* [PATCH v3] membarrier: Document scheduler barrier requirements
@ 2017-09-19 22:02 Mathieu Desnoyers
2017-09-19 22:53 ` Paul E. McKenney
2017-09-21 12:25 ` Peter Zijlstra
0 siblings, 2 replies; 4+ messages in thread
From: Mathieu Desnoyers @ 2017-09-19 22:02 UTC (permalink / raw)
To: Paul E . McKenney
Cc: linux-kernel, Mathieu Desnoyers, Peter Zijlstra, Boqun Feng,
Andrew Hunter, Maged Michael, gromer, Avi Kivity,
Benjamin Herrenschmidt, Paul Mackerras, Michael Ellerman,
Dave Watson
Document the membarrier requirement on having a full memory barrier in
__schedule() after coming from user-space, before storing to rq->curr.
It is provided by smp_mb__after_spinlock() in __schedule().
Document that membarrier requires a full barrier on transition from
kernel thread to userspace thread. We currently have an implicit barrier
from atomic_dec_and_test() in mmdrop() that ensures this.
The x86 switch_mm_irqs_off() full barrier is currently provided by many
cpumask update operations as well as write_cr3(). Document that
write_cr3() provides this barrier.
Changes since v1:
- Update comments to match reality for code paths which are after
storing to rq->curr, before returning to user-space.
Changes since v2:
- Update changelog (smp_mb__before_spinlock -> smp_mb__after_spinlock).
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: Andrew Hunter <ahh@google.com>
CC: Maged Michael <maged.michael@gmail.com>
CC: gromer@google.com
CC: Avi Kivity <avi@scylladb.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Dave Watson <davejwatson@fb.com>
---
arch/x86/mm/tlb.c | 5 +++++
include/linux/sched/mm.h | 5 +++++
kernel/sched/core.c | 9 +++++++++
3 files changed, 19 insertions(+)
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 1ab3821f9e26..74f94fe4aded 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
}
#endif
+ /*
+ * The membarrier system call requires a full memory barrier
+ * before returning to user-space, after storing to rq->curr.
+ * Writing to CR3 provides that full memory barrier.
+ */
if (real_prev == next) {
VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
next->context.ctx_id);
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3a19c253bdb1..766cc47c4d7c 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
extern void __mmdrop(struct mm_struct *);
static inline void mmdrop(struct mm_struct *mm)
{
+ /*
+ * The implicit full barrier implied by atomic_dec_and_test is
+ * required by the membarrier system call before returning to
+ * user-space, after storing to rq->curr.
+ */
if (unlikely(atomic_dec_and_test(&mm->mm_count)))
__mmdrop(mm);
}
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 18a6966567da..7977b25acf54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct *prev)
finish_arch_post_lock_switch();
fire_sched_in_preempt_notifiers(current);
+ /*
+ * When transitioning from a kernel thread to a userspace
+ * thread, mmdrop()'s implicit full barrier is required by the
+ * membarrier system call, because the current active_mm can
+ * become the current mm without going through switch_mm().
+ */
if (mm)
mmdrop(mm);
if (unlikely(prev_state == TASK_DEAD)) {
@@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
* Make sure that signal_pending_state()->signal_pending() below
* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
* done by the caller to avoid the race with signal_wake_up().
+ *
+ * The membarrier system call requires a full memory barrier
+ * after coming from user-space, before storing to rq->curr.
*/
rq_lock(rq, &rf);
smp_mb__after_spinlock();
--
2.11.0
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH v3] membarrier: Document scheduler barrier requirements
2017-09-19 22:02 [PATCH v3] membarrier: Document scheduler barrier requirements Mathieu Desnoyers
@ 2017-09-19 22:53 ` Paul E. McKenney
2017-09-21 12:25 ` Peter Zijlstra
1 sibling, 0 replies; 4+ messages in thread
From: Paul E. McKenney @ 2017-09-19 22:53 UTC (permalink / raw)
To: Mathieu Desnoyers
Cc: linux-kernel, Peter Zijlstra, Boqun Feng, Andrew Hunter,
Maged Michael, gromer, Avi Kivity, Benjamin Herrenschmidt,
Paul Mackerras, Michael Ellerman, Dave Watson
On Tue, Sep 19, 2017 at 06:02:05PM -0400, Mathieu Desnoyers wrote:
> Document the membarrier requirement on having a full memory barrier in
> __schedule() after coming from user-space, before storing to rq->curr.
