Commit 5bc78502 authored by Mathieu Desnoyers's avatar Mathieu Desnoyers Committed by Peter Zijlstra
Browse files

sched: fix exit_mm vs membarrier (v4) 



exit_mm should issue memory barriers after user-space memory accesses,
before clearing current->mm, to order user-space memory accesses
performed prior to exit_mm before clearing tsk->mm, which has the
effect of skipping the membarrier private expedited IPIs.

exit_mm should also update the runqueue's membarrier_state so
membarrier global expedited IPIs are not sent when they are not
needed.

The membarrier system call can be issued concurrently with do_exit
if we have thread groups created with CLONE_VM but not CLONE_THREAD.

Here is the scenario I have in mind:

Two thread groups are created, A and B. Thread group B is created by
issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD.
Let's assume we have a single thread within each thread group (Thread A
and Thread B).

The AFAIU we can have:

Userspace variables:

int x = 0, y = 0;

CPU 0                   CPU 1
Thread A                Thread B
(in thread group A)     (in thread group B)

x = 1
barrier()
y = 1
exit()
exit_mm()
current->mm = NULL;
                        r1 = load y
                        membarrier()
                          skips CPU 0 (no IPI) because its current mm is NULL
                        r2 = load x
                        BUG_ON(r1 == 1 && r2 == 0)

Signed-off-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201020134715.13909-2-mathieu.desnoyers@efficios.com
parent 45da7a2b

include/linux/sched/mm.h

+5 −0
Original line number Diff line number Diff line
@@ -347,6 +347,8 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm) 


extern void membarrier_exec_mmap(struct mm_struct *mm);



extern void membarrier_update_current_mm(struct mm_struct *next_mm);



#else

#ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS

static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
@@ -361,6 +363,9 @@ static inline void membarrier_exec_mmap(struct mm_struct *mm) 
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)

{

}

static inline void membarrier_update_current_mm(struct mm_struct *next_mm)

{

}

#endif



#endif /* _LINUX_SCHED_MM_H */

kernel/exit.c

+15 −1
Original line number Diff line number Diff line
@@ -475,10 +475,24 @@ static void exit_mm(void) 
	BUG_ON(mm != current->active_mm);

	/* more a memory barrier than a real lock */

	task_lock(current);

	/*

	 * When a thread stops operating on an address space, the loop

	 * in membarrier_private_expedited() may not observe that

	 * tsk->mm, and the loop in membarrier_global_expedited() may

	 * not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED

	 * rq->membarrier_state, so those would not issue an IPI.

	 * Membarrier requires a memory barrier after accessing

	 * user-space memory, before clearing tsk->mm or the

	 * rq->membarrier_state.

	 */

	smp_mb__after_spinlock();

	local_irq_disable();

	current->mm = NULL;

	mmap_read_unlock(mm);

	membarrier_update_current_mm(NULL);

	enter_lazy_tlb(mm, current);

	local_irq_enable();

	task_unlock(current);

	mmap_read_unlock(mm);

	mm_update_next_owner(mm);

	mmput(mm);

	if (test_thread_flag(TIF_MEMDIE))

kernel/sched/membarrier.c

+12 −0
Original line number Diff line number Diff line
@@ -76,6 +76,18 @@ void membarrier_exec_mmap(struct mm_struct *mm) 
	this_cpu_write(runqueues.membarrier_state, 0);

}



void membarrier_update_current_mm(struct mm_struct *next_mm)

{

	struct rq *rq = this_rq();

	int membarrier_state = 0;



	if (next_mm)

		membarrier_state = atomic_read(&next_mm->membarrier_state);

	if (READ_ONCE(rq->membarrier_state) == membarrier_state)

		return;

	WRITE_ONCE(rq->membarrier_state, membarrier_state);

}



static int membarrier_global_expedited(void)

{

	int cpu;

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