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Latch sequence counters are a multiversion concurrency control mechanism where the seqcount_t counter even/odd value is used to switch between two data storage copies. This allows the seqcount_t read path to safely interrupt its write side critical section (e.g. from NMIs). Initially, latch sequence counters were implemented as a single write function, raw_write_seqcount_latch(), above plain seqcount_t. The read path was expected to use plain seqcount_t raw_read_seqcount(). A specialized read function was later added, raw_read_seqcount_latch(), and became the standardized way for latch read paths. Having unique read and write APIs meant that latch sequence counters are basically a data type of their own -- just inappropriately overloading plain seqcount_t. The seqcount_latch_t data type was thus introduced at seqlock.h. Use that new data type instead of seqcount_raw_spinlock_t. This ensures that only latch-safe APIs are to be used with the sequence counter. Note that the use of seqcount_raw_spinlock_t was not very useful in the first place. Only the "raw_" subset of seqcount_t APIs were used at timekeeping.c. This subset was created for contexts where lockdep cannot be used. seqcount_LOCKTYPE_t's raison d'être -- verifying that the seqcount_t writer serialization lock is held -- cannot thus be done. References: |
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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.