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From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 02 May 2018 16:07:48 +0000
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Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
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On Wed, May 2, 2018 at 9:03 AM Mathieu Desnoyers <
mathieu.desnoyers@efficios.com> wrote:

> ----- On May 1, 2018, at 11:53 PM, Daniel Colascione dancol@google.com
wrote:
> [...]
> >
> > I think a small enhancement to rseq would let us build a perfect
userspace
> > mutex, one that spins on lock-acquire only when the lock owner is
running
> > and that sleeps otherwise, freeing userspace from both specifying ad-hoc
> > spin counts and from trying to detect situations in which spinning is
> > generally pointless.
> >
> > It'd work like this: in the per-thread rseq data structure, we'd
include a
> > description of a futex operation for the kernel would perform (in the
> > context of the preempted thread) upon preemption, immediately before
> > schedule(). If the futex operation itself sleeps, that's no problem: we
> > will have still accomplished our goal of running some other thread
instead
> > of the preempted thread.

> Hi Daniel,

> I agree that the problem you are aiming to solve is important. Let's see
> what prevents the proposed rseq implementation from doing what you
envision.

> The main issue here is touching userspace immediately before schedule().
> At that specific point, it's not possible to take a page fault. In the
proposed
> rseq implementation, we get away with it by raising a task struct flag,
and using
> it in a return to userspace notifier (where we can actually take a
fault), where
> we touch the userspace TLS area.

> If we can find a way to solve this limitation, then the rest of your
design
> makes sense to me.

Thanks for taking a look!

Why couldn't we take a page fault just before schedule? The reason we can't
take a page fault in atomic context is that doing so might call schedule.
Here, we're about to call schedule _anyway_, so what harm does it do to
call something that might call schedule? If we schedule via that call, we
can skip the manual schedule we were going to perform.