Hi Mathieu,
On Fri, Aug 14, 2020 at 12:43:56PM -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.
>
> 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)
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
> Changes since v1:
> - Use smp_mb__after_spinlock rather than smp_mb.
> - Document race scenario in commit message.
> ---
> kernel/exit.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 733e80f334e7..fe64e6e28dd5 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -475,6 +475,14 @@ static void exit_mm(void)
> BUG_ON(mm != current->active_mm);
> /* more a memory barrier than a real lock */
> task_lock(current);
> + /*
> + * When a thread stops operating on an address space, the loop
> + * in membarrier_{private,global}_expedited() may not observe
Is it accurate to say that the correctness of
membarrier_global_expedited() relies on the observation of ->mm? Because
IIUC membarrier_global_expedited() loop doesn't check ->mm.
Regards,
Boqun
> + * that tsk->mm, and not issue an IPI. Membarrier requires a
> + * memory barrier after accessing user-space memory, before
> + * clearing tsk->mm.
> + */
> + smp_mb__after_spinlock();
> current->mm = NULL;
> mmap_read_unlock(mm);
> enter_lazy_tlb(mm, current);
> --
> 2.11.0
>
