Re: [Xen-devel] Race condition with scheduler runqueues

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 27.02.2013 07:19, Juergen Gross wrote:
> On 19.02.2013 10:28, Jan Beulich wrote:
> > > > > On 18.02.13 at 19:11, Andrew Cooper<andrew.cooper3@xxxxxxxxxx> wrote:
> > > Hello,
> > >
> > > Our testing has discovered a crash (pagefault at 0x0000000000000008)
> > > which I have tracked down to bad __runq_remove() in csched_vcpu_sleep()
> > > in sched_credit.c (because a static function of the same name also
> > > exists in sched_credit2.c, which confused matters to start with)
> > >
> > > The test case was a loop of localhost migrate of a 1vcpu HVM win8
> > > domain. The test case itself has passed many times in the past on the
> > > same Xen codebase (Xen-4.1.3), indicating that it is very rare. There
> > > does not appear to be any relevant changes between the version of Xen in
> > > the test and xen-4.1-testing.
> > >
> > > The failure itself is because of a XEN_DOMCTL_scheduler_op (trace below)
> > > from dom0, targeting the VCPU of the migrating domain.
> > >
> > > (XEN) Xen call trace:
> > > (XEN) [<ffff82c480116a14>] csched_vcpu_sleep+0x44/0x70
> > > (XEN) 0[<ffff82c480120117>] vcpu_sleep_nosync+0xe7/0x3b0
> > > (XEN) 12[<ffff82c4801203e9>] vcpu_sleep_sync+0x9/0x50
> > > (XEN) 14[<ffff82c48011fd4c>] sched_adjust+0xac/0x230
> > > (XEN) 24[<ffff82c480102bc1>] do_domctl+0x731/0x1130
> > > (XEN) 64[<ffff82c4802013c4>] compat_hypercall+0x74/0x80
> > >
> > > The relevant part of csched_vcpu_sleep() is
> > >
> > > else if ( __vcpu_on_runq(svc) )
> > > __runq_remove(svc);
> > >
> > > which disassembles to
> > >
> > > ffff82c480116a01: 49 8b 10 mov (%r8),%rdx
> > > ffff82c480116a04: 4c 39 c2 cmp %r8,%rdx
> > > ffff82c480116a07: 75 07 jne ffff82c480116a10
> > > <csched_vcpu_sleep+0x40>
> > > ffff82c480116a09: f3 c3 repz retq
> > > ffff82c480116a0b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> > > ffff82c480116a10: 49 8b 40 08 mov 0x8(%r8),%rax
> > > ffff82c480116a14: 48 89 42 08 mov %rax,0x8(%rdx) #
> > > <- Pagefault here
> > > ffff82c480116a18: 48 89 10 mov %rdx,(%rax)
> > > ffff82c480116a1b: 4d 89 40 08 mov %r8,0x8(%r8)
> > > ffff82c480116a1f: 4d 89 00 mov %r8,(%r8)
> > >
> > > The relevant crash registers from the pagefault are:
> > > rax: 0000000000000000
> > > rdx: 0000000000000000
> > > r8: ffff83080c89ed90
> > >
> > > If I am reading the code correctly, this means that runq->next is NULL,
> > > so we fail list_empty() and erroneously pass __vcpu_on_runq(). We then
> > > fail with a fault when trying to update runq->prev, which is also NULL.
> > >
> > > The only place I can spot in the code where the runq->{next,prev} could
> > > conceivably be NULL is in csched_alloc_vdata() between the memset() and
> > > INIT_LIST_HEAD(). This is logically sensible in combination with the
> > > localhost migrate loop, and I cant immediately see anything to prevent
> > > this race happening.
> > But that doesn't make sense: csched_alloc_vdata() doesn't store
> > svc into vc->sched_priv; that's being done by the generic
> > scheduler code once the actor returns.
> >
> > So I'd rather suspect a stale pointer being used, which is easily
> > possible when racing with sched_move_domain() (as opposed to
> > schedule_cpu_switch(), where the new pointer gets stored
> > _before_ de-allocating the old one).
> >
> > However, sched_move_domain() (as well as schedule_cpu_switch())
> > get called only from CPU pools code, and I would guess CPU pools
> > aren't involved here, and you don't in parallel soft offline/online
> > pCPU-s (as I'm sure you otherwise would have mentioned it).
> >
> > But wait - libxl__domain_make() _unconditionally_ calls
> > xc_cpupool_movedomain(), as does XendDomainInfo's
> > _constructDomain(). The reason for this escapes me - Jürgen? Yet
> > I'd expect the pool ID matching check to short cut the resulting
> > sysctl, i.e. sched_move_domain() ought to not be reached anyway
> > (worth verifying of course).
> >
> > The race there nevertheless ought to be fixed.
> Something like the attached patch?
>
> Not tested thoroughly yet.
Argh. Sent an old version, sorry. This one should be better.

Juergen

--
Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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