Linux kernel for uConsole
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88ecd153be
Since seqlocks in the Linux kernel do not require the use of marked
atomic accesses in critical sections, we teach KCSAN to assume such
accesses are atomic. KCSAN currently also pretends that writes to
`sequence` are atomic, although currently plain writes are used (their
corresponding reads are READ_ONCE).
Further, to avoid false positives in the absence of clear ending of a
seqlock reader critical section (only when using the raw interface),
KCSAN assumes a fixed number of accesses after start of a seqlock
critical section are atomic.
=== Commentary on design around absence of clear begin/end markings ===
Seqlock usage via seqlock_t follows a predictable usage pattern, where
clear critical section begin/end is enforced. With subtle special cases
for readers needing to be flat atomic regions, e.g. because usage such
as in:
- fs/namespace.c:__legitimize_mnt - unbalanced read_seqretry
- fs/dcache.c:d_walk - unbalanced need_seqretry
But, anything directly accessing seqcount_t seems to be unpredictable.
Filtering for usage of read_seqcount_retry not following 'do { .. }
while (read_seqcount_retry(..));':
$ git grep 'read_seqcount_retry' | grep -Ev 'while \(|seqlock.h|Doc|\* '
=> about 1/3 of the total read_seqcount_retry usage.
Just looking at fs/namei.c, we conclude that it is non-trivial to
prescribe and migrate to an interface that would force clear begin/end
seqlock markings for critical sections.
As such, we concluded that the best design currently, is to simply
ensure that KCSAN works well with the existing code.
Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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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.