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Patch series "cpumask: Fix invalid uniprocessor assumptions", v4. On uniprocessor builds, it is currently assumed that any cpumask will contain the single CPU: cpu0. This assumption is used to provide optimised implementations. The current assumption also appears to be wrong, by ignoring the fact that users can provide empty cpumasks. This can result in bugs as explained in [1] - for_each_cpu() will run one iteration of the loop even when passed an empty cpumask. This series introduces some basic tests, and updates the optimisations for uniprocessor builds. The x86 patch was written after the kernel test robot [2] ran into a failed build. I have tried to list the files potentially affected by the changes to cpumask.h, in an attempt to find any other cases that fail on !SMP. I've gone through some of the files manually, and ran a few cross builds, but nothing else popped up. I (build) checked about half of the potientally affected files, but I do not have the resources to do them all. I hope we can fix other issues if/when they pop up later. [1] https://lore.kernel.org/all/20220530082552.46113-1-sander@svanheule.net/ [2] https://lore.kernel.org/all/202206060858.wA0FOzRy-lkp@intel.com/ This patch (of 5): The maps to keep track of shared caches between CPUs on SMP systems are declared in asm/smp.h, among them specifically cpu_llc_shared_map. These maps are externally defined in cpu/smpboot.c. The latter is only compiled on CONFIG_SMP=y, which means the declared extern symbols from asm/smp.h do not have a corresponding definition on uniprocessor builds. The inline cpu_llc_shared_mask() function from asm/smp.h refers to the map declaration mentioned above. This function is referenced in cacheinfo.c inside for_each_cpu() loop macros, to provide cpumask for the loop. On uniprocessor builds, the symbol for the cpu_llc_shared_map does not exist. However, the current implementation of for_each_cpu() also (wrongly) ignores the provided mask. By sheer luck, the compiler thus optimises out this unused reference to cpu_llc_shared_map, and the linker therefore does not require the cpu_llc_shared_mask to actually exist on uniprocessor builds. Only on SMP bulids does smpboot.o exist to provide the required symbols. To no longer rely on compiler optimisations for successful uniprocessor builds, move the definitions of cpu_llc_shared_map and cpu_l2c_shared_map from smpboot.c to cacheinfo.c. Link: https://lkml.kernel.org/r/cover.1656777646.git.sander@svanheule.net Link: https://lkml.kernel.org/r/e8167ddb570f56744a3dc12c2149a660a324d969.1656777646.git.sander@svanheule.net Signed-off-by: Sander Vanheule <sander@svanheule.net> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Cc: Marco Elver <elver@google.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Valentin Schneider <vschneid@redhat.com> Cc: Yury Norov <yury.norov@gmail.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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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.