From: ebiederm@xmission.com (Eric W. Biederman)
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Oleg Nesterov <oleg@redhat.com>,
Russell King - ARM Linux admin <linux@armlinux.org.uk>,
Chris Metcalf <cmetcalf@ezchip.com>,
Christoph Lameter <cl@linux.com>, Kirill Tkhai <tkhai@yandex.ru>,
Mike Galbraith <efault@gmx.de>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 2/3] task: RCU protect tasks on the runqueue
Date: Tue, 03 Sep 2019 11:44:59 -0500 [thread overview]
Message-ID: <87k1apqqgk.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20190903074718.GT2386@hirez.programming.kicks-ass.net> (Peter Zijlstra's message of "Tue, 3 Sep 2019 09:47:18 +0200")
Peter Zijlstra <peterz@infradead.org> writes:
> On Tue, Sep 03, 2019 at 09:41:17AM +0200, Peter Zijlstra wrote:
>> On Mon, Sep 02, 2019 at 11:52:01PM -0500, Eric W. Biederman wrote:
>>
>> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> > index 2b037f195473..802958407369 100644
>> > --- a/kernel/sched/core.c
>> > +++ b/kernel/sched/core.c
>>
>> > @@ -3857,7 +3857,7 @@ static void __sched notrace __schedule(bool preempt)
>> >
>> > if (likely(prev != next)) {
>> > rq->nr_switches++;
>> > - rq->curr = next;
>> > + rcu_assign_pointer(rq->curr, next);
>> > /*
>> > * The membarrier system call requires each architecture
>> > * to have a full memory barrier after updating
>>
>> This one is sad; it puts a (potentially) expensive barrier in here. And
>> I'm not sure I can explain the need for it. That is, we've not changed
>> @next before this and don't need to 'publish' it as such.
>>
>> Can we use RCU_INIT_POINTER() or simply WRITE_ONCE(), here?
>
> That is, I'm thinking we qualify for point 3 (both a and b) of
> RCU_INIT_POINTER().
I don't think point (b) is a concern on any widely visible architecture.
After taking a quick skim through the users it does appear to me that
we almost definitely have changes to the task_struct since the last time
another cpu say that structure (3 a) and that we have cases where
reading stale values in the task_struct will result in incorrect
operation of the code.
The concern of point (b) is the old alpha caching case where you could
dereference a pointer and get a stale copy of the data structure. This
is a concern when an you are following the pointer from another cpu.
From my quick skim the cases I can see where point (b) might apply are
in fair.c:task_numa_compare lots of fields in task_struct are read. It
looks like reading a stale (old/wrong) value of cur->numa_group could be
very inexplicable and weird. Similarly in the membarrier code reading a
pre-exec version of cur->mm could give completely inexplicable results.
Finally in rcuwait_wake_up reading a stale version of the process
cur->state could cause incorrect or missed wake ups in wake_up_process.
There might already be enough barriers in the scheduler that the barrier
in rcu_update_pointer is redundant. The comment about membarrier at
least suggests that for processes that return to userspace we have a
full memory barrier.
So with a big fat comment explaining why it is safe we could potentially
use RCU_INIT_POINTER. I currently don't see where the appropriate
barriers are so I can not write that comment or with a clear conscious
write the code to use RCU_INIT_POINTER instead of rcu_assign_pointer.
