Messages in this thread Patch in this message | | | From | Mathieu Desnoyers <> | Subject | [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v3) | Date | Thu, 24 Sep 2020 13:25:07 -0400 |
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Add comments and memory barrier to kthread_use_mm and kthread_unuse_mm to allow the effect of membarrier(2) to apply to kthreads accessing user-space memory as well.
Given that no prior kthread use this guarantee and that it only affects kthreads, adding this guarantee does not affect user-space ABI.
Refine the check in membarrier_global_expedited to exclude runqueues running the idle thread rather than all kthreads from the IPI cpumask.
Now that membarrier_global_expedited can IPI kthreads, the scheduler also needs to update the runqueue's membarrier_state when entering lazy TLB state.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: Will Deacon <will@kernel.org> Cc: Paul E. McKenney <paulmck@kernel.org> Cc: Nicholas Piggin <npiggin@gmail.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Andrew Morton <akpm@linux-foundation.org> --- Changes since v1: - Add WARN_ON_ONCE(current->mm) in play_idle_precise (PeterZ), - Use smp_mb__after_spinlock rather than smp_mb after task_lock.
Changes since v2: - Update the rq's membarrier state on kthread use/unuse mm, - The scheduler must use membarrier_switch_mm for lazy TLB case as well, now that global expedited membarrier IPIs kthreads. --- kernel/kthread.c | 21 +++++++++++++++++++++ kernel/sched/core.c | 3 ++- kernel/sched/idle.c | 1 + kernel/sched/membarrier.c | 8 ++------ kernel/sched/sched.h | 5 +++-- 5 files changed, 29 insertions(+), 9 deletions(-)
diff --git a/kernel/kthread.c b/kernel/kthread.c index 3edaa380dc7b..a396734d31f3 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -1248,6 +1248,7 @@ void kthread_use_mm(struct mm_struct *mm) tsk->active_mm = mm; } tsk->mm = mm; + membarrier_update_current_mm(mm); switch_mm_irqs_off(active_mm, mm, tsk); local_irq_enable(); task_unlock(tsk); @@ -1255,8 +1256,19 @@ void kthread_use_mm(struct mm_struct *mm) finish_arch_post_lock_switch(); #endif + /* + * When a kthread starts operating on an address space, the loop + * in membarrier_{private,global}_expedited() may not observe + * that tsk->mm, and not issue an IPI. Membarrier requires a + * memory barrier after storing to tsk->mm, before accessing + * user-space memory. A full memory barrier for membarrier + * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by + * mmdrop(), or explicitly with smp_mb(). + */ if (active_mm != mm) mmdrop(active_mm); + else + smp_mb(); to_kthread(tsk)->oldfs = force_uaccess_begin(); } @@ -1276,9 +1288,18 @@ void kthread_unuse_mm(struct mm_struct *mm) force_uaccess_end(to_kthread(tsk)->oldfs); task_lock(tsk); + /* + * When a kthread stops operating on an address space, the loop + * in membarrier_{private,global}_expedited() may not observe + * that tsk->mm, and not issue an IPI. Membarrier requires a + * memory barrier after accessing user-space memory, before + * clearing tsk->mm. + */ + smp_mb__after_spinlock(); sync_mm_rss(mm); local_irq_disable(); tsk->mm = NULL; + membarrier_update_current_mm(NULL); /* active_mm is still 'mm' */ enter_lazy_tlb(mm, tsk); local_irq_enable(); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2d95dc3f4644..bab6f4f2809f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3736,6 +3736,8 @@ context_switch(struct rq *rq, struct task_struct *prev, */ arch_start_context_switch(prev); + membarrier_switch_mm(rq, prev->mm, next->mm); + /* * kernel -> kernel lazy + transfer active * user -> kernel lazy + mmgrab() active @@ -3752,7 +3754,6 @@ context_switch(struct rq *rq, struct task_struct *prev, else prev->active_mm = NULL; } else { // to user - membarrier_switch_mm(rq, prev->active_mm, next->mm); /* * sys_membarrier() requires an smp_mb() between setting * rq->curr / membarrier_switch_mm() and returning to userspace. diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index f324dc36fc43..f0d81a5ea471 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -338,6 +338,7 @@ void play_idle_precise(u64 duration_ns, u64 latency_ns) WARN_ON_ONCE(!(current->flags & PF_KTHREAD)); WARN_ON_ONCE(!(current->flags & PF_NO_SETAFFINITY)); WARN_ON_ONCE(!duration_ns); + WARN_ON_ONCE(current->mm); rcu_sleep_check(); preempt_disable(); diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c index 8bc8b8a888b7..e5246580201b 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -112,13 +112,9 @@ static int membarrier_global_expedited(void) MEMBARRIER_STATE_GLOBAL_EXPEDITED)) continue; - /* - * Skip the CPU if it runs a kernel thread. The scheduler - * leaves the prior task mm in place as an optimization when - * scheduling a kthread. - */ + /* Skip the CPU if it runs the idle thread. */ p = rcu_dereference(cpu_rq(cpu)->curr); - if (p->flags & PF_KTHREAD) + if (is_idle_task(p)) continue; __cpumask_set_cpu(cpu, tmpmask); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 28709f6b0975..d884959d4280 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -2592,12 +2592,13 @@ static inline void membarrier_switch_mm(struct rq *rq, struct mm_struct *prev_mm, struct mm_struct *next_mm) { - int membarrier_state; + int membarrier_state = 0; if (prev_mm == next_mm) return; - membarrier_state = atomic_read(&next_mm->membarrier_state); + if (next_mm) + membarrier_state = atomic_read(&next_mm->membarrier_state); if (READ_ONCE(rq->membarrier_state) == membarrier_state) return; -- 2.17.1
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