Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state racy load (v2)

[Will Deacon]

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@xxxxxxxxxx> 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