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Conditional atomic operations (e.g. cmpxchg()) only provide ordering
when the condition holds; when the condition does not hold, the location
is not modified and relaxed ordering is provided. Where ordering is
needed for failed conditional atomics, it is necessary to use
smp_mb__before_atomic() and/or smp_mb__after_atomic().
This is explained tersely in memory-barriers.txt, and is implied but not
explicitly stated in the kerneldoc comments for the conditional
operations. The lack of an explicit statement has lead to some off-list
queries about the ordering semantics of failing conditional operations,
so evidently this is confusing.
Update the kerneldoc comments to explicitly describe the lack of ordering
for failed conditional atomic operations.
For most conditional atomic operations, this is written as:
| If (${condition}), atomically updates @v to (${new}) with ${desc_order} ordering.
| Otherwise, @v is not modified and relaxed ordering is provided.
For the try_cmpxchg() operations, this is written as:
| If (${condition}), atomically updates @v to @new with ${desc_order} ordering.
| Otherwise, @v is not modified, @old is updated to the current value of @v,
| and relaxed ordering is provided.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Link: https://lore.kernel.org/r/20240209124010.2096198-1-mark.rutland@arm.com
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.