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From: Will Deacon <will@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	paulmck <paulmck@linux.ibm.com>, Ingo Molnar <mingo@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Russell King, ARM Linux" <linux@armlinux.org.uk>,
	Chris Metcalf <cmetcalf@ezchip.com>, Chris Lameter <cl@linux.com>,
	Kirill Tkhai <tkhai@yandex.ru>, Mike Galbraith <efault@gmx.de>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state racy load (v2)
Date: Thu, 12 Sep 2019 16:47:35 +0100	[thread overview]
Message-ID: <20190912154734.j3mmjmqf2iltbenm@willie-the-truck> (raw)
In-Reply-To: <CAHk-=wgC8YspwtUeaV9ZwHjZDfXaJaT7i4v==Dp-vnVOF5i6qg@mail.gmail.com>

On Thu, Sep 12, 2019 at 03:24:35PM +0100, Linus Torvalds wrote:
> On Thu, Sep 12, 2019 at 2:48 PM Will Deacon <will@kernel.org> wrote:
> >
> > So the man page for sys_membarrier states that the expedited variants "never
> > block", which feels pretty strong. Do any other system calls claim to
> > provide this guarantee without a failure path if blocking is necessary?
> 
> The traditional semantics for "we don't block" is that "we block on
> memory allocations and locking and user accesses etc, but  we don't
> wait for our own IO".
> 
> So there may be new IO started (and waited on) as part of allocating
> new memory etc, or in just paging in user memory, but the IO that the
> operation _itself_ explicitly starts is not waited on.

Thanks, that makes sense, and I'd be inclined to suggest an update to the
sys_membarrier manpage to make this more clear since the "never blocks"
phrasing doesn't seem to be used like this for other system calls.

> No system call should ever be considered "atomic" in any sense. If
> you're doing RT, you should maybe expect "getpid()" to not ever block,
> but that's just about the exclusive list of truly nonblocking system
> calls, and even that can be preempted.

In which case, why can't we just use GFP_KERNEL for the cpumask allocation
instead of GFP_NOWAIT and then remove the failure path altogether? Mathieu?

Will

  reply	other threads:[~2019-09-12 15:47 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-06  3:12 [RFC PATCH 0/4] Membarrier fixes/cleanups Mathieu Desnoyers
2019-09-06  3:12 ` [RFC PATCH 1/4] Fix: sched/membarrier: private expedited registration check Mathieu Desnoyers
2019-09-06  3:12 ` [RFC PATCH 2/4] Cleanup: sched/membarrier: remove redundant check Mathieu Desnoyers
2019-09-06  3:12 ` [RFC PATCH 3/4] Cleanup: sched/membarrier: only sync_core before usermode for same mm Mathieu Desnoyers
2019-09-06  7:41   ` Peter Zijlstra
2019-09-06 13:40     ` Mathieu Desnoyers
2019-09-06  3:13 ` [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state racy load Mathieu Desnoyers
2019-09-06  8:23   ` Peter Zijlstra
2019-09-08 13:49     ` [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state racy load (v2) Mathieu Desnoyers
2019-09-08 16:51       ` Linus Torvalds
2019-09-10  9:48         ` Mathieu Desnoyers
2019-09-12 13:48           ` Will Deacon
2019-09-12 14:24             ` Linus Torvalds
2019-09-12 15:47               ` Will Deacon [this message]
2019-09-13 14:22                 ` Mathieu Desnoyers
2019-09-19 16:26                   ` Will Deacon
2019-09-19 17:33                     ` Mathieu Desnoyers
2019-09-09 11:00       ` Oleg Nesterov
2019-09-13 15:20         ` Mathieu Desnoyers
2019-09-13 16:04           ` Oleg Nesterov
2019-09-13 17:07             ` Mathieu Desnoyers

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