Re: [PATCH v2] membarrier: Document scheduler barrier requirements

[Mathieu Desnoyers]

----- On Sep 19, 2017, at 5:47 PM, Andrea Parri parri.andrea@xxxxxxxxx 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@xxxxxxxxxxxx>
>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>> CC: Boqun Feng <boqun.feng@xxxxxxxxx>
>> CC: Andrew Hunter <ahh@xxxxxxxxxx>
>> CC: Maged Michael <maged.michael@xxxxxxxxx>
>> CC: gromer@xxxxxxxxxx
>> CC: Avi Kivity <avi@xxxxxxxxxxxx>
>> CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
>> CC: Paul Mackerras <paulus@xxxxxxxxx>
>> CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> CC: Dave Watson <davejwatson@xxxxxx>
>> ---
>>  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