> It is provided by smp_mb__after_spinlock() in __schedule().
>
> Document that membarrier requires a full barrier on transition from
> kernel thread to userspace thread. We currently have an implicit barrier
> from atomic_dec_and_test() in mmdrop() that ensures this.
>
> The x86 switch_mm_irqs_off() full barrier is currently provided by many
> cpumask update operations as well as write_cr3(). Document that
> write_cr3() provides this barrier.
>
> Changes since v1:
> - Update comments to match reality for code paths which are after
> storing to rq->curr, before returning to user-space.
> Changes since v2:
> - Update changelog (smp_mb__before_spinlock -> smp_mb__after_spinlock).
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
I replaced the version that I had queued with this one, thank you!
Thanx, Paul
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Andrew Hunter <ahh@google.com>
> CC: Maged Michael <maged.michael@gmail.com>
> CC: gromer@google.com
> CC: Avi Kivity <avi@scylladb.com>
> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> CC: Paul Mackerras <paulus@samba.org>
> CC: Michael Ellerman <mpe@ellerman.id.au>
> CC: Dave Watson <davejwatson@fb.com>
> ---
> arch/x86/mm/tlb.c | 5 +++++
> include/linux/sched/mm.h | 5 +++++
> kernel/sched/core.c | 9 +++++++++
> 3 files changed, 19 insertions(+)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1ab3821f9e26..74f94fe4aded 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> }
> #endif
>
> + /*
> + * The membarrier system call requires a full memory barrier
> + * before returning to user-space, after storing to rq->curr.
> + * Writing to CR3 provides that full memory barrier.
> + */
> if (real_prev == next) {
> VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> next->context.ctx_id);
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 3a19c253bdb1..766cc47c4d7c 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
> extern void __mmdrop(struct mm_struct *);
> static inline void mmdrop(struct mm_struct *mm)
> {
> + /*
> + * The implicit full barrier implied by atomic_dec_and_test is
> + * required by the membarrier system call before returning to
> + * user-space, after storing to rq->curr.
> + */
> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
> __mmdrop(mm);
> }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 18a6966567da..7977b25acf54 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> finish_arch_post_lock_switch();
>
> fire_sched_in_preempt_notifiers(current);
> + /*
> + * When transitioning from a kernel thread to a userspace
> + * thread, mmdrop()'s implicit full barrier is required by the
> + * membarrier system call, because the current active_mm can
> + * become the current mm without going through switch_mm().
> + */
> if (mm)
> mmdrop(mm);
> if (unlikely(prev_state == TASK_DEAD)) {
> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
> * Make sure that signal_pending_state()->signal_pending() below
> * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> * done by the caller to avoid the race with signal_wake_up().
> + *
> + * The membarrier system call requires a full memory barrier
> + * after coming from user-space, before storing to rq->curr.
> */
> rq_lock(rq, &rf);
> smp_mb__after_spinlock();
> --
> 2.11.0
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v3] membarrier: Document scheduler barrier requirements
2017-09-19 22:02 [PATCH v3] membarrier: Document scheduler barrier requirements Mathieu Desnoyers
2017-09-19 22:53 ` Paul E. McKenney
@ 2017-09-21 12:25 ` Peter Zijlstra
2017-09-22 15:26 ` Mathieu Desnoyers
1 sibling, 1 reply; 4+ messages in thread
From: Peter Zijlstra @ 2017-09-21 12:25 UTC (permalink / raw)
To: Mathieu Desnoyers
Cc: Paul E . McKenney, linux-kernel, Boqun Feng, Andrew Hunter,
Maged Michael, gromer, Avi Kivity, Benjamin Herrenschmidt,
Paul Mackerras, Michael Ellerman, Dave Watson
On Tue, Sep 19, 2017 at 06:02:05PM -0400, Mathieu Desnoyers wrote:
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1ab3821f9e26..74f94fe4aded 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> }
> #endif
>
> + /*
> + * The membarrier system call requires a full memory barrier
> + * before returning to user-space, after storing to rq->curr.
> + * Writing to CR3 provides that full memory barrier.
> + */
> if (real_prev == next) {
> VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> next->context.ctx_id);
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 3a19c253bdb1..766cc47c4d7c 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
> extern void __mmdrop(struct mm_struct *);
> static inline void mmdrop(struct mm_struct *mm)
> {
> + /*
> + * The implicit full barrier implied by atomic_dec_and_test is
> + * required by the membarrier system call before returning to
> + * user-space, after storing to rq->curr.
> + */
> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
> __mmdrop(mm);
> }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 18a6966567da..7977b25acf54 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> finish_arch_post_lock_switch();
>
> fire_sched_in_preempt_notifiers(current);
> + /*
> + * When transitioning from a kernel thread to a userspace
> + * thread, mmdrop()'s implicit full barrier is required by the
> + * membarrier system call, because the current active_mm can
> + * become the current mm without going through switch_mm().
> + */
> if (mm)
> mmdrop(mm);
> if (unlikely(prev_state == TASK_DEAD)) {
I would also put a comment in context_switch() that explains we either
pass through switch_mm() or do mmdrop().
And I think that for the weak archs that don't have native RELEASE we
actually rely on rq_unlock() for the smp_mb().
So there's 4 schemes:
- switch_mm()/mmdrop() (x86,s390, sparc?)
- finish_lock_switch() (weak, !release)
- switch_to (arm64)
- member arch hook (ppc)
And I don't think that's spelled out clearly enough.
> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
> * Make sure that signal_pending_state()->signal_pending() below
> * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> * done by the caller to avoid the race with signal_wake_up().
> + *
> + * The membarrier system call requires a full memory barrier
> + * after coming from user-space, before storing to rq->curr.
> */
> rq_lock(rq, &rf);
> smp_mb__after_spinlock();
Right, this is the only part that's actually trivial :-)
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v3] membarrier: Document scheduler barrier requirements
2017-09-21 12:25 ` Peter Zijlstra
@ 2017-09-22 15:26 ` Mathieu Desnoyers
0 siblings, 0 replies; 4+ messages in thread
From: Mathieu Desnoyers @ 2017-09-22 15:26 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Paul E. McKenney, linux-kernel, Boqun Feng, Andrew Hunter,
maged michael, gromer, Avi Kivity, Benjamin Herrenschmidt,
Paul Mackerras, Michael Ellerman, Dave Watson
----- On Sep 21, 2017, at 8:25 AM, Peter Zijlstra peterz@infradead.org wrote:
> On Tue, Sep 19, 2017 at 06:02:05PM -0400, Mathieu Desnoyers wrote:
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 1ab3821f9e26..74f94fe4aded 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct
>> mm_struct *next,
>> }
>> #endif
>>
>> + /*
>> + * The membarrier system call requires a full memory barrier
>> + * before returning to user-space, after storing to rq->curr.
>> + * Writing to CR3 provides that full memory barrier.
>> + */
>> if (real_prev == next) {
>> VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
>> next->context.ctx_id);
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index 3a19c253bdb1..766cc47c4d7c 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
>> extern void __mmdrop(struct mm_struct *);
>> static inline void mmdrop(struct mm_struct *mm)
>> {
>> + /*
>> + * The implicit full barrier implied by atomic_dec_and_test is
>> + * required by the membarrier system call before returning to
>> + * user-space, after storing to rq->curr.
>> + */
>> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
>> __mmdrop(mm);
>> }
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 18a6966567da..7977b25acf54 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct
>> *prev)
>> finish_arch_post_lock_switch();
>>
>> fire_sched_in_preempt_notifiers(current);
>> + /*
>> + * When transitioning from a kernel thread to a userspace
>> + * thread, mmdrop()'s implicit full barrier is required by the
>> + * membarrier system call, because the current active_mm can
>> + * become the current mm without going through switch_mm().
>> + */
>> if (mm)
>> mmdrop(mm);
>> if (unlikely(prev_state == TASK_DEAD)) {
>
>
> I would also put a comment in context_switch() that explains we either
> pass through switch_mm() or do mmdrop().
>
> And I think that for the weak archs that don't have native RELEASE we
> actually rely on rq_unlock() for the smp_mb().
>
> So there's 4 schemes:
>
> - switch_mm()/mmdrop() (x86,s390, sparc?)
> - finish_lock_switch() (weak, !release)
> - switch_to (arm64)
> - member arch hook (ppc)
>
> And I don't think that's spelled out clearly enough.
>
>> @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt)
>> * Make sure that signal_pending_state()->signal_pending() below
>> * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
>> * done by the caller to avoid the race with signal_wake_up().
>> + *
>> + * The membarrier system call requires a full memory barrier
>> + * after coming from user-space, before storing to rq->curr.
>> */
>> rq_lock(rq, &rf);
>> smp_mb__after_spinlock();
>
> Right, this is the only part that's actually trivial :-)
Does something like this work ? (except for tabs vs spaces)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 08095bb1cfe6..6254f87645de 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2760,6 +2760,13 @@ context_switch(struct rq *rq, struct task_struct *prev,
*/
arch_start_context_switch(prev);
+ /*
+ * If mm is non-NULL, we pass through switch_mm(). If mm is
+ * NULL, we will pass through mmdrop() in finish_task_switch().
+ * Both of these contain the full memory barrier required by
+ * membarrier after storing to rq->curr, before returning to
+ * user-space.
+ */
if (!mm) {
next->active_mm = oldmm;
mmgrab(oldmm);
@@ -3346,16 +3353,17 @@ static void __sched notrace __schedule(bool preempt)
/*
* The membarrier system call requires each architecture
* to have a full memory barrier after updating
- * rq->curr, before returning to user-space. For TSO
- * (e.g. x86), the architecture must provide its own
- * barrier in switch_mm(). For weakly ordered machines
- * for which spin_unlock() acts as a full memory
- * barrier, finish_lock_switch() in common code takes
- * care of this barrier. For weakly ordered machines for
- * which spin_unlock() acts as a RELEASE barrier (only
- * arm64 and PowerPC), arm64 has a full barrier in
- * switch_to(), and PowerPC has a full barrier in
- * membarrier_arch_sched_in().
+ * rq->curr, before returning to user-space.
+ *
+ * Here are the schemes providing that barrier on the
+ * various architectures:
+ * - mm ? switch_mm() : mmdrop() for x86, s390, sparc,
+ * - finish_lock_switch() for weakly-ordered
+ * architectures where spin_unlock is a full barrier,
+ * - switch_to() for arm64 (weakly-ordered, spin_unlock
+ * is a RELEASE barrier),
+ * - membarrier_arch_sched_in() for PowerPC,
+ * (weakly-ordered, spin_unlock is a RELEASE barrier).
*/
++*switch_count;
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
^ permalink raw reply related [flat|nested] 4+ messages in thread
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2017-09-19 22:02 [PATCH v3] membarrier: Document scheduler barrier requirements Mathieu Desnoyers
2017-09-19 22:53 ` Paul E. McKenney
2017-09-21 12:25 ` Peter Zijlstra
2017-09-22 15:26 ` Mathieu Desnoyers
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