fbeb558b0d
Some new assertions pointed out that the existing code has nested rt_mutex wait state in the futex code. Specifically, the futex_lock_pi() cancel case uses spin_lock() while there still is a rt_waiter enqueued for this task, resulting in a state where there are two waiters for the same task (and task_struct::pi_blocked_on gets scrambled). The reason to take hb->lock at this point is to avoid the wake_futex_pi() EAGAIN case. This happens when futex_top_waiter() and rt_mutex_top_waiter() state becomes inconsistent. The current rules are such that this inconsistency will not be observed. Notably the case that needs to be avoided is where futex_lock_pi() and futex_unlock_pi() interleave such that unlock will fail to observe a new waiter. *However* the case at hand is where a waiter is leaving, in this case the race means a waiter that is going away is not observed -- which is harmless, provided this race is explicitly handled. This is a somewhat dangerous proposition because the converse race is not observing a new waiter, which must absolutely not happen. But since the race is valid this cannot be asserted. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Link: https://lkml.kernel.org/r/20230915151943.GD6743@noisy.programming.kicks-ass.net
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.