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 load_cr3(). However, since load_cr3() may be done lazily in the future, move the documentation from load_cr3() to cpumask_set_cpu(). [ Applies on top of "membarrier: expedited private command (v4)". ] 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 | 7 +++++-- include/linux/sched/mm.h | 4 ++++ kernel/sched/core.c | 9 +++++++++ 3 files changed, 18 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index bb103d693f33..a79e72691cf1 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -105,6 +105,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, this_cpu_write(cpu_tlbstate.loaded_mm, next); WARN_ON_ONCE(cpumask_test_cpu(cpu, mm_cpumask(next))); + /* + * The full memory barrier implied by mm_cpumask update + * operations is required by the membarrier system call. + */ cpumask_set_cpu(cpu, mm_cpumask(next)); /* @@ -132,8 +136,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, * due to instruction fetches or for no reason at all, * and neither LOCK nor MFENCE orders them. * Fortunately, load_cr3() is serializing and gives the - * ordering guarantee we need. This full barrier is also - * required by the membarrier system call. + * ordering guarantee we need. */ load_cr3(next->pgd); diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index 2b24a6974847..fe29d06e2800 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 4f85494620d7..0e36d9960d91 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2649,6 +2649,12 @@ 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)) { @@ -3290,6 +3296,9 @@ 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. */ smp_mb__before_spinlock(); rq_lock(rq, &rf); -- 2.11.0