Eric
next prev parent reply other threads:[~2019-09-03 16:45 UTC|newest]
Thread overview: 75+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-30 14:08 [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value Russell King - ARM Linux admin
2019-08-30 15:24 ` Oleg Nesterov
2019-08-30 15:30 ` Linus Torvalds
2019-08-30 15:40 ` Russell King - ARM Linux admin
2019-08-30 15:43 ` Linus Torvalds
2019-08-30 15:41 ` Linus Torvalds
2019-08-30 16:09 ` Oleg Nesterov
2019-08-30 16:21 ` Linus Torvalds
2019-08-30 16:44 ` Oleg Nesterov
2019-08-30 16:58 ` Linus Torvalds
2019-08-30 19:36 ` Eric W. Biederman
2019-09-02 13:40 ` Oleg Nesterov
2019-09-02 13:53 ` Peter Zijlstra
2019-09-02 14:44 ` Oleg Nesterov
2019-09-02 16:20 ` Peter Zijlstra
2019-09-02 17:04 ` Eric W. Biederman
2019-09-02 17:34 ` Linus Torvalds
2019-09-03 4:50 ` [PATCH 0/3] task: Making tasks on the runqueue rcu protected Eric W. Biederman
2019-09-03 4:51 ` [PATCH 1/3] task: Add a count of task rcu users Eric W. Biederman
2019-09-04 14:36 ` Oleg Nesterov
2019-09-04 14:44 ` Frederic Weisbecker
2019-09-04 15:32 ` Oleg Nesterov
2019-09-04 16:33 ` Frederic Weisbecker
2019-09-04 18:20 ` Linus Torvalds
2019-09-05 14:59 ` Frederic Weisbecker
2019-09-03 4:52 ` [PATCH 2/3] task: RCU protect tasks on the runqueue Eric W. Biederman
2019-09-03 7:41 ` Peter Zijlstra
2019-09-03 7:47 ` Peter Zijlstra
2019-09-03 16:44 ` Eric W. Biederman [this message]
2019-09-03 17:08 ` Linus Torvalds
2019-09-03 18:13 ` Eric W. Biederman
2019-09-03 19:18 ` Linus Torvalds
2019-09-03 20:06 ` Peter Zijlstra
2019-09-03 21:32 ` Paul E. McKenney
2019-09-05 20:02 ` Eric W. Biederman
2019-09-05 20:55 ` Paul E. McKenney
2019-09-06 7:07 ` Peter Zijlstra
2019-09-09 12:22 ` Eric W. Biederman
2019-09-25 7:36 ` Peter Zijlstra
2019-09-27 8:10 ` [tip: sched/urgent] tasks, sched/core: RCUify the assignment of rq->curr tip-bot2 for Eric W. Biederman
2019-09-03 19:42 ` [PATCH 2/3] task: RCU protect tasks on the runqueue Peter Zijlstra
2019-09-14 12:31 ` [PATCH v2 1/4] task: Add a count of task rcu users Eric W. Biederman
2019-09-14 12:31 ` [PATCH v2 2/4] task: Ensure tasks are available for a grace period after leaving the runqueue Eric W. Biederman
2019-09-14 12:32 ` [PATCH v2 3/4] task: With a grace period after finish_task_switch, remove unnecessary code Eric W. Biederman
2019-09-04 14:22 ` [PATCH 2/3] task: RCU protect tasks on the runqueue Frederic Weisbecker
2019-09-03 4:52 ` [PATCH 3/3] task: Clean house now that tasks on the runqueue are rcu protected Eric W. Biederman
2019-09-03 9:45 ` kbuild test robot
2019-09-03 13:06 ` Oleg Nesterov
2019-09-03 13:58 ` [PATCH 0/3] task: Making tasks on the runqueue " Oleg Nesterov
2019-09-03 15:44 ` Linus Torvalds
2019-09-03 19:46 ` Peter Zijlstra
[not found] ` <87muf7f4bf.fsf_-_@x220.int.ebiederm.org>
2019-09-14 12:33 ` [PATCH v2 1/4] task: Add a count of task rcu users Eric W. Biederman
2019-09-15 13:54 ` Paul E. McKenney
2019-09-27 8:10 ` [tip: sched/urgent] tasks: Add a count of task RCU users tip-bot2 for Eric W. Biederman
2019-09-14 12:33 ` [PATCH v2 2/4] task: Ensure tasks are available for a grace period after leaving the runqueue Eric W. Biederman
2019-09-15 14:07 ` Paul E. McKenney
2019-09-15 14:09 ` Paul E. McKenney
2019-09-27 8:10 ` [tip: sched/urgent] tasks, sched/core: " tip-bot2 for Eric W. Biederman
2019-09-14 12:34 ` [PATCH v2 3/4] task: With a grace period after finish_task_switch, remove unnecessary code Eric W. Biederman
2019-09-15 14:32 ` Paul E. McKenney
2019-09-15 17:07 ` Linus Torvalds
2019-09-15 18:47 ` Paul E. McKenney
2019-09-27 8:10 ` [tip: sched/urgent] tasks, sched/core: With a grace period after finish_task_switch(), " tip-bot2 for Eric W. Biederman
2019-09-14 12:35 ` [PATCH v2 4/4] task: RCUify the assignment of rq->curr Eric W. Biederman
2019-09-15 14:41 ` Paul E. McKenney
2019-09-15 17:59 ` Eric W. Biederman
2019-09-15 18:25 ` Eric W. Biederman
2019-09-15 18:48 ` Paul E. McKenney
2019-09-20 23:02 ` Frederic Weisbecker
2019-09-26 1:49 ` Eric W. Biederman
2019-09-26 12:42 ` Frederic Weisbecker
2019-09-14 17:43 ` [PATCH v2 0/4] task: Making tasks on the runqueue rcu protected Linus Torvalds
2019-09-17 17:38 ` Eric W. Biederman
2019-09-25 7:51 ` Peter Zijlstra
2019-09-26 1:11 ` Eric W. Biederman
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