[RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

[RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

[Mathieu Desnoyers]

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@efficios.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-mm@kvack.org
---
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
+	 * 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

Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

[Boqun Feng]

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@efficios.com>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: linux-mm@kvack.org
> ---
> 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
> 

Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

[Mathieu Desnoyers]

----- On Aug 16, 2020, at 11:23 AM, Boqun Feng boqun.feng@gmail.com wrote:

> 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@efficios.com>
>> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Paul E. McKenney <paulmck@kernel.org>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Alan Stern <stern@rowland.harvard.edu>
>> Cc: linux-mm@kvack.org
>> ---
>> 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.

Good point, I was wrong. Will instead reword as:

        /*
         * When a thread stops operating on an address space, the loop
         * in membarrier_private_expedited() may not observe that
         * tsk->mm, and the loop in membarrier_global_expedited() may
         * not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED
         * rq->membarrier_state, so those would not issue an IPI.
         * Membarrier requires a memory barrier after accessing
         * user-space memory, before clearing tsk->mm or the
         * rq->membarrier_state.
         */

And I'll make sure exit_mm clears this_rq()->membarrier_state as well.

Thanks,

Mathieu

> 
> 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

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