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SubjectRe: [PATCH v3] membarrier: Document scheduler barrier requirements
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 :-)

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