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

[Mathieu Desnoyers]

----- On Sep 12, 2019, at 11:47 AM, Will Deacon will@xxxxxxxxxx wrote:

> 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.

The current wording from membarrier(2) is:

              The  "expedited" commands complete faster than the non-expedited
              ones; they never block, but have the downside of  causing  extra
              overhead.

We could simply remove the "; they never block" part then ?

> 
>> 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?

Looking at:

#define GFP_KERNEL      (__GFP_RECLAIM | __GFP_IO | __GFP_FS)

I notice that it does not include __GFP_NOFAIL. What prevents GFP_KERNEL from
failing, and where is this guarantee documented ?

Regarding __GFP_NOFAIL, its use seems to be discouraged in linux/gfp.h:

 * %__GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
 * cannot handle allocation failures. The allocation could block
 * indefinitely but will never return with failure. Testing for
 * failure is pointless.
 * New users should be evaluated carefully (and the flag should be
 * used only when there is no reasonable failure policy) but it is
 * definitely preferable to use the flag rather than opencode endless
 * loop around allocator.
 * Using this flag for costly allocations is _highly_ discouraged.

So I am reluctant to use it.

But if we can agree on the right combination of flags that guarantees there
is no failure, I would be perfectly fine with using them to remove the fallback
code.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com