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cpu_caps_cleared[] and cpu_caps_set[] are arrays of type u32 and therefore naturally aligned to 4 bytes, which is also unsigned long aligned on 32-bit, but not on 64-bit. The array pointer is handed into atomic bit operations. If the access 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 these arrays 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 ] 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> Reviewed-by: Borislav Petkov <bp@suse.de> Link: https://lkml.kernel.org/r/20190916223958.27048-2-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.