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__d_add() and __d_move() wake up waiters on dentry::d_wait from within the i_seq_dir write held region. This violates the PREEMPT_RT constraints as the wake up acquires wait_queue_head::lock which is a "sleeping" spinlock on RT. There is no requirement to do so. __d_lookup_unhash() has cleared DCACHE_PAR_LOOKUP and dentry::d_wait and returned the now unreachable wait queue head pointer to the caller, so the actual wake up can be postponed until the i_dir_seq write side critical section is left. The only requirement is that dentry::lock is held across the whole sequence including the wake up. The previous commit includes an analysis why this is considered safe. Move the wake up past end_dir_add() which leaves the i_dir_seq write side critical section and enables preemption. For non RT kernels there is no difference because preemption is still disabled due to dentry::lock being held, but it shortens the time between wake up and unlocking dentry::lock, which reduces the contention for the woken up waiter. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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.