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RFC locking: Fix ww_mutex algorithm inconsistency.

Thomas Hellstrom dri-devel lkml 1 version New

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#Subject and filesSizeRepliesApplied
1/2locking: Implement an algorithm choice for Wound-Wait mutexesDocumentation/locking/ww-mutex-design.txt drivers/dma-buf/reservation.c drivers/gpu/drm/drm_modeset_lock.c +5 more+154 -330 replies
2/2drm: Change deadlock-avoidance algorithm for the modeset locks.drivers/gpu/drm/drm_modeset_lock.c+1 -10 replies

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Cover letter [PATCH RFC 0/2] locking: Fix ww_mutex algorithm inconsistency.

The algorithm used for linux Wound/Wait mutexes, is actually not Wound/Wait but Wait/Die. See for example

http://www.mathcs.emory.edu/~cheung/Courses/554/Syllabus/8-recv+serial/deadlock-compare.html

Rather than renaming them across the tree to something like Wait/Die mutexes or Deadlock Avoidance mutexes, this patch set implements also the Wound/Wait algorithm It shouldn't touch the binary ordinary mutex paths when compiled with otpimization.
We use Wound/Wait for the modeset locks which in theory may benefit slightly from Wound/Wait rather than Wait/Die, but Wait/Die is actually superior for larger number of simultaneous contending transactions so we keep the original implementation as a choice, and don't touch the algorithm used by the reservation objects.

Performance- and functional testing has been done using

git://people.freedesktop.org/~thomash/ww_mutex_test

Using WW_BUILTIN and 4 and 16 competing threads.

Another option is of course to ignore the the naming / algorithm inconsistency.

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