Re: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)
Hi,
On Tue, Oct 20, 2020 at 10:59:58AM -0400, Mathieu Desnoyers wrote:
> - On Oct 20, 2020, at 10:36 AM, Peter Zijlstra pet...@infradead.org wrote:
>
> > On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
> >> +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);
> >> +}
> >
> > This is suspisioucly similar to membarrier_switch_mm().
> >
> > Would something like so make sense?
>
> Very much yes. Do you want me to re-send the series, or you
> want to fold this in as you merge it ?
>
> Thanks,
>
> Mathieu
>
> >
> > ---
> > --- a/kernel/sched/membarrier.c
> > +++ b/kernel/sched/membarrier.c
> > @@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru
> >
> > 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);
> > + membarrier_switch_mm(this_rq(), NULL, next_mm);
> > }
> >
> > static int membarrier_global_expedited(void)
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index d2621155393c..3d589c2ffd28 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq
> > *rq,
> > struct mm_struct *prev_mm,
> > struct mm_struct *next_mm)
> > {
> > - int membarrier_state;
> > + int membarrier_state = 0;
> >
> > if (prev_mm == next_mm)
Unless I'm missing something subtle, in exit_mm(),
membarrier_update_current_mm() is called with @next_mm == NULL, and
inside membarrier_update_current_mm(), membarrier_switch_mm() is called
wiht @prev_mm == NULL. As a result, the branch above is taken, so
membarrier_update_current_mm() becomes a nop. I think we should use the
previous value of current->mm as the @prev_mm, something like below
maybe?
void update_current_mm(struct mm_struct *next_mm)
{
struct mm_struct *prev_mm;
unsigned long flags;
local_irq_save(flags);
prev_mm = current->mm;
current->mm = next_mm;
membarrier_switch_mm(this_rq(), prev_mm, next_mm);
local_irq_restore(flags);
}
, and replace all settings for "current->mm" in kernel with
update_current_mm().
Thoughts?
Regards,
Boqun
> > return;
> >
> > - membarrier_state = atomic_read(&next_mm->membarrier_state);
> > + if (next_mm)
> > + membarrier_state = atomic_read(&next_mm->membarrier_state);
> > +
> > if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> > return;
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
Re: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)
- On Oct 20, 2020, at 10:36 AM, Peter Zijlstra pet...@infradead.org wrote:
> On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
>> +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);
>> +}
>
> This is suspisioucly similar to membarrier_switch_mm().
>
> Would something like so make sense?
Very much yes. Do you want me to re-send the series, or you
want to fold this in as you merge it ?
Thanks,
Mathieu
>
> ---
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru
>
> 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);
> + membarrier_switch_mm(this_rq(), NULL, next_mm);
> }
>
> static int membarrier_global_expedited(void)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index d2621155393c..3d589c2ffd28 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq *rq,
> struct mm_struct *prev_mm,
> struct mm_struct *next_mm)
> {
> - int membarrier_state;
> + int membarrier_state = 0;
>
> if (prev_mm == next_mm)
> return;
>
> - membarrier_state = atomic_read(&next_mm->membarrier_state);
> + if (next_mm)
> + membarrier_state = atomic_read(&next_mm->membarrier_state);
> +
> if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> return;
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
Re: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)
On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
> +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);
> +}
This is suspisioucly similar to membarrier_switch_mm().
Would something like so make sense?
---
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru
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);
+ membarrier_switch_mm(this_rq(), NULL, next_mm);
}
static int membarrier_global_expedited(void)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d2621155393c..3d589c2ffd28 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq *rq,
struct mm_struct *prev_mm,
struct mm_struct *next_mm)
{
- int membarrier_state;
+ int membarrier_state = 0;
if (prev_mm == next_mm)
return;
- membarrier_state = atomic_read(&next_mm->membarrier_state);
+ if (next_mm)
+ membarrier_state = atomic_read(&next_mm->membarrier_state);
+
if (READ_ONCE(rq->membarrier_state) == membarrier_state)
return;
[PATCH 1/3] sched: fix exit_mm vs membarrier (v4)
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 AThread 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
Cc: Peter Zijlstra (Intel)
Cc: Boqun Feng
Cc: Will Deacon
Cc: Paul E. McKenney
Cc: Nicholas Piggin
Cc: Andy Lutomirski
Cc: Thomas Gleixner
Cc: Linus Torvalds
Cc: Alan Stern
Cc: linux...@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.
Changes since v3:
- Disable interrupts around call to membarrier_update_current_mm, which
is required to access the runqueue's fields.
---
include/linux/sched/mm.h | 5 +
kernel/exit.c | 16 +++-
kernel/sched/membarrier.c | 12
3 files changed, 32 insertions(+), 1 deletion(-)
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..18ca74c07085 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -475,10 +475,24 @@ 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();
+ local_irq_disable();
current->mm = NULL;
- mmap_read_unlock(mm);
+ membarrier_update_current_mm(NULL);
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))
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
