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Michael Larabel reported [1] a nginx performance regression in v6.15-rc3 and bisected it to commit51339d99c0("locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type") The problem is the _Generic() usage with a default association that masks the fact that "local_trylock_t *" association is not being selected as expected. Replacing the default with the only other expected type "local_lock_t *" reveals the underlying problem: include/linux/local_lock_internal.h:174:26: error: ‘_Generic’ selector of type ‘__seg_gs local_lock_t *’ is not compatible with any association The local_locki's are part of __percpu structures and thus the __percpu attribute is needed to associate the type properly. Add the attribute and keep the default replaced to turn any further mismatches into compile errors. The failure to recognize local_try_lock_t in __local_lock_release() means that a local_trylock[_irqsave]() operation will set tl->acquired to 1 (there's no _Generic() part in the trylock code), but then local_unlock[_irqrestore]() will not set tl->acquired back to 0, so further trylock operations will always fail on the same cpu+lock, while non-trylock operations continue to work - a lockdep_assert() is also not being executed in the _Generic() part of local_lock() code. This means consume_stock() and refill_stock() operations will fail deterministically, resulting in taking the slow paths and worse performance. Fixes:51339d99c0("locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type") Reported-by: Michael Larabel <Michael@phoronix.com> Closes: https://www.phoronix.com/review/linux-615-nginx-regression/2 [1] Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 reStructuredText 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.