[PATCH -v2] locking/mutex: Document that mutex_unlock() is non-atomic

[Ingo Molnar]

* Jann Horn <jannh@xxxxxxxxxx> wrote:

> On Thu, Nov 30, 2023 at 10:53 PM Waiman Long <longman@xxxxxxxxxx> wrote:
> > On 11/30/23 15:48, Jann Horn wrote:
> > > I have seen several cases of attempts to use mutex_unlock() to release an
> > > object such that the object can then be freed by another task.
> > > My understanding is that this is not safe because mutex_unlock(), in the
> > > MUTEX_FLAG_WAITERS && !MUTEX_FLAG_HANDOFF case, accesses the mutex
> > > structure after having marked it as unlocked; so mutex_unlock() requires
> > > its caller to ensure that the mutex stays alive until mutex_unlock()
> > > returns.
> > >
> > > If MUTEX_FLAG_WAITERS is set and there are real waiters, those waiters
> > > have to keep the mutex alive, I think; but we could have a spurious
> > > MUTEX_FLAG_WAITERS left if an interruptible/killable waiter bailed
> > > between the points where __mutex_unlock_slowpath() did the cmpxchg
> > > reading the flags and where it acquired the wait_lock.
> >
> > Could you clarify under what condition a concurrent task can decide to
> > free the object holding the mutex? Is it !mutex_is_locked() or after a
> > mutex_lock()/mutex_unlock sequence?
> 
> I mean a mutex_lock()+mutex_unlock() sequence.
> 
> > mutex_is_locked() will return true if the mutex has waiter even if it
> > is currently free.
> 
> I don't understand your point, and maybe I also don't understand what
> you mean by "free". Isn't mutex_is_locked() defined such that it only
> looks at whether a mutex has an owner, and doesn't look at the waiter
> list?

Yeah, mutex_is_locked() is not a sufficient check - and mutexes have no 
implicit refcount properties like spinlocks. Once you call a mutex API, you 
have to guarantee the lifetime of the object until the function returns.

I.e. entering a mutex_lock()-ed critical section cannot be used to 
guarantee that all mutex_unlock() instances have stopped using the mutex.
I agree that this is a bit unintuitive, and differs from spinlocks.

I've clarified all this a bit more in the final patch (added a 'fully' 
qualifier, etc.), and made the changelog more assertive - see the attached 
patch.

Thanks,

	Ingo

=======================>
From: Jann Horn <jannh@xxxxxxxxxx>
Date: Thu, 30 Nov 2023 21:48:17 +0100
Subject: [PATCH] locking/mutex: Document that mutex_unlock() is non-atomic

I have seen several cases of attempts to use mutex_unlock() to release an
object such that the object can then be freed by another task.

This is not safe because mutex_unlock(), in the
MUTEX_FLAG_WAITERS && !MUTEX_FLAG_HANDOFF case, accesses the mutex
structure after having marked it as unlocked; so mutex_unlock() requires
its caller to ensure that the mutex stays alive until mutex_unlock()
returns.

If MUTEX_FLAG_WAITERS is set and there are real waiters, those waiters
have to keep the mutex alive, but we could have a spurious
MUTEX_FLAG_WAITERS left if an interruptible/killable waiter bailed
between the points where __mutex_unlock_slowpath() did the cmpxchg
reading the flags and where it acquired the wait_lock.

( With spinlocks, that kind of code pattern is allowed and, from what I
  remember, used in several places in the kernel. )

Document this, such a semantic difference between mutexes and spinlocks
is fairly unintuitive.

[ mingo: Made the changelog a bit more assertive, refined the comments. ]

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231130204817.2031407-1-jannh@xxxxxxxxxx
---
 Documentation/locking/mutex-design.rst | 6 ++++++
 kernel/locking/mutex.c                 | 5 +++++
 2 files changed, 11 insertions(+)

diff --git a/Documentation/locking/mutex-design.rst b/Documentation/locking/mutex-design.rst
index 78540cd7f54b..7572339b2f12 100644
--- a/Documentation/locking/mutex-design.rst
+++ b/Documentation/locking/mutex-design.rst
@@ -101,6 +101,12 @@ features that make lock debugging easier and faster:
     - Detects multi-task circular deadlocks and prints out all affected
       locks and tasks (and only those tasks).
 
+Releasing a mutex is not an atomic operation: Once a mutex release operation
+has begun, another context may be able to acquire the mutex before the release
+operation has fully completed. The mutex user must ensure that the mutex is not
+destroyed while a release operation is still in progress - in other words,
+callers of mutex_unlock() must ensure that the mutex stays alive until
+mutex_unlock() has returned.
 
 Interfaces
 ----------
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 2deeeca3e71b..cbae8c0b89ab 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -532,6 +532,11 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
  * This function must not be used in interrupt context. Unlocking
  * of a not locked mutex is not allowed.
  *
+ * The caller must ensure that the mutex stays alive until this function has
+ * returned - mutex_unlock() can NOT directly be used to release an object such
+ * that another concurrent task can free it.
+ * Mutexes are different from spinlocks & refcounts in this aspect.
+ *
  * This function is similar to (but not equivalent to) up().
  */
 void __sched mutex_unlock(struct mutex *lock)