Date: Wed, 27 Jan 2016 19:14:26 +0100 From: Olivier Houchard <cognet@ci0.org> To: John Baldwin <jhb@freebsd.org> Cc: Wojciech Macek <wma@semihalf.com>, hackers@freebsd.org, freebsd-arm@freebsd.org, arm64-dev <arm64-dev@semihalf.com> Subject: Re: SCHED_ULE race condition, fix proposal Message-ID: <20160127181426.GA48838@ci0.org> In-Reply-To: <2587742.rOiGAYXjN1@ralph.baldwin.cx> References: <CANsEV8e2QbW1Y83eC-d3GczWu4Lu91jDK14Xa1FkL=Y2s%2BRBMQ@mail.gmail.com> <2587742.rOiGAYXjN1@ralph.baldwin.cx>
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Hi,
I may be reading the arm64 code wrong, but shouldn't :
ldr x0, =_C_LABEL(blocked_lock)
ldr x2, [x0]
be just:
ldr x2, =_C_LABEL(blocked_lock)
Regards,
Olivier
On Wed, Jan 27, 2016 at 09:51:12AM -0800, John Baldwin wrote:
> On Wednesday, January 27, 2016 06:18:16 PM Wojciech Macek wrote:
> > Hello,
> >
> > I've encountered a very nasty race condition during debugging armv8 HWPMC.
> > It seems that ULE scheduler can execute the same thread on two different
> > CPUs at the same time...
> >
> > Here is the scenario.
> > The PMC driver must execute some of the code on the CPU0. To ensure that, a
> > process migration is triggered as following:
> >
> >
> > thread_lock(curthread);
> > sched_bind(curthread, cpu);
> > thread_unlock(curthread);
> >
> > KASSERT(curthread->td_oncpu == cpu,
> > ("[pmc,%d] CPU not bound [cpu=%d, curr=%d]", __LINE__,
> > cpu, curthread->td_oncpu));
> >
> >
> > That causes the context switch and (finally) execution of sched_switch()
> > function. The code correctly detects migration and calls
> > sched_switch_migrate. That function is supposed to add current thread to
> > the runqueue of another CPU ("tdn" variable). So it does:
> >
> > tdq_lock_pair(tdn, tdq);
> > tdq_add(tdn, td, flags);
> > tdq_notify(tdn, td);
> > TDQ_UNLOCK(tdn);
> > spinlock_exit();
> >
> >
> > But that sometimes is causing a crash, because the other CPU is staring to
> > process mi_switch as soon as the IPI arrives (via tdq_notify) and the
> > runqueue lock is released. The problem is, that the thread does not contain
> > valid register set, because its context was not yet stored - that happens
> > later in machine dependent cpu_switch function. In another words, the
> > sched_switch run on the CPU we want the thread to migrate onto restores
> > thread context before it was actually stored on another core - that causes
> > setting regs/pc/lt to some junk data and crash.
> >
> >
> > I'd like to discuss a possible solution for this. I think it would be
> > reasonable to extend cpu_switch to be capable of releasing a lock as the
> > last thing it does after storing everything into the PCB. We could then
> > remove the "TDQ_UNLOCK(tdn);" from the sched_switch_migrate and be sure
> > that in the situation of migration nobody is allowed to touch the target
> > runqueue until the migrating process finishes storing its context.
> >
> > But first I'd like to discuss some possible alternatives and maybe find
> > another solution, because any change in this area will impact all supported
> > architectures.
>
> This belongs on hackers, not developers@.
>
> cpu_switch() already does what you describe though in a slightly different
> way. The thread_lock() of a thread being switched out is set to blocked_lock.
> cpu_switch() on the new CPU will always spin until cpu_switch updates
> thread_lock of the old thread to point to the proper runq lock after saving
> its state in the pcb. arm64 does this here:
>
> /*
> * Release the old thread. This doesn't need to be a store-release
> * as the above dsb instruction will provide release semantics.
> */
> str x2, [x0, #TD_LOCK]
> #if defined(SCHED_ULE) && defined(SMP)
> /* Read the value in blocked_lock */
> ldr x0, =_C_LABEL(blocked_lock)
> ldr x2, [x0]
> 1:
> ldar x3, [x1, #TD_LOCK]
> cmp x3, x2
> b.eq 1b
> #endif
>
> Note the thread_lock_block() call just above the block you noted from
> sched_switch_migrate() to see where td_lock is set to &blocked_lock.
>
> If the comment about 'dsb' above is wrong that might explain why you see
> stale state in the PCB after seeing the new value of td_lock.
>
> --
> John Baldwin
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