* [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)
[not found] <20200924172508.8724-1-mathieu.desnoyers@efficios.com>
@ 2020-09-24 17:25 ` Mathieu Desnoyers
2020-10-07 14:29 ` Peter Zijlstra
0 siblings, 1 reply; 4+ messages in thread
From: Mathieu Desnoyers @ 2020-09-24 17:25 UTC (permalink / raw)
To: Peter Zijlstra, Boqun Feng
Cc: linux-kernel, Mathieu Desnoyers, Will Deacon, Paul E . McKenney,
Nicholas Piggin, Andy Lutomirski, Thomas Gleixner,
Linus Torvalds, Alan Stern, linux-mm
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.
exit_mm should also update the runqueue's membarrier_state so
membarrier global expedited IPIs are not sent when they are not
needed.
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: Boqun Feng <boqun.feng@gmail.com>
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.
Changes since v2:
- Introduce membarrier_update_current_mm,
- Use membarrier_update_current_mm to update rq's membarrier_state from
exit_mm.
---
include/linux/sched/mm.h | 5 +++++
kernel/exit.c | 12 ++++++++++++
kernel/sched/membarrier.c | 12 ++++++++++++
3 files changed, 29 insertions(+)
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index f889e332912f..5dd7f56baaba 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -370,6 +370,8 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
extern void membarrier_exec_mmap(struct mm_struct *mm);
+extern void membarrier_update_current_mm(struct mm_struct *next_mm);
+
#else
#ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS
static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
@@ -384,6 +386,9 @@ static inline void membarrier_exec_mmap(struct mm_struct *mm)
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
{
}
+static inline void membarrier_update_current_mm(struct mm_struct *next_mm)
+{
+}
#endif
#endif /* _LINUX_SCHED_MM_H */
diff --git a/kernel/exit.c b/kernel/exit.c
index 733e80f334e7..0767a2dbf245 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -475,7 +475,19 @@ 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_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.
+ */
+ smp_mb__after_spinlock();
current->mm = NULL;
+ membarrier_update_current_mm(NULL);
mmap_read_unlock(mm);
enter_lazy_tlb(mm, current);
task_unlock(current);
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 168479a7d61b..8bc8b8a888b7 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -63,6 +63,18 @@ void membarrier_exec_mmap(struct mm_struct *mm)
this_cpu_write(runqueues.membarrier_state, 0);
}
+void membarrier_update_current_mm(struct mm_struct *next_mm)
+{
+ struct rq *rq = this_rq();
+ int membarrier_state = 0;
+
+ if (next_mm)
+ membarrier_state = atomic_read(&next_mm->membarrier_state);
+ if (READ_ONCE(rq->membarrier_state) == membarrier_state)
+ return;
+ WRITE_ONCE(rq->membarrier_state, membarrier_state);
+}
+
static int membarrier_global_expedited(void)
{
int cpu;
--
2.17.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)
2020-09-24 17:25 ` [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3) Mathieu Desnoyers
@ 2020-10-07 14:29 ` Peter Zijlstra
2020-10-07 14:57 ` Mathieu Desnoyers
0 siblings, 1 reply; 4+ messages in thread
From: Peter Zijlstra @ 2020-10-07 14:29 UTC (permalink / raw)
To: Mathieu Desnoyers
Cc: Boqun Feng, linux-kernel, Will Deacon, Paul E . McKenney,
Nicholas Piggin, Andy Lutomirski, Thomas Gleixner,
Linus Torvalds, Alan Stern, linux-mm
On Thu, Sep 24, 2020 at 01:25:06PM -0400, Mathieu Desnoyers wrote:
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 733e80f334e7..0767a2dbf245 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -475,7 +475,19 @@ 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_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.
> + */
> + smp_mb__after_spinlock();
> current->mm = NULL;
> + membarrier_update_current_mm(NULL);
> mmap_read_unlock(mm);
> enter_lazy_tlb(mm, current);
> task_unlock(current);
This site seems to be lacking in IRQ disabling. As proposed it will
explode on RT.
Something like so to match kthread_unuse_mm().
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -486,11 +486,13 @@ static void exit_mm(void)
* rq->membarrier_state.
