[tip: locking/core] locking/mutex: Document that mutex_unlock() is non-atomic

[tip-bot2 for Jann Horn]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     a51749ab34d9e5dec548fe38ede7e01e8bb26454
Gitweb:        https://git.kernel.org/tip/a51749ab34d9e5dec548fe38ede7e01e8bb26454
Author:        Jann Horn <jannh@xxxxxxxxxx>
AuthorDate:    Thu, 30 Nov 2023 21:48:17 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 01 Dec 2023 11:27:43 +01:00

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 78540cd..7572339 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 2deeeca..cbae8c0 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)