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i_dir_seq is a sequence counter with a lock which is represented by the lowest bit. The writer atomically updates the counter which ensures that it can be modified by only one writer at a time. This requires preemption to be disabled across the write side critical section. On !PREEMPT_RT kernels this is implicit by the caller acquiring dentry::lock. On PREEMPT_RT kernels spin_lock() does not disable preemption which means that a preempting writer or reader would live lock. It's therefore required to disable preemption explicitly. An alternative solution would be to replace i_dir_seq with a seqlock_t for PREEMPT_RT, but that comes with its own set of problems due to arbitrary lock nesting. A pure sequence count with an associated spinlock is not possible because the locks held by the caller are not necessarily related. As the critical section is small, disabling preemption is a sensible solution. Reported-by: Oleg.Karfich@wago.com Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Link: https://lkml.kernel.org/r/20220613140712.77932-2-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.