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Date: Fri, 22 Sep 2017 15:26:38 +0000 (UTC)
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Boqun Feng <boqun.feng@...il.com>,
Andrew Hunter <ahh@...gle.com>,
maged michael <maged.michael@...il.com>,
gromer <gromer@...gle.com>, Avi Kivity <avi@...lladb.com>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Michael Ellerman <mpe@...erman.id.au>,
Dave Watson <davejwatson@...com>
Subject: Re: [PATCH v3] membarrier: Document scheduler barrier requirements
----- On Sep 21, 2017, at 8:25 AM, Peter Zijlstra peterz@...radead.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
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