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

From: Mathieu Desnoyers
Date: Thu Oct 05 2017 - 17:41:50 EST

Threads targeting the same VM but which belong to different thread
groups is a tricky case. It has a few consequences:

It turns out that we cannot rely on get_nr_threads(p) to count the
number of threads using a VM. We can use
(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
instead to skip the synchronize_sched() for cases where the VM only has
a single user, and that user only has a single thread.

It also turns out that we cannot use for_each_thread() to set
thread flags in all threads using a VM, as it only iterates on the
thread group.

Therefore, test the membarrier state variable directly rather than
relying on thread flags. This means
membarrier_register_private_expedited() needs to set the
MEMBARRIER_STATE_SWITCH_MM flag, issue synchronize_sched(), and only
then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows private
expedited membarrier commands to succeed. membarrier_arch_switch_mm()
now tests for the MEMBARRIER_STATE_SWITCH_MM flag.

Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CC: Boqun Feng <boqun.feng@xxxxxxxxx>
CC: Andrew Hunter <ahh@xxxxxxxxxx>
CC: Maged Michael <maged.michael@xxxxxxxxx>
CC: gromer@xxxxxxxxxx
CC: Avi Kivity <avi@xxxxxxxxxxxx>
CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
CC: Paul Mackerras <paulus@xxxxxxxxx>
CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
CC: Dave Watson <davejwatson@xxxxxx>
CC: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
CC: Will Deacon <will.deacon@xxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
CC: Nicholas Piggin <npiggin@xxxxxxxxx>
CC: linuxppc-dev@xxxxxxxxxxxxxxxx
CC: linux-arch@xxxxxxxxxxxxxxx
---
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