Jul 2015 10:30:38 +0100
> > Paolo Bonzini <pbonz...@redhat.com> wrote:
> >
> >>
> >>
> >> On 17/07/2015 06:44, Paolo Bonzini wrote:
> >>>
> >>>
> >>> On 16/07/2015 21:05, Richard W.M. Jones wrote:
> >>>>
> >>>> Sorry to spoil things, but I'm still seeing this bug, although it is
> >>>> now a lot less frequent with your patch. I would estimate it happens
> >>>> more often than 1 in 5 runs with qemu.git, and probably 1 in 200 runs
> >>>> with qemu.git + the v2 patch series.
> >>>>
> >>>> It's the exact same hang in both cases.
> >>>>
> >>>> Is it possible that this patch doesn't completely close any race?
> >>>>
> >>>> Still, it is an improvement, so there is that.
> >>>
> >>> Would seem at first glance like a different bug.
> >>>
> >>> Interestingly, adding some "tracing" (qemu_clock_get_ns) makes the bug
> >>> more likely: now it reproduces in about 10 tries. Of course :) adding
> >>> other kinds of tracing instead make it go away again (>50 tries).
> >>>
> >>> Perhaps this:
> >>>
> >>> i/o thread vcpu thread worker thread
> >>> ---------------------------------------------------------------------
> >>> lock_iothread
> >>> notify_me = 1
> >>> ...
> >>> unlock_iothread
> >>> lock_iothread
> >>> notify_me = 3
> >>> ppoll
> >>> notify_me = 1
> >>> bh->scheduled = 1
> >>> event_notifier_set
> >>> event_notifier_test_and_clear
> >>> ppoll
> >>> ^^ hang
> >>>
> >>> In the exact shape above, it doesn't seem too likely to happen, but
> >>> perhaps there's another simpler case. Still, the bug exists.
> >>>
> >>> The above is not really related to notify_me. Here the notification is
> >>> not being optimized away! So I wonder if this one has been there forever.
> >>>
> >>> Fam suggested putting the event_notifier_test_and_clear before
> >>> aio_bh_poll(), but it does not work. I'll look more close
> >>>
> >>> However, an unconditional event_notifier_test_and_clear is pretty
> >>> expensive. On one hand, obviously correctness comes first. On the
> >>> other hand, an expensive operation at the wrong place can mask the race
> >>> very easily; I'll let the fix run for a while, but I'm not sure if a
> >>> successful test really says anything useful.
> >>
> >> So it may not be useful, but still successful test is successful. :)
> >> The following patch, which also includes the delta between v2 and v3
> >> of this series, survived 674 reboots before hitting a definitely
> >> unrelated problem:
> >>
> >> error: kvm run failed Function not implemented
> >> PC=00000000bf671210 SP=00000000c00001f0
> >> X00=000000000a003e70 X01=0000000000000000 X02=00000000bf680548
> >> X03=0000000000000030
> >> X04=00000000bbb5fc18 X05=00000000004b7770 X06=00000000bf721930
> >> X07=000000000000009a
> >> X08=00000000bf716858 X09=0000000000000090 X10=0000000000000000
> >> X11=0000000000000046
> >> X12=00000000a007e03a X13=0000000000000000 X14=0000000000000000
> >> X15=0000000000000000
> >> X16=00000000bf716df0 X17=0000000000000000 X18=0000000000000000
> >> X19=00000000bf6f5f18
> >> X20=0000000000000000 X21=0000000000000000 X22=0000000000000000
> >> X23=0000000000000000
> >> X24=0000000000000000 X25=0000000000000000 X26=0000000000000000
> >> X27=0000000000000000
> >> X28=0000000000000000 X29=0000000000000000 X30=0000000000000000
> >> PSTATE=60000305 (flags -ZC-)
> >>
> >> For the record, this is the kvm_run struct:
> >>
> >> $6 = {request_interrupt_window = 0 '\000', padding1 =
> >> "\000\000\000\000\000\000", exit_reason = 0,
> >> ready_for_interrupt_injection = 0 '\000', if_flag = 0 '\000', flags = 0,
> >> cr8 = 0, apic_base = 0, {hw = {
> >> hardware_exit_reason = 150994968}, fail_entry =
> >> {hardware_entry_failure_reason = 150994968}, ex = {
> >> exception = 150994968, error_code = 0}, io = {direction = 24 '\030',
> >> size = 0 '\000', port = 2304,
> >> count = 0, data_offset = 144}, debug = {arch = {<No data fields>}},
> >> mmio = {phys_addr = 150994968,
> >> data = "\220\000\000\000\000\000\000", len = 4, is_write = 0
> >> '\000'}, hypercall = {nr = 150994968,
> >> args = {144, 4, 0, 0, 0, 0}, ret = 0, longmode = 0, pad = 0},
> >> tpr_access = {rip = 150994968,
> >> is_write = 144, pad = 0}, s390_sieic = {icptcode = 24 '\030', ipa =
> >> 2304, ipb = 0},
> >> s390_reset_flags = 150994968, s390_ucontrol = {trans_exc_code =
> >> 150994968, pgm_code = 144}, dcr = {
> >> dcrn = 150994968, data = 0, is_write = 144 '\220'}, internal =
> >> {suberror = 150994968, ndata = 0,
> >> data = {144, 4, 0 <repeats 14 times>}}, osi = {gprs = {150994968,
> >> 144, 4, 0 <repeats 29 times>}},
> >> papr_hcall = {nr = 150994968, ret = 144, args = {4, 0, 0, 0, 0, 0, 0,
> >> 0, 0}}, s390_tsch = {
> >> subchannel_id = 24, subchannel_nr = 2304, io_int_parm = 0,
> >> io_int_word = 144, ipb = 0,
> >> dequeued = 4 '\004'}, epr = {epr = 150994968}, system_event = {type
> >> = 150994968, flags = 144},
> >> s390_stsi = {addr = 150994968, ar = 144 '\220', reserved = 0 '\000',
> >> fc = 0 '\000', sel1 = 0 '\000',
> >> sel2 = 0},
> >> padding =
> >> "\030\000\000\t\000\000\000\000\220\000\000\000\000\000\000\000\004",
> >> '\000' <repeats 238 times>}, kvm_valid_regs = 0, kvm_dirty_regs = 0, s =
> >> {regs = {<No data fields>},
> >> padding = '\000' <repeats 2047 times>}}
> >>
> >> Marc, does it ring any bell?
> >
> > Well, this is an example of a guest accessing non-memory using an
> > instruction that we cannot safely emulate - not an IO accessor (load
> > multiple, for example).
> >
> > In this case, we kill the guest (we could as well inject a fault).
> >
> > This vcpu seems to be accessing 0x9000018 (the mmio structure points
> > there), but I can't immediately relate it to the content of the
> > registers.
>
> [VIRT_UART] = { 0x09000000, 0x00001000 },
>
Still: there is nothing in the registers that remotely points to that
area. X0 is the closest, but it'd take a big negative offset to get
there.
Is that a Linux kernel? or something else?
M.
--
Jazz is not dead. It just smells funny.
