Messages in this thread Patch in this message | | | Date | Mon, 24 Aug 2020 11:27:49 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2) |
| |
----- On Aug 16, 2020, at 11:29 AM, Boqun Feng boqun.feng@gmail.com wrote:
> On Fri, Aug 14, 2020 at 12:43:57PM -0400, Mathieu Desnoyers wrote: >> 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 <mathieu.desnoyers@efficios.com> >> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> >> 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. >> --- >> 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(); > > Similar question here: could smp_mb() guarantee the correctness of > GLOBAL_EXPEDITED? Don't you need membarrier_switch_mm() here and in > kthread_unuse_mm(), too? > > Am I miss something here?
I think you have a good point there. Which brings me to wonder why we don't have membarrier_switch_mm() when entering/leaving lazy TLB mode. This means an IPI can be sent to a kthread even if it does not use an mm, just because the membarrier state in the runqueue is that of the active_mm.
Thoughts ?
Thanks,
Mathieu
> > Regards, > Boqun > >> >> 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
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
| |