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futex_lock_pi() and __fixup_pi_state_owner() acquire the futex_q::lock_ptr without holding a reference assuming the previously obtained hash bucket and the assigned lock_ptr are still valid. This isn't the case once the private hash can be resized and becomes invalid after the reference drop. Introduce futex_q_lockptr_lock() to lock the hash bucket recorded in futex_q::lock_ptr. The lock pointer is read in a RCU section to ensure that it does not go away if the hash bucket has been replaced and the old pointer has been observed. After locking the pointer needs to be compared to check if it changed. If so then the hash bucket has been replaced and the user has been moved to the new one and lock_ptr has been updated. The lock operation needs to be redone in this case. The locked hash bucket is not returned. A special case is an early return in futex_lock_pi() (due to signal or timeout) and a successful futex_wait_requeue_pi(). In both cases a valid futex_q::lock_ptr is expected (and its matching hash bucket) but since the waiter has been removed from the hash this can no longer be guaranteed. Therefore before the waiter is removed and a reference is acquired which is later dropped by the waiter to avoid a resize. Add futex_q_lockptr_lock() and use it. Acquire an additional reference in requeue_pi_wake_futex() and futex_unlock_pi() while the futex_q is removed, denote this extra reference in futex_q::drop_hb_ref and let the waiter drop the reference in this case. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20250416162921.513656-11-bigeasy@linutronix.de |
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| README | ||
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 reStructuredText 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.