From: Andy Lutomirski <luto@amacapital.net>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Chris Metcalf <cmetcalf@mellanox.com>,
Gilad Ben Yossef <giladb@mellanox.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Christoph Lameter <cl@linux.com>,
Viresh Kumar <viresh.kumar@linaro.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Daniel Lezcano <daniel.lezcano@linaro.org>,
Francis Giraldeau <francis.giraldeau@gmail.com>,
Andi Kleen <andi@firstfloor.org>, Arnd Bergmann <arnd@arndb.de>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: task isolation discussion at Linux Plumbers
Date: Fri, 11 Nov 2016 09:00:37 -0800 [thread overview]
Message-ID: <CALCETrX+esCwaDuV6Y81FxW04+=7KvvY8-m2siyKjU7qYZhRDA@mail.gmail.com> (raw)
In-Reply-To: <20161110045213.GY4127@linux.vnet.ibm.com>
On Wed, Nov 9, 2016 at 8:52 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Wed, Nov 09, 2016 at 05:44:02PM -0800, Andy Lutomirski wrote:
>> On Wed, Nov 9, 2016 at 9:38 AM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>>
>> Are you planning on changing rcu_nmi_enter()? It would make it easier
>> to figure out how they interact if I could see the code.
>
> It already calls rcu_dynticks_eqs_exit(), courtesy of the earlier
> consolidation patches.
>
>> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> > index dbf20b058f48..342c8ee402d6 100644
>> > --- a/kernel/rcu/tree.c
>> > +++ b/kernel/rcu/tree.c
>>
>>
>> > /*
>> > @@ -305,17 +318,22 @@ static void rcu_dynticks_eqs_enter(void)
>> > static void rcu_dynticks_eqs_exit(void)
>> > {
>> > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>> > + int seq;
>> >
>> > /*
>> > - * CPUs seeing atomic_inc() must see prior idle sojourns,
>> > + * CPUs seeing atomic_inc_return() must see prior idle sojourns,
>> > * and we also must force ordering with the next RCU read-side
>> > * critical section.
>> > */
>> > - smp_mb__before_atomic(); /* See above. */
>> > - atomic_inc(&rdtp->dynticks);
>> > - smp_mb__after_atomic(); /* See above. */
>> > + seq = atomic_inc_return(&rdtp->dynticks);
>> > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
>> > - !(atomic_read(&rdtp->dynticks) & 0x1));
>> > + !(seq & RCU_DYNTICK_CTRL_CTR));
>>
>> I think there's still a race here. Suppose we're running this code on
>> cpu n and...
>>
>> > + if (seq & RCU_DYNTICK_CTRL_MASK) {
>> > + rcu_eqs_special_exit();
>> > + /* Prefer duplicate flushes to losing a flush. */
>> > + smp_mb__before_atomic(); /* NMI safety. */
>>
>> ... another CPU changes the page tables and calls rcu_eqs_special_set(n) here.
>
> But then rcu_eqs_special_set() will return false because we already
> exited the extended quiescent state at the atomic_inc_return() above.
>
> That should tell the caller to send an IPI.
>
>> That CPU expects that we will flush prior to continuing, but we won't.
>> Admittedly it's highly unlikely that any stale TLB entries would be
>> created yet, but nothing rules it out.
>
> That said, 0day is having some heartburn from this, so I must have broken
> something somewhere. My own tests of course complete just fine...
>
>> > + atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
>> > + }
>>
>> Maybe the way to handle it is something like:
>>
>> this_cpu_write(rcu_nmi_needs_eqs_special, 1);
>> barrier();
>>
>> /* NMI here will call rcu_eqs_special_exit() regardless of the value
>> in dynticks */
>>
>> atomic_and(...);
>> smp_mb__after_atomic();
>> rcu_eqs_special_exit();
>>
>> barrier();
>> this_cpu_write(rcu_nmi_needs_eqs_special, 0);
>>
>>
>> Then rcu_nmi_enter() would call rcu_eqs_special_exit() if the dynticks
>> bit is set *or* rcu_nmi_needs_eqs_special is set.
>>
>> Does that make sense?
>
> I believe that rcu_eqs_special_set() returning false covers this, but
> could easily be missing something.
I think you're right. I'll stare at it some more when I do the actual
TLB flush patch.
