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Subject[PATCH v2 0/3] qrwlock: Introducing a queue read/write lock implementation
- Improve lock fastpath performance.
- Optionally provide classic read/write lock behavior for backward
- Use xadd instead of cmpxchg for fair reader code path to make it
immute to reader contention.
- Run more performance testing.

As mentioned in the LWN article, the
classic read/write lock suffer from an unfairness problem that it is
possible for a stream of incoming readers to block a waiting writer
from getting the lock for a long time. Also, a waiting reader/writer
contending a rwlock in local memory will have a higher chance of
acquiring the lock than a reader/writer in remote node.

This patch set introduces a queue-based read/write lock implementation
that can largely solve this unfairness problem while, at the same
time, can optionally provide the classic read/write lock behavior
for those locks that need it. This new queue read/write lock is both
fairer than the classic read/write lock and will be faster in lock
contention situations.

The queue write lock can also be used as a replacement for ticket
spinlocks that are highly contended if lock size increase is not
an issue.

There is no change in the interface. By just selecting the QUEUE_RWLOCK
config parameter during the configuration phase, the classic read/write
lock will be replaced by the new queue read/write lock. This will
made the systems more deterministic and faster in lock contention
situations. In uncontended cases, the queue read/write lock may be
a bit slower than the classic one depending on the exact mix of read
and write locking primitives. Given the fact that locking overhead is
typically a very small percentage of the total CPU time in uncontended
cases, there won't be any noticeable degraduation in performance with
this replacement.

This patch set currently provides queue read/write lock support on
x86 architecture only. Support for other architectures can be added
later on once proper testing is done.

Signed-off-by: Waiman Long <>

Waiman Long (3):
qrwlock: A queue read/write lock implementation
qrwlock x86: Enable x86 to use queue read/write lock
qrwlock: Optionally enable classic read/write lock behavior

arch/x86/Kconfig | 3 +
arch/x86/include/asm/spinlock.h | 2 +
arch/x86/include/asm/spinlock_types.h | 4 +
include/asm-generic/qrwlock.h | 251 +++++++++++++++++++++++++++++++++
include/linux/rwlock.h | 15 ++
include/linux/rwlock_types.h | 12 ++-
include/linux/spinlock_types.h | 4 +
lib/Kconfig | 23 +++
lib/Makefile | 1 +
lib/qrwlock.c | 194 +++++++++++++++++++++++++
lib/spinlock_debug.c | 20 +++
11 files changed, 528 insertions(+), 1 deletions(-)
create mode 100644 include/asm-generic/qrwlock.h
create mode 100644 lib/qrwlock.c

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