From 943f0edb754fac195043c620b44f920e4fb76ec8 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Sun, 15 Aug 2021 23:28:03 +0200
Subject: [PATCH] locking/rt: Add base code for RT rw_semaphore and rwlock
Git-commit: 943f0edb754fac195043c620b44f920e4fb76ec8
Patch-mainline: v5.15-rc1
References: bsc#1190137 bsc#1189998
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 <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211302.957920571@linutronix.de
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
include/linux/rwbase_rt.h | 39 ++++++
kernel/locking/rwbase_rt.c | 263 +++++++++++++++++++++++++++++++++++++
2 files changed, 302 insertions(+)
create mode 100644 include/linux/rwbase_rt.h
create mode 100644 kernel/locking/rwbase_rt.c
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 <linux/rtmutex.h>
+#include <linux/atomic.h>
+
+#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 */
diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
new file mode 100644
index 000000000000..4ba15088e640
--- /dev/null
+++ b/kernel/locking/rwbase_rt.c
@@ -0,0 +1,263 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * RT-specific reader/writer semaphores and reader/writer locks
+ *
+ * down_write/write_lock()
+ * 1) Lock rtmutex
+ * 2) Remove the reader BIAS to force readers into the slow path
+ * 3) Wait until all readers have left the critical section
+ * 4) Mark it write locked
+ *
+ * up_write/write_unlock()
+ * 1) Remove the write locked marker
+ * 2) Set the reader BIAS, so readers can use the fast path again
+ * 3) Unlock rtmutex, to release blocked readers
+ *
+ * down_read/read_lock()
+ * 1) Try fast path acquisition (reader BIAS is set)
+ * 2) Take tmutex::wait_lock, which protects the writelocked flag
+ * 3) If !writelocked, acquire it for read
+ * 4) If writelocked, block on tmutex
+ * 5) unlock rtmutex, goto 1)
+ *
+ * up_read/read_unlock()
+ * 1) Try fast path release (reader count != 1)
+ * 2) Wake the writer waiting in down_write()/write_lock() #3
+ *
+ * down_read/read_lock()#3 has the consequence, that rw semