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

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

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 - JÃrgen? 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.


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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