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The field 'transition_task' of policy structure is used to track the
task which is performing the frequency transition. Using this field to
print a warning once detect a case where the same task is calling
_begin() again before completing the preivous frequency transition via
the _end().
However, there is a potential race condition in _end() and _begin() APIs
while updating the field 'transition_task' of policy, the scenario is
depicted below:
Task A Task B
/* 1st freq transition */
Invoke _begin() {
...
...
}
/* 2nd freq transition */
Invoke _begin() {
... //waiting for A to
... //clear
... //transition_ongoing
... //in _end() for
... //the 1st transition
|
Change the frequency |
|
Invoke _end() { |
... |
... |
transition_ongoing = false; V
transition_ongoing = true;
transition_task = current;
transition_task = NULL;
... //A overwrites the task
... //performing the transition
... //result in error warning.
}
To fix this race condition, the transition_lock of policy structure is
now acquired before updating policy structure in _end() API. Which ensure
that only one task can update the 'transition_task' field at a time.
Link: https://lore.kernel.org/all/b3c61d8a-d52d-3136-fbf0-d1de9f1ba411@huawei.com/
Fixes: ca654dc3a9 ("cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end")
Signed-off-by: Liao Chang <liaochang1@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.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.