linux-kernel - Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state

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Date:   Sat, 12 May 2018 13:26:47 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Byungchul Park <byungchul.park@....com>,
        jiangshanlai@...il.com, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com, peterz@...radead.org
Subject: Re: [PATCH] rcu: Report a quiescent state when it's exactly in the
 state

On Sat, 12 May 2018 07:41:19 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> Don't get me wrong, this discussion was quite useful to me.  We probably
> need to at least change the comments, and perhaps the code as well.  But
> I agree that we need input from Peter and Steven to make much more forward
> progress.

It's the weekend so I skimmed more than read this thread, but I will
just add this.

The table Joel posted is interesting, and perhaps we should keep things
consistent with that. But that said, with respect to task-RCU, as
nothing on a trampoline should ever call cond_resched() (and perhaps I
should add code in lockdep that verifies this), we just want a
quiescent state that tells us that the task has left the trampoline. A
cond_resched() should be one of those points that does.

It really has nothing to do with scheduling or preemption. The issue is
that if a task is on a trampoline and gets preempted, there's no
knowing when it is off that trampoline where we can free it. We need to
have places in the kernel that we know is a quiescent state to move
task-RCU forward. cond_resched() seems to be one of them. schedule
itself can not be, because it can be called from an interrupt preempting
a task while it is on the trampoline.

-- Steve