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

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Date:   Tue, 4 Aug 2020 19:01:53 +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 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..