*/
smp_mb__after_spinlock();
+ local_irq_disable()
current->mm = NULL;
membarrier_update_current_mm(NULL);
- mmap_read_unlock(mm);
enter_lazy_tlb(mm, current);
+ local_irq_enable();
task_unlock(current);
+ mmap_read_unlock(mm);
mm_update_next_owner(mm);
mmput(mm);
if (test_thread_flag(TIF_MEMDIE))
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)
2020-10-07 14:29 ` Peter Zijlstra
@ 2020-10-07 14:57 ` Mathieu Desnoyers
2020-10-07 15:09 ` Peter Zijlstra
0 siblings, 1 reply; 4+ messages in thread
From: Mathieu Desnoyers @ 2020-10-07 14:57 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Boqun Feng, linux-kernel, Will Deacon, paulmck, Nicholas Piggin,
Andy Lutomirski, Thomas Gleixner, Linus Torvalds, Alan Stern,
linux-mm
----- On Oct 7, 2020, at 10:29 AM, Peter Zijlstra peterz@infradead.org wrote:
> On Thu, Sep 24, 2020 at 01:25:06PM -0400, Mathieu Desnoyers wrote:
>> diff --git a/kernel/exit.c b/kernel/exit.c
>> index 733e80f334e7..0767a2dbf245 100644
>> --- a/kernel/exit.c
>> +++ b/kernel/exit.c
>> @@ -475,7 +475,19 @@ 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_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.
>> + */
>> + smp_mb__after_spinlock();
>> current->mm = NULL;
>> + membarrier_update_current_mm(NULL);
>> mmap_read_unlock(mm);
>> enter_lazy_tlb(mm, current);
>> task_unlock(current);
>
> This site seems to be lacking in IRQ disabling. As proposed it will
> explode on RT.
Right, so irq off is needed for accessing this_rq()'s fields safely,
correct ?
I'll fold that fix in my patch for the next round, thanks!
Mathieu
>
> Something like so to match kthread_unuse_mm().
>
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -486,11 +486,13 @@ static void exit_mm(void)
> * rq->membarrier_state.
> */
> smp_mb__after_spinlock();
> + local_irq_disable()
> current->mm = NULL;
> membarrier_update_current_mm(NULL);
> - mmap_read_unlock(mm);
> enter_lazy_tlb(mm, current);
> + local_irq_enable();
> task_unlock(current);
> + mmap_read_unlock(mm);
> mm_update_next_owner(mm);
> mmput(mm);
> if (test_thread_flag(TIF_MEMDIE))
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)
2020-10-07 14:57 ` Mathieu Desnoyers
@ 2020-10-07 15:09 ` Peter Zijlstra
0 siblings, 0 replies; 4+ messages in thread
From: Peter Zijlstra @ 2020-10-07 15:09 UTC (permalink / raw)
To: Mathieu Desnoyers
Cc: Boqun Feng, linux-kernel, Will Deacon, paulmck, Nicholas Piggin,
Andy Lutomirski, Thomas Gleixner, Linus Torvalds, Alan Stern,
linux-mm
On Wed, Oct 07, 2020 at 10:57:52AM -0400, Mathieu Desnoyers wrote:
> ----- On Oct 7, 2020, at 10:29 AM, Peter Zijlstra peterz@infradead.org wrote:
>
> > On Thu, Sep 24, 2020 at 01:25:06PM -0400, Mathieu Desnoyers wrote:
> >> diff --git a/kernel/exit.c b/kernel/exit.c
> >> index 733e80f334e7..0767a2dbf245 100644
> >> --- a/kernel/exit.c
> >> +++ b/kernel/exit.c
> >> @@ -475,7 +475,19 @@ 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_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.
> >> + */
> >> + smp_mb__after_spinlock();
> >> current->mm = NULL;
> >> + membarrier_update_current_mm(NULL);
> >> mmap_read_unlock(mm);
> >> enter_lazy_tlb(mm, current);
> >> task_unlock(current);
> >
> > This site seems to be lacking in IRQ disabling. As proposed it will
> > explode on RT.
>
> Right, so irq off is needed for accessing this_rq()'s fields safely,
> correct ?
Yes, but also we're having IRQs disabled on ever other site that mucks
with ->mm these days.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2020-09-24 17:25 ` [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3) Mathieu Desnoyers
2020-10-07 14:29 ` Peter Zijlstra
2020-10-07 14:57 ` Mathieu Desnoyers
2020-10-07 15:09 ` Peter Zijlstra
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