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Subject[PATCH v6 0/5] qrwlock: Introducing a queue read/write lock implementation
- Modify queue_read_can_lock() to avoid false positive result.
- Move the two slowpath functions' performance tuning change from
patch 4 to patch 1.
- Add a new optional patch to use the new smp_store_release() function
if that is merged.

- Fix wrong definitions for QW_MASK_FAIR & QW_MASK_UNFAIR macros.
- Add an optional patch 4 which should only be applied after the
mcs_spinlock.h header file is merged.

- Optimize the fast path with better cold cache behavior and
- Removing some testing code.
- Make x86 use queue rwlock with no user configuration.

- Make read lock stealing the default and fair rwlock an option with
a different initializer.
- In queue_read_lock_slowpath(), check irq_count() and force spinning
and lock stealing in interrupt context.
- Unify the fair and classic read-side code path, and make write-side
to use cmpxchg with 2 different writer states. This slows down the
write lock fastpath to make the read side more efficient, but is
still slightly faster than a spinlock.

- 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 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 if the lock owners
choose to use the fair variant of the lock.

The queue rwlock has two variants selected at initialization time
- unfair (with read lock stealing) and fair (to both readers and
writers). The unfair rwlock is the default.

The read lock slowpath will check if the reader is in an interrupt
context. If so, it will force lock spinning and stealing without
waiting in a queue. This is to ensure the read lock will be granted
as soon as possible.

Even the unfair rwlock is fairer than the current version as there
is a higher chance for writers to get the lock and is fair among
the writers.

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.

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

The optional patch 4 has a dependency on the the mcs_spinlock.h
header file which has not been merged yet. So this patch should only
be applied after the mcs_spinlock.h header file is merged.

The optional patch 5 use the new smp_store_release() and
smp_load_acquire() functions which are being reviewed & not merged yet.

Waiman Long (5):
qrwlock: A queue read/write lock implementation
qrwlock x86: Enable x86 to use queue read/write lock
qrwlock: Enable fair queue read/write lock
qrwlock: Use the mcs_spinlock helper functions for MCS queuing
qrwlock: Use smp_store_release() in write_unlock()

arch/x86/Kconfig | 1 +
arch/x86/include/asm/spinlock.h | 2 +
arch/x86/include/asm/spinlock_types.h | 4 +
include/asm-generic/qrwlock.h | 245 +++++++++++++++++++++++++++++++++
include/linux/rwlock.h | 15 ++
include/linux/rwlock_types.h | 13 ++
kernel/Kconfig.locks | 7 +
lib/Makefile | 1 +
lib/qrwlock.c | 205 +++++++++++++++++++++++++++
lib/spinlock_debug.c | 19 +++
10 files changed, 512 insertions(+), 0 deletions(-)
create mode 100644 include/asm-generic/qrwlock.h
create mode 100644 lib/qrwlock.c

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