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?
