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.desnoy...@efficios.com>
> Cc: Peter Zijlstra (Intel) <pet...@infradead.org>
> Cc: Will Deacon <w...@kernel.org>
> Cc: Paul E. McKenney <paul...@kernel.org>
> Cc: Nicholas Piggin <npig...@gmail.com>
> Cc: Andy Lutomirski <l...@amacapital.net>
> Cc: Andrew Morton <a...@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?
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
>
