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McKenney" , Andy Lutomirski , Andrew Morton , Alan Stern , Nicholas Piggin , Mathieu Desnoyers Subject: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2) Date: Fri, 14 Aug 2020 12:43:57 -0400 Message-Id: <20200814164358.4783-3-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20200814164358.4783-1-mathieu.desnoyers@efficios.com> References: <20200814164358.4783-1-mathieu.desnoyers@efficios.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Signed-off-by: Mathieu Desnoyers Cc: Peter Zijlstra (Intel) Cc: Will Deacon Cc: Paul E. McKenney Cc: Nicholas Piggin Cc: Andy Lutomirski Cc: Andrew Morton --- 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. --- kernel/kthread.c | 19 +++++++++++++++++++ kernel/sched/idle.c | 1 + kernel/sched/membarrier.c | 8 ++------ 3 files changed, 22 insertions(+), 6 deletions(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 3edaa380dc7b..77aaaa7bc8d9 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -1255,8 +1255,19 @@ 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(); } @@ -1276,6 +1287,14 @@ 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; diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index 6bf34986f45c..3443ee8335d0 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -341,6 +341,7 @@ 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 168479a7d61b..8a294483074d 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -100,13 +100,9 @@ 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); -- 2.11.0