From 943f0edb754fac195043c620b44f920e4fb76ec8 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Sun, 15 Aug 2021 23:28:03 +0200 Subject: locking/rt: Add base code for RT rw_semaphore and rwlock On PREEMPT_RT, rw_semaphores and rwlocks are substituted with an rtmutex and a reader count. The implementation is writer unfair, as it is not feasible to do priority inheritance on multiple readers, but experience has shown that real-time workloads are not the typical workloads which are sensitive to writer starvation. The inner workings of rw_semaphores and rwlocks on RT are almost identical except for the task state and signal handling. rw_semaphores are not state preserving over a contention, they are expected to enter and leave with state == TASK_RUNNING. rwlocks have a mechanism to preserve the state of the task at entry and restore it after unblocking taking potential non-lock related wakeups into account. rw_semaphores can also be subject to signal handling interrupting a blocked state, while rwlocks ignore signals. To avoid code duplication, provide a shared implementation which takes the small difference vs. state and signals into account. The code is included into the relevant rw_semaphore/rwlock base code and compiled for each use case separately. Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/r/20210815211302.957920571@linutronix.de --- include/linux/rwbase_rt.h | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) create mode 100644 include/linux/rwbase_rt.h (limited to 'include/linux/rwbase_rt.h') diff --git a/include/linux/rwbase_rt.h b/include/linux/rwbase_rt.h new file mode 100644 index 000000000000..1d264dd08625 --- /dev/null +++ b/include/linux/rwbase_rt.h @@ -0,0 +1,39 @@ +// SPDX-License-Identifier: GPL-2.0-only +#ifndef _LINUX_RWBASE_RT_H +#define _LINUX_RWBASE_RT_H + +#include +#include + +#define READER_BIAS (1U << 31) +#define WRITER_BIAS (1U << 30) + +struct rwbase_rt { + atomic_t readers; + struct rt_mutex_base rtmutex; +}; + +#define __RWBASE_INITIALIZER(name) \ +{ \ + .readers = ATOMIC_INIT(READER_BIAS), \ + .rtmutex = __RT_MUTEX_BASE_INITIALIZER(name.rtmutex), \ +} + +#define init_rwbase_rt(rwbase) \ + do { \ + rt_mutex_base_init(&(rwbase)->rtmutex); \ + atomic_set(&(rwbase)->readers, READER_BIAS); \ + } while (0) + + +static __always_inline bool rw_base_is_locked(struct rwbase_rt *rwb) +{ + return atomic_read(&rwb->readers) != READER_BIAS; +} + +static __always_inline bool rw_base_is_contended(struct rwbase_rt *rwb) +{ + return atomic_read(&rwb->readers) > 0; +} + +#endif /* _LINUX_RWBASE_RT_H */ -- cgit v1.2.3