Revision b11e7b81bf94f633ae44f12a3d2e198277781952 authored by Linus Torvalds on 27 September 2015, 00:56:50 UTC, committed by Linus Torvalds on 27 September 2015, 00:56:50 UTC
Pull staging driver fixes from Greg KH: "Here are some tiny staging driver and documentation fixes for 4.3-rc3. All of these resolve reported issues that people have found and have been in the linux-next tree for a while with no problems" * tag 'staging-4.3-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging: MAINTAINERS: Update email address for Martyn Welch staging: ion: fix corruption of ion_import_dma_buf staging: dgap: Remove myself from the MAINTAINERS file staging: most: Add dependency to HAS_IOMEM staging: unisys: remove reference of visorutil staging: unisys: visornic: handle error return from device registration staging: unisys: stop device registration before visorbus registration staging: unisys: visorbus: Unregister driver on error staging: unisys: visornic: Fix receive bytes statistics staging: unisys: unregister netdev when create debugfs fails staging: fbtft: replace master->setup() with spi_setup() staging: fbtft: fix 9-bit SPI support detection staging/lustre: change Lustre URLs and mailing list staging/android: Update ION TODO per LPC discussion Staging: most: MOST and MOSTCORE should depend on HAS_DMA staging: most: fix HDM_USB dependencies and build errors
percpu-rw-semaphore.txt
Percpu rw semaphores
--------------------
Percpu rw semaphores is a new read-write semaphore design that is
optimized for locking for reading.
The problem with traditional read-write semaphores is that when multiple
cores take the lock for reading, the cache line containing the semaphore
is bouncing between L1 caches of the cores, causing performance
degradation.
Locking for reading is very fast, it uses RCU and it avoids any atomic
instruction in the lock and unlock path. On the other hand, locking for
writing is very expensive, it calls synchronize_rcu() that can take
hundreds of milliseconds.
The lock is declared with "struct percpu_rw_semaphore" type.
The lock is initialized percpu_init_rwsem, it returns 0 on success and
-ENOMEM on allocation failure.
The lock must be freed with percpu_free_rwsem to avoid memory leak.
The lock is locked for read with percpu_down_read, percpu_up_read and
for write with percpu_down_write, percpu_up_write.
The idea of using RCU for optimized rw-lock was introduced by
Eric Dumazet <eric.dumazet@gmail.com>.
The code was written by Mikulas Patocka <mpatocka@redhat.com>
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