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sched: membarrier: cover kthread_use_mm (v4)

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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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201020134715.13909-3-mathieu.desnoyers@efficios.com
This commit is contained in:
Mathieu Desnoyers 2020-10-20 09:47:14 -04:00 committed by Peter Zijlstra
parent 5bc7850232
commit 618758ed3a
3 changed files with 25 additions and 4 deletions
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21
kernel/kthread.c
View File

@ -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();

1
kernel/sched/idle.c
View File

@ -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();

7
kernel/sched/membarrier.c
View File

@ -126,12 +126,11 @@ static int membarrier_global_expedited(void)
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 a kernel thread which is not using
* a task mm.
*/
p = rcu_dereference(cpu_rq(cpu)->curr);
if (p->flags & PF_KTHREAD)
if (!p->mm)
continue;
__cpumask_set_cpu(cpu, tmpmask);