--Andy
next prev parent reply other threads:[~2016-11-11 17:01 UTC|newest]
Thread overview: 80+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-16 21:19 [PATCH v15 00/13] support "task_isolation" mode Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 01/13] vmstat: add quiet_vmstat_sync function Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 02/13] vmstat: add vmstat_idle function Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 03/13] lru_add_drain_all: factor out lru_add_drain_needed Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 04/13] task_isolation: add initial support Chris Metcalf
2016-08-29 16:33 ` Peter Zijlstra
2016-08-29 16:40 ` Chris Metcalf
2016-08-29 16:48 ` Peter Zijlstra
2016-08-29 16:53 ` Chris Metcalf
2016-08-30 7:59 ` Peter Zijlstra
2016-08-30 7:58 ` Peter Zijlstra
2016-08-30 15:32 ` Chris Metcalf
2016-08-30 16:30 ` Andy Lutomirski
2016-08-30 17:02 ` Chris Metcalf
2016-08-30 18:43 ` Andy Lutomirski
2016-08-30 19:37 ` Chris Metcalf
2016-08-30 19:50 ` Andy Lutomirski
2016-09-02 14:04 ` Chris Metcalf
2016-09-02 17:28 ` Andy Lutomirski
2016-09-09 17:40 ` Chris Metcalf
2016-09-12 17:41 ` Andy Lutomirski
2016-09-12 19:25 ` Chris Metcalf
2016-09-27 14:22 ` Frederic Weisbecker
2016-09-27 14:39 ` Peter Zijlstra
2016-09-27 14:51 ` Frederic Weisbecker
2016-09-27 14:48 ` Paul E. McKenney
2016-09-30 16:59 ` Chris Metcalf
2016-09-01 10:06 ` Peter Zijlstra
2016-09-02 14:03 ` Chris Metcalf
2016-09-02 16:40 ` Peter Zijlstra
2017-02-02 16:13 ` Eugene Syromiatnikov
2017-02-02 18:12 ` Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 05/13] task_isolation: track asynchronous interrupts Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 06/13] arch/x86: enable task isolation functionality Chris Metcalf
2016-08-30 21:46 ` Andy Lutomirski
2016-08-16 21:19 ` [PATCH v15 07/13] arm64: factor work_pending state machine to C Chris Metcalf
2016-08-17 8:05 ` Will Deacon
2016-08-16 21:19 ` [PATCH v15 08/13] arch/arm64: enable task isolation functionality Chris Metcalf
2016-08-26 16:25 ` Catalin Marinas
2016-08-16 21:19 ` [PATCH v15 09/13] arch/tile: " Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 10/13] arm, tile: turn off timer tick for oneshot_stopped state Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 11/13] task_isolation: support CONFIG_TASK_ISOLATION_ALL Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 12/13] task_isolation: add user-settable notification signal Chris Metcalf
2016-08-16 21:19 ` [PATCH v15 13/13] task_isolation self test Chris Metcalf
2016-08-17 19:37 ` [PATCH] Fix /proc/stat freezes (was [PATCH v15] "task_isolation" mode) Christoph Lameter
2016-08-20 1:42 ` Chris Metcalf
2016-09-28 13:16 ` Frederic Weisbecker
2016-08-29 16:27 ` Ping: [PATCH v15 00/13] support "task_isolation" mode Chris Metcalf
2016-09-07 21:11 ` Francis Giraldeau
2016-09-07 21:39 ` Francis Giraldeau
2016-09-08 16:21 ` Francis Giraldeau
2016-09-12 16:01 ` Chris Metcalf
2016-09-12 16:14 ` Peter Zijlstra
2016-09-12 21:15 ` Rafael J. Wysocki
2016-09-13 0:05 ` Rafael J. Wysocki
2016-09-13 16:00 ` Francis Giraldeau
2016-09-13 0:20 ` Francis Giraldeau
2016-09-13 16:12 ` Chris Metcalf
2016-09-27 14:49 ` Frederic Weisbecker
2016-09-27 14:35 ` Frederic Weisbecker
2016-09-30 17:07 ` Chris Metcalf
2016-11-05 4:04 ` task isolation discussion at Linux Plumbers Chris Metcalf
2016-11-05 16:05 ` Christoph Lameter
2016-11-07 16:55 ` Thomas Gleixner
2016-11-07 18:36 ` Thomas Gleixner
2016-11-07 19:12 ` Rik van Riel
2016-11-07 19:16 ` Will Deacon
2016-11-07 19:18 ` Rik van Riel
2016-11-11 20:54 ` Luiz Capitulino
2016-11-09 1:40 ` Paul E. McKenney
2016-11-09 11:14 ` Andy Lutomirski
2016-11-09 17:38 ` Paul E. McKenney
2016-11-09 18:57 ` Will Deacon
2016-11-09 19:11 ` Paul E. McKenney
2016-11-10 1:44 ` Andy Lutomirski
2016-11-10 4:52 ` Paul E. McKenney
2016-11-10 5:10 ` Paul E. McKenney
2016-11-11 17:00 ` Andy Lutomirski [this message]
2016-11-09 11:07 ` Frederic Weisbecker
2016-12-19 14:37 ` Paul E. McKenney
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