linux-kernel - Re: [RFC PATCH 2/2] sched: membarrier: cover kthread_use

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Message-ID: <20200805105920.GB35926@hirez.programming.kicks-ass.net>
Date:   Wed, 5 Aug 2020 12:59:20 +0200
From:   peterz@...radead.org
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>, paulmck <paulmck@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 2/2] sched: membarrier: cover kthread_use_mm

On Tue, Aug 04, 2020 at 07:01:53PM +0200, peterz@...radead.org wrote:
> On Tue, Aug 04, 2020 at 10:59:33AM -0400, Mathieu Desnoyers wrote:
> > ----- On Aug 4, 2020, at 10:51 AM, Peter Zijlstra peterz@...radead.org wrote:
> > > On Tue, Jul 28, 2020 at 12:00:10PM -0400, Mathieu Desnoyers wrote:
> 
> > >>  	task_lock(tsk);
> > >> +	/*
> > >> +	 * When a kthread stops operating on an address space, the loop
> > >> +	 * in membarrier_{private,global}_expedited() may not observe
> > >> +	 * that tsk->mm, and not issue an IPI. Membarrier requires a
> > >> +	 * memory barrier after accessing user-space memory, before
> > >> +	 * clearing tsk->mm.
> > >> +	 */
> > >> +	smp_mb();
> > >>  	sync_mm_rss(mm);
> > >>  	local_irq_disable();
> > > 
> > > Would it make sense to put the smp_mb() inside the IRQ disable region?
> > 
> > I've initially placed it right after task_lock so we could eventually
> > have a smp_mb__after_non_raw_spinlock or something with a much better naming,
> > which would allow removing the extra barrier when it is implied by the
> > spinlock.
> 
> Oh, right, fair enough. I'll go think about if smp_mb__after_spinlock()
> will work for mutexes too.
> 
> It basically needs to upgrade atomic*_acquire() to smp_mb(). So that's
> all architectures that have their own _acquire() and an actual
> smp_mb__after_atomic().
> 
> Which, from the top of my head are only arm64, power and possibly riscv.
> And if I then git-grep smp_mb__after_spinlock, all those seem to be
> covered.
> 
> But let me do a better audit..

All I could find is csky, which, afaict, defines a superfluous
smp_mb__after_spinlock.

The relevant architectures are indeed power, arm64 and riscv, they all
have custom acquire/release and all define smp_mb__after_spinlock()
appropriately.

Should we rename it to smp_mb__after_acquire() ?