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Several files under Documentation/*.txt describe some type of locking API. Move them to locking/ subdir and add to the locking/index.rst index file. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Link: https://lore.kernel.org/r/dd833a10bbd0b2c1461d78913f5ec28a7e27f00b.1588345503.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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28 lines
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====================
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Percpu rw semaphores
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====================
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Percpu rw semaphores is a new read-write semaphore design that is
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optimized for locking for reading.
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The problem with traditional read-write semaphores is that when multiple
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cores take the lock for reading, the cache line containing the semaphore
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is bouncing between L1 caches of the cores, causing performance
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degradation.
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Locking for reading is very fast, it uses RCU and it avoids any atomic
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instruction in the lock and unlock path. On the other hand, locking for
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writing is very expensive, it calls synchronize_rcu() that can take
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hundreds of milliseconds.
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The lock is declared with "struct percpu_rw_semaphore" type.
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The lock is initialized percpu_init_rwsem, it returns 0 on success and
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-ENOMEM on allocation failure.
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The lock must be freed with percpu_free_rwsem to avoid memory leak.
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The lock is locked for read with percpu_down_read, percpu_up_read and
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for write with percpu_down_write, percpu_up_write.
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The idea of using RCU for optimized rw-lock was introduced by
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Eric Dumazet <eric.dumazet@gmail.com>.
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The code was written by Mikulas Patocka <mpatocka@redhat.com>
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