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SubjectRe: [PATCH v2] membarrier: Document scheduler barrier requirements
----- On Sep 19, 2017, at 5:47 PM, Andrea Parri parri.andrea@gmail.com wrote:

> On Tue, Sep 19, 2017 at 03:56:31PM -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().
>
> It is smp_mb__after_spinlock(). (Yes: I missed it in my previous email.)

OK. Will send a v3 fixing the changelog.

Thanks,

Mathieu

>
> Andrea
>
>
>>
>> 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.
>>
>> 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

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

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