Re: [RFC][PATCH 1/4] locking/mutex: Use try_cmpxchg()

[Xu, Yanfei]

On 6/30/21 11:35 PM, Peter Zijlstra wrote:
> For simpler and better code.
>
> Signed-off-by: Peter Zijlstra (Intel)<peterz@xxxxxxxxxxxxx>
> ---
>   kernel/locking/mutex.c |   27 ++++++---------------------
>   1 file changed, 6 insertions(+), 21 deletions(-)

Hi Peter,

I read the mutex codes today, and find there seems something wrong for 
the patch. Should we consider the race condition as blow?

From 4035f50c96e17cbe3febab768b64da5c000e5b76 Mon Sep 17 00:00:00 2001
From: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
Date: Mon, 5 Jul 2021 17:56:58 +0800
Subject: [PATCH] locking/mutex: fix the endless loop when racing against
 mutex.owner

if a race condition happened on mutex.owner after we fetch its value,
atomic_long_try_cmpxchg_acquire/release invoked on &mutex.owner will
return false. Then we need to reassign the temporary variable which
saves mutex.owner value if in loop, or it will lead an endless loop.

Fixes: 9265e48a579d ("locking/mutex: Use try_cmpxchg()")

Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
---
 kernel/locking/mutex.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 5e6a811ac733..ec6b6724c118 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -95,12 +95,12 @@ static inline unsigned long __owner_flags(unsigned 
long owner)

 static inline struct task_struct *__mutex_trylock_common(struct mutex 
*lock, bool handoff)
 {
-       unsigned long owner, curr = (unsigned long)current;
+       unsigned long flags, owner, task, curr = (unsigned long)current;

-       owner = atomic_long_read(&lock->owner);
        for (;;) { /* must loop, can race against a flag */
-               unsigned long flags = __owner_flags(owner);
-               unsigned long task = owner & ~MUTEX_FLAGS;
+               owner = atomic_long_read(&lock->owner);
+               flags = __owner_flags(owner);
+               task = owner & ~MUTEX_FLAGS;

                if (task) {
                        if (flags & MUTEX_FLAG_PICKUP) {
@@ -231,10 +231,10 @@ __mutex_remove_waiter(struct mutex *lock, struct 
mutex_waiter *waiter)
  */
 static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
 {
-       unsigned long owner = atomic_long_read(&lock->owner);
+       unsigned long owner, new;

        for (;;) {
-               unsigned long new;
+               owner = atomic_long_read(&lock->owner);

                MUTEX_WARN_ON(__owner_task(owner) != current);
                MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP);
@@ -1227,8 +1227,9 @@ static noinline void __sched 
__mutex_unlock_slowpath(struct mutex *lock, unsigne
         * Except when HANDOFF, in that case we must not clear the 
owner field,
         * but instead set it to the top waiter.
         */
-       owner = atomic_long_read(&lock->owner);
        for (;;) {
+               owner = atomic_long_read(&lock->owner);
+
                MUTEX_WARN_ON(__owner_task(owner) != current);
                MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP);

--
2.29.2