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__before_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.
Signed-off-by: Mathieu Desnoyers <[email protected]>
CC: Peter Zijlstra <[email protected]>
CC: Paul E. McKenney <[email protected]>
CC: Boqun Feng <[email protected]>
CC: Andrew Hunter <[email protected]>
CC: Maged Michael <[email protected]>
CC: [email protected]
CC: Avi Kivity <[email protected]>
CC: Benjamin Herrenschmidt <[email protected]>
CC: Paul Mackerras <[email protected]>
CC: Michael Ellerman <[email protected]>
CC: Dave Watson <[email protected]>
---
arch/x86/mm/tlb.c | 5 +++++
include/linux/sched/mm.h | 4 ++++
kernel/sched/core.c | 9 +++++++++
3 files changed, 18 insertions(+)
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 1ab3821f9e26..fa3bbe048af0 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
+ * after coming from user-space, before 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 df4005e2c4cf..f3bc261fe7c7 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,10 @@ 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.
+ */
if (unlikely(atomic_dec_and_test(&mm->mm_count)))
__mmdrop(mm);
}
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c5c1b2c51807..48d524b18868 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2648,6 +2648,12 @@ 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)) {
@@ -3289,6 +3295,9 @@ 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
On Mon, Sep 18, 2017 at 02:01:22PM -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__before_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.
>
> Signed-off-by: Mathieu Desnoyers <[email protected]>
Queued for review, thank you Mathieu!
Thanx, Paul
> CC: Peter Zijlstra <[email protected]>
> CC: Paul E. McKenney <[email protected]>
> CC: Boqun Feng <[email protected]>
> CC: Andrew Hunter <[email protected]>
> CC: Maged Michael <[email protected]>
> CC: [email protected]
> CC: Avi Kivity <[email protected]>
> CC: Benjamin Herrenschmidt <[email protected]>
> CC: Paul Mackerras <[email protected]>
> CC: Michael Ellerman <[email protected]>
> CC: Dave Watson <[email protected]>
> ---
> arch/x86/mm/tlb.c | 5 +++++
> include/linux/sched/mm.h | 4 ++++
> kernel/sched/core.c | 9 +++++++++
> 3 files changed, 18 insertions(+)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1ab3821f9e26..fa3bbe048af0 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
> + * after coming from user-space, before 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 df4005e2c4cf..f3bc261fe7c7 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,10 @@ 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.
> + */
> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
> __mmdrop(mm);
> }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c5c1b2c51807..48d524b18868 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2648,6 +2648,12 @@ 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)) {
> @@ -3289,6 +3295,9 @@ 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
>
On Mon, Sep 18, 2017 at 02:01:22PM -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__before_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.
>
> Signed-off-by: Mathieu Desnoyers <[email protected]>
> CC: Peter Zijlstra <[email protected]>
> CC: Paul E. McKenney <[email protected]>
> CC: Boqun Feng <[email protected]>
> CC: Andrew Hunter <[email protected]>
> CC: Maged Michael <[email protected]>
> CC: [email protected]
> CC: Avi Kivity <[email protected]>
> CC: Benjamin Herrenschmidt <[email protected]>
> CC: Paul Mackerras <[email protected]>
> CC: Michael Ellerman <[email protected]>
> CC: Dave Watson <[email protected]>
> ---
> arch/x86/mm/tlb.c | 5 +++++
> include/linux/sched/mm.h | 4 ++++
> kernel/sched/core.c | 9 +++++++++
> 3 files changed, 18 insertions(+)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 1ab3821f9e26..fa3bbe048af0 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
> + * after coming from user-space, before storing to rq->curr.
I'm confused: isn't this case covered by the
rq_lock(rq, &rf);
smp_mb__after_spinlock();
below (all archs)? You meant "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 df4005e2c4cf..f3bc261fe7c7 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -38,6 +38,10 @@ 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", right?
(your commit says "on transition from kernel thread _to_ userspace"
regarding mmdrop()).
Andrea
> + */
> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
> __mmdrop(mm);
> }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c5c1b2c51807..48d524b18868 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2648,6 +2648,12 @@ 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)) {
> @@ -3289,6 +3295,9 @@ 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
>
----- On Sep 18, 2017, at 8:26 PM, Andrea Parri [email protected] wrote:
> On Mon, Sep 18, 2017 at 02:01:22PM -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__before_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.
>>
>> Signed-off-by: Mathieu Desnoyers <[email protected]>
>> CC: Peter Zijlstra <[email protected]>
>> CC: Paul E. McKenney <[email protected]>
>> CC: Boqun Feng <[email protected]>
>> CC: Andrew Hunter <[email protected]>
>> CC: Maged Michael <[email protected]>
>> CC: [email protected]
>> CC: Avi Kivity <[email protected]>
>> CC: Benjamin Herrenschmidt <[email protected]>
>> CC: Paul Mackerras <[email protected]>
>> CC: Michael Ellerman <[email protected]>
>> CC: Dave Watson <[email protected]>
>> ---
>> arch/x86/mm/tlb.c | 5 +++++
>> include/linux/sched/mm.h | 4 ++++
>> kernel/sched/core.c | 9 +++++++++
>> 3 files changed, 18 insertions(+)
>>
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 1ab3821f9e26..fa3bbe048af0 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
>> + * after coming from user-space, before storing to rq->curr.
>
> I'm confused: isn't this case covered by the
>
> rq_lock(rq, &rf);
> smp_mb__after_spinlock();
>
> below (all archs)? You meant "before returning to user-space, after
> storing to rq->curr"?
Right, my bad.
>
>
>> + * 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 df4005e2c4cf..f3bc261fe7c7 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -38,6 +38,10 @@ 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", right?
Yes.
I'll send an updated patch to Paul. Thanks for the review!
Mathieu
>
> (your commit says "on transition from kernel thread _to_ userspace"
> regarding mmdrop()).
>
> Andrea
>
>
>> + */
>> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
>> __mmdrop(mm);
>> }
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index c5c1b2c51807..48d524b18868 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2648,6 +2648,12 @@ 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)) {
>> @@ -3289,6 +3295,9 @@ 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
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com