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From: Boqun Feng <boqun.feng@gmail.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
	fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH rcu 04/18] rcu: Weaken ->dynticks accesses and updates
Date: Fri, 30 Jul 2021 13:56:41 +0800	[thread overview]
Message-ID: <YQOUmZmAZQIhjEWC@boqun-archlinux> (raw)
In-Reply-To: <20210729105331.GA301667@lothringen>

On Thu, Jul 29, 2021 at 12:53:31PM +0200, Frederic Weisbecker wrote:
> On Thu, Jul 29, 2021 at 03:58:04PM +0800, Boqun Feng wrote:
> > > The following litmus test, also adapted from the one supplied off-list
> > > by Frederic Weisbecker, models the RCU grace-period kthread detecting
> > > a non-idle CPU that is concurrently transitioning to idle:
> > > 
> > > 	C dynticks-into-idle
> > > 
> > > 	{
> > > 		DYNTICKS=1; (* Initially non-idle. *)
> > > 	}
> > > 
> > > 	P0(int *X, int *DYNTICKS)
> > > 	{
> > > 		int dynticks;
> > > 
> > > 		// Non-idle.
> > > 		WRITE_ONCE(*X, 1);
> > > 		dynticks = READ_ONCE(*DYNTICKS);
> > > 		smp_store_release(DYNTICKS, dynticks + 1);
> > > 		smp_mb();
> > 
> > this smp_mb() is not needed, as we rely on the release-acquire pair to
> > provide the ordering.
> > 
> > This means that if we use different implementations (one w/ smp_mb(),
> > another w/o) rcu_dynticks_inc() for idle-to-nonidle and nonidle-to-idle,
> > we could save a smp_mb(). Thoughts?
> 
> That's exactly what I wanted to propose but everybody was sober. Namely order
> only the RCU read side critical sections before/after idle together:
> 
>      READ side critical section
>      //enter idle
>      //exit idle
>      smp_mb()
>      READ side critical section
> 
> instead of ordering the RCU read side critical section before idle - with the RCU
> idle extended quiescent state - with the RCU read side critical section after idle:
> 
>      READ side critical section
>      //enter idle
>      smp_mb();
>      //exit idle
>      smp_mb()
>      READ side critical section
> 
> So the side effect now is that if the write side waits for the reader to
> report a quiescent state and scans its dynticks state and see it's not yet in
> RCU idle mode, then later on when the read side enters in RCU idle mode we
> expect it to see the write side updates.
> But after the barrier removal the reader will only see the write side update
> once we exit RCU idle mode.
> 
> So the following may happen:
> 
> 	P0(int *X, int *Y, int *DYNTICKS)
> 	{
> 		int y;
> 
> 		WRITE_ONCE(*X, 1);
> 		smp_store_release(DYNTICKS, 1); // rcu_eqs_enter
> 		//smp_mb() not there anymore
> 		y = READ_ONCE(*Y);
> 		smp_store_release(DYNTICKS, 2); // rcu_eqs_exit()
> 		smp_mb();
> 	}
> 
> 	P1(int *X, int *Y, int *DYNTICKS)
> 	{
> 		int x;
> 		int dynticks;
> 		
> 		WRITE_ONCE(*Y, 1);
> 		smp_mb();
> 		dynticks = smp_load_acquire(DYNTICKS);
> 		x = READ_ONCE(*X);
> 	}
> 
> 	exists (1:x=0 /\ 0:y=0)
> 

Thanks for the detailed explanation ;-)

> Theoretically it shouldn't matter because the RCU idle mode isn't
> supposed to perform RCU reads. But theoretically again once a CPU

Right, in LOCKDEP=y kernel, rcu_read_lock_held() requires
rcu_is_watching(), so rcu_dereference() is not allowed in idle mode,
unless using RCU_NONIDLE() or rcu_irq_enter_irqson() to temporarily exit
the idle mode.

> has reported a quiescent state, any further read is expected to see
> the latest updates from the write side.

Yes, but in your above case, doesn't P0 already reach to a quiescent
state even before WRITE_ONCE()? IOW, that case is similar to the
following:

	P0(int *X, int *Y)
	{
		// in QS

		WRITE_ONCE(*X, 1);
		y = READ_ONCE(*Y);
	}

	P1(int *X, int *Y)
	{
		WRITE_ONCE(*Y, 1);
		synchronize_rcu();
		x = READ_ONCE(*X);
	}

	exists (1:x=0 /\ 0:y=0)

And RCU doesn't guarantee the READ_ONCE() on P0 sees the WRITE_ONCE() on
P1.

> 
> So I don't know what to think. In practice I believe it's not a big deal
> because RCU idle mode code is usually a fragile path that just handles
> cpuidle code to put the CPU in/out low power mode. But what about dragons...

My current thought is that if the cpuidle code requires some ordering
with synchronize_rcu(), RCU_NONIDLE() should be used, and ordering can
be guaranteed in this case (RCU_NONIDLE() has a rcu_eqs_exit() in it).
Otherwise, it's a bug.

So looks like we can drop that smp_mb() in rcu_eqs_enter()? At least, we
can say something in the doc to prevent people from relying on the
ordering between normal reads in RCU idle mode and synchronize_rcu().

Thoughts?

Regards,
Boqun

  reply	other threads:[~2021-07-30  5:57 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-21 20:20 [PATCH rcu 0/18] Miscellaneous fixes for v5.15 Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 01/18] rcu: Fix to include first blocked task in stall warning Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 02/18] rcu: Fix stall-warning deadlock due to non-release of rcu_node ->lock Paul E. McKenney
2021-08-03 14:24   ` Qais Yousef
2021-08-03 15:52     ` Paul E. McKenney
2021-08-03 16:12       ` Qais Yousef
2021-08-03 16:28         ` Paul E. McKenney
2021-08-03 16:33           ` Qais Yousef
2021-08-04 13:50           ` Qais Yousef
2021-08-04 22:33             ` Paul E. McKenney
2021-08-06  9:56               ` Qais Yousef
2021-08-06  9:57   ` Qais Yousef
2021-08-06 11:43     ` Paul E. McKenney
2021-08-06 12:33       ` Qais Yousef
2021-07-21 20:21 ` [PATCH rcu 03/18] rcu: Remove special bit at the bottom of the ->dynticks counter Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 04/18] rcu: Weaken ->dynticks accesses and updates Paul E. McKenney
2021-07-21 20:41   ` Linus Torvalds
2021-07-21 21:25     ` Paul E. McKenney
2021-07-28 17:37   ` [PATCH v2 " Paul E. McKenney
2021-07-28 17:58     ` Linus Torvalds
2021-07-28 18:12       ` Mathieu Desnoyers
2021-07-28 18:32         ` Linus Torvalds
2021-07-28 18:39           ` Mathieu Desnoyers
2021-07-28 18:46         ` Paul E. McKenney
2021-07-28 18:46       ` Paul E. McKenney
2021-07-28 18:57         ` Linus Torvalds
2021-07-28 18:23     ` Mathieu Desnoyers
2021-07-28 18:58       ` Paul E. McKenney
2021-07-28 19:45         ` Paul E. McKenney
2021-07-28 20:03           ` Mathieu Desnoyers
2021-07-28 20:28             ` Paul E. McKenney
2021-07-29 14:41               ` Mathieu Desnoyers
2021-07-29 15:57                 ` Paul E. McKenney
2021-07-29 17:41                   ` Mathieu Desnoyers
2021-07-29 18:05                     ` Paul E. McKenney
2021-07-29 18:42                       ` Mathieu Desnoyers
2021-07-28 20:37     ` Josh Triplett
2021-07-28 20:47       ` Paul E. McKenney
2021-07-28 22:23         ` Frederic Weisbecker
2021-07-29  1:07           ` Paul E. McKenney
2021-07-29  7:58   ` [PATCH " Boqun Feng
2021-07-29 10:53     ` Frederic Weisbecker
2021-07-30  5:56       ` Boqun Feng [this message]
2021-07-30 17:18         ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 05/18] rcu: Mark accesses to ->rcu_read_lock_nesting Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 06/18] rculist: Unify documentation about missing list_empty_rcu() Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 07/18] rcu/tree: Handle VM stoppage in stall detection Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 08/18] rcu: Do not disable GP stall detection in rcu_cpu_stall_reset() Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 09/18] rcu: Start timing stall repetitions after warning complete Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 10/18] srcutiny: Mark read-side data races Paul E. McKenney
2021-07-29  8:23   ` Boqun Feng
2021-07-29 13:36     ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 11/18] rcu: Mark lockless ->qsmask read in rcu_check_boost_fail() Paul E. McKenney
2021-07-29  8:54   ` Boqun Feng
2021-07-29 14:03     ` Paul E. McKenney
2021-07-30  2:28       ` Boqun Feng
2021-07-30  3:26         ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 12/18] rcu: Make rcu_gp_init() and rcu_gp_fqs_loop noinline to conserve stack Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 13/18] rcu: Remove trailing spaces and tabs Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 14/18] rcu: Mark accesses in tree_stall.h Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 15/18] rcu: Remove useless "ret" update in rcu_gp_fqs_loop() Paul E. McKenney
2021-08-03 16:48   ` Joe Perches
2021-08-03 17:10     ` Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 16/18] rcu: Use per_cpu_ptr to get the pointer of per_cpu variable Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 17/18] rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU Paul E. McKenney
2021-07-21 20:21 ` [PATCH rcu 18/18] rcu: Print human-readable message for schedule() in RCU reader Paul E. McKenney

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