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The x86_capability array in cpuinfo_x86 is of type u32 and thus is naturally aligned to 4 bytes. But, set_bit() and clear_bit() require the array to be aligned to size of unsigned long (i.e. 8 bytes on 64-bit systems). The array pointer is handed into atomic bit operations. If the access is not aligned to unsigned long then the atomic bit operations can end up crossing a cache line boundary, which causes the CPU to do a full bus lock as it can't lock both cache lines at once. The bus lock operation is heavy weight and can cause severe performance degradation. The upcoming #AC split lock detection mechanism will issue warnings for this kind of access. Force the alignment of the array to unsigned long. This avoids the massive code changes which would be required when converting the array data type to unsigned long. [ tglx: Rewrote changelog so it contains information WHY this is required ] Suggested-by: David Laight <David.Laight@aculab.com> Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> Signed-off-by: Tony Luck <tony.luck@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lkml.kernel.org/r/20190916223958.27048-4-tony.luck@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.