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When a user gives an invalid command to change a private flag which is not supported, either because it is read-only, or the device is not capable of the feature, we simply ignore the request. A naive solution would simply be to report error codes when one of the flags was not supported. However, this causes problems because it makes the operation not atomic. If a user requests multiple private flags together at once we could end up changing one before failing at the second flag. We can do a bit better if we instead update a temporary copy of the flags variable in the loop, and then copy it into place after. If we aren't careful this has the pitfall of potentially silently overwriting any changes caused by other threads. Avoid this by using cmpxchg64 which will compare and swap the flags variable only if it currently matched the old value. We'll report -EAGAIN in the (hopefully rare!) case where the cmpxchg64 fails. This ensures that we can properly report when flags are not supported in an atomic fashion without the risk of overwriting other threads changes. Signed-off-by: Jacob Keller <jacob.e.keller@intel.com> Tested-by: Andrew Bowers <andrewx.bowers@intel.com> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.