Re: [RFC PATCH for 4.14 2/2] Fix: membarrier: Handle CLONE_VM + !CLONE_THREAD correctly on powerpc

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Paul E. McKenney <redacted>
CC: Boqun Feng <redacted>
CC: Andrew Hunter <redacted>
CC: Maged Michael <redacted>
CC: gromer@google.com
CC: Avi Kivity <redacted>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <redacted>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Dave Watson <redacted>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: Will Deacon <redacted>
CC: Andy Lutomirski <luto@kernel.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Nicholas Piggin <npiggin@gmail.com>
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-arch@vger.kernel.org
---
arch/powerpc/include/asm/membarrier.h | 21 ++-------------------
arch/powerpc/kernel/membarrier.c      | 17 ++++-------------
include/linux/mm_types.h              |  2 +-
include/linux/sched/mm.h              | 28 ++++++----------------------
kernel/fork.c                         |  2 --
kernel/sched/membarrier.c             | 16 +++++++++++++---
6 files changed, 26 insertions(+), 60 deletions(-)

diff --git a/arch/powerpc/include/asm/membarrier.h

b/arch/powerpc/include/asm/membarrier.h
index 61152a7a3cf9..0951646253d9 100644

--- a/arch/powerpc/include/asm/membarrier.h
+++ b/arch/powerpc/include/asm/membarrier.h

@@ -11,8 +11,8 @@ static inline void membarrier_arch_switch_mm(struct mm_struct

*prev,
	 * when switching from userspace to kernel is not needed after
	 * store to rq->curr.
	 */
-	if (likely(!test_ti_thread_flag(task_thread_info(tsk),
-			TIF_MEMBARRIER_PRIVATE_EXPEDITED) || !prev))
+	if (likely(!(atomic_read(&next->membarrier_state)
+			& MEMBARRIER_STATE_SWITCH_MM) || !prev))
		return;

	/*

@@ -21,23 +21,6 @@ static inline void membarrier_arch_switch_mm(struct mm_struct

*prev,
	 */
	smp_mb();
}
-static inline void membarrier_arch_fork(struct task_struct *t,
-		unsigned long clone_flags)
-{
-	/*
-	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
-	 * fork is protected by siglock. membarrier_arch_fork is called
-	 * with siglock held.
-	 */
-	if (test_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED))
-		set_ti_thread_flag(task_thread_info(t),
-				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
-}
-static inline void membarrier_arch_execve(struct task_struct *t)
-{
-	clear_ti_thread_flag(task_thread_info(t),
-			TIF_MEMBARRIER_PRIVATE_EXPEDITED);
-}
void membarrier_arch_register_private_expedited(struct task_struct *t);

#endif /* _ASM_POWERPC_MEMBARRIER_H */

diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
index b0d79a5f5981..4795ad59b833 100644
--- a/arch/powerpc/kernel/membarrier.c
+++ b/arch/powerpc/kernel/membarrier.c

@@ -19,24 +19,15 @@

#include <linux/thread_info.h>
#include <linux/spinlock.h>
#include <linux/rcupdate.h>
+#include <linux/atomic.h>

void membarrier_arch_register_private_expedited(struct task_struct *p)
{
-	struct task_struct *t;
+	struct mm_struct *mm = p->mm;

-	if (get_nr_threads(p) == 1) {
-		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+	atomic_or(MEMBARRIER_STATE_SWITCH_MM, &mm->membarrier_state);
+	if (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
		return;
-	}
-	/*
-	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
-	 * fork is protected by siglock.
-	 */
-	spin_lock(&p->sighand->siglock);
-	for_each_thread(p, t)
-		set_ti_thread_flag(task_thread_info(t),
-				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
-	spin_unlock(&p->sighand->siglock);
	/*
	 * Ensure all future scheduler executions will observe the new
	 * thread flag state for this process.

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5e0fe8ce053b..1861ea8dba77 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -446,7 +446,7 @@ struct mm_struct {

	struct core_state *core_state; /* coredumping support */
#ifdef CONFIG_MEMBARRIER
-	int membarrier_private_expedited;
+	atomic_t membarrier_state;
#endif
#ifdef CONFIG_AIO
	spinlock_t			ioctx_lock;

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index b2767ecb21a8..2fb6089efcd7 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h

@@ -212,35 +212,23 @@ static inline void memalloc_noreclaim_restore(unsigned int

flags)

#ifdef CONFIG_MEMBARRIER

+enum {
+	MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY	= (1U << 0),
+	MEMBARRIER_STATE_SWITCH_MM			= (1U << 1),
+};
+
#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
#include <asm/membarrier.h>
#else
-static inline void membarrier_arch_fork(struct task_struct *t,
-		unsigned long clone_flags)
-{
-}
-static inline void membarrier_arch_execve(struct task_struct *t)
-{
-}
static inline void membarrier_arch_register_private_expedited(
		struct task_struct *p)
{
}
#endif

-static inline void membarrier_fork(struct task_struct *t,
-		unsigned long clone_flags)
-{
-	/*
-	 * Prior copy_mm() copies the membarrier_private_expedited field
-	 * from current->mm to t->mm.
-	 */
-	membarrier_arch_fork(t, clone_flags);
-}
static inline void membarrier_execve(struct task_struct *t)
{
-	t->mm->membarrier_private_expedited = 0;
-	membarrier_arch_execve(t);
+	atomic_set(&t->mm->membarrier_state, 0);
}
#else
#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS

@@ -249,10 +237,6 @@ static inline void membarrier_arch_switch_mm(struct

mm_struct *prev,
{
}
#endif
-static inline void membarrier_fork(struct task_struct *t,
-		unsigned long clone_flags)
-{
-}
static inline void membarrier_execve(struct task_struct *t)
{
}

diff --git a/kernel/fork.c b/kernel/fork.c
index bd4a93915e08..10646182440f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c

@@ -1840,8 +1840,6 @@ static __latent_entropy struct task_struct *copy_process(

	 */
	copy_seccomp(p);

-	membarrier_fork(p, clone_flags);
-
	/*
	 * Process group and session signals need to be delivered to just the
	 * parent before the fork or both the parent and the child after the

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index f06949a279ca..23bd256f8458 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c

@@ -18,6 +18,7 @@

#include <linux/membarrier.h>
#include <linux/tick.h>
#include <linux/cpumask.h>
+#include <linux/atomic.h>

#include "sched.h"	/* for cpu_rq(). */

@@ -40,7 +41,8 @@ static int membarrier_private_expedited(void)

	bool fallback = false;
	cpumask_var_t tmpmask;

-	if (!READ_ONCE(current->mm->membarrier_private_expedited))
+	if (!(atomic_read(&current->mm->membarrier_state)
+			& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
		return -EPERM;

	if (num_online_cpus() == 1)

@@ -104,11 +106,19 @@ static int membarrier_private_expedited(void)

static void membarrier_register_private_expedited(void)
{
	struct task_struct *p = current;
+	struct mm_struct *mm = p->mm;

-	if (READ_ONCE(p->mm->membarrier_private_expedited))
+	/*
+	 * We need to consider threads belonging to different thread
+	 * groups, which use the same mm. (CLONE_VM but not
+	 * CLONE_THREAD).
+	 */
+	if (atomic_read(&mm->membarrier_state)
+			& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)
		return;
	membarrier_arch_register_private_expedited(p);
-	WRITE_ONCE(p->mm->membarrier_private_expedited, 1);
+	atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
+			&mm->membarrier_state);
}

/**
--
2.11.0