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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@google.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Link: https://lore.kernel.org/r/20231130204817.2031407-1-jannh@google.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.