From: "Paul E. McKenney" <paulmck@kernel.org>
To: Zqiang <qiang1.zhang@intel.com>
Cc: frederic@kernel.org, quic_neeraju@quicinc.com,
joel@joelfernandes.org, rcu@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] rcu-tasks: Make RCU Tasks Trace checking for userspace execution
Date: Mon, 18 Jul 2022 08:21:32 -0700 [thread overview]
Message-ID: <20220718152132.GE1790663@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20220718001610.263700-1-qiang1.zhang@intel.com>
On Mon, Jul 18, 2022 at 08:16:10AM +0800, Zqiang wrote:
> For RCU tasks trace, the userspace execution is also a valid quiescent
> state, if the task is in userspace, the ->trc_reader_nesting should be
> zero and if the ->trc_reader_special.b.need_qs is not set, set the
> tasks ->trc_reader_special.b.need_qs is TRC_NEED_QS_CHECKED, this cause
> grace-period kthread remove it from holdout list if it remains here.
>
> This commit add rcu_tasks_trace_qs() to rcu_flavor_sched_clock_irq()
> when the kernel built with no PREEMPT_RCU.
>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
The looks plausible to me, but can you tell me how this avoids the
following sequence of events?
o CPU 0 takes a scheduling-clock interrupt. Just before this
point CPU 0 was running in user context, thus as you say
should not be in an RCU Tasks quiescent state.
o CPU 0 enters an RCU Tasks Trace read-side critical section.
o CPU 1 starts a new RCU Tasks Trace grace period.
o CPU 0 reaches the newly added rcu_note_voluntary_context_switch().
Except that the quiescent state implied by userspace execution
was before the new grace period, and thus does not apply to it.
(Yes, I know, if this is a bug in this patch, the bug already exists
due to the call in rcu_flavor_sched_clock_irq() for !PREEMPT kernels,
but if this change is safe, it should be possible to explain why.)
Thanx, Paul
> ---
> v1->v2:
> Fix build error due to undeclared rcu_tasks_trace_qs(), note in no-PREEMPT_RCU
> kernel, the RCU Tasks is replaced by RCU, so rcu_note_voluntary_context_switch()
> only include rcu_tasks_trace_qs().
>
> kernel/rcu/tree_plugin.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 4152816dd29f..5fb0b2dd24fd 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -976,7 +976,7 @@ static void rcu_flavor_sched_clock_irq(int user)
> * neither access nor modify, at least not while the
> * corresponding CPU is online.
> */
> -
> + rcu_note_voluntary_context_switch(current);
> rcu_qs();
> }
> }
> --
> 2.25.1
>
next prev parent reply other threads:[~2022-07-18 15:21 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-18 0:16 [PATCH v2] rcu-tasks: Make RCU Tasks Trace checking for userspace execution Zqiang
2022-07-18 15:21 ` Paul E. McKenney [this message]
2022-07-18 23:54 ` Zhang, Qiang1
2022-07-19 0:26 ` Paul E. McKenney
2022-07-19 4:34 ` Zhang, Qiang1
2022-07-19 14:38 ` Paul E. McKenney
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