Messages in this thread | | | Date | Tue, 4 Aug 2020 10:48:41 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier |
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----- On Aug 4, 2020, at 10:34 AM, Peter Zijlstra peterz@infradead.org wrote:
> On Tue, Jul 28, 2020 at 12:00:09PM -0400, Mathieu Desnoyers wrote: >> exit_mm should issue memory barriers after user-space memory accesses, >> before clearing current->mm, to order user-space memory accesses >> performed prior to exit_mm before clearing tsk->mm, which has the >> effect of skipping the membarrier private expedited IPIs. >> >> The membarrier system call can be issued concurrently with do_exit >> if we have thread groups created with CLONE_VM but not CLONE_THREAD. > > I'm still wonder what the exact failure case is though; exit_mm() is on > the exit path (as the name very much implies) and the thread is about to > die. The context switch that follows guarantees a full barrier before we > run anything else again.
Here is the scenario I have in mind:
Two thread groups are created, A and B. Thread group B is created by issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD. Let's assume we have a single thread within each thread group (Thread A and Thread B).
The AFAIU we can have:
Userspace variables:
int x = 0, y = 0;
CPU 0 CPU 1 Thread A Thread B (in thread group A) (in thread group B)
x = 1 barrier() y = 1 exit() exit_mm() current->mm = NULL; r1 = load y membarrier() skips CPU 0 (no IPI) because its current mm is NULL r2 = load x BUG_ON(r1 == 1 && r2 == 0)
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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