Messages in this thread | | | Date | Thu, 12 Sep 2019 16:47:35 +0100 | From | Will Deacon <> | Subject | Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state racy load (v2) |
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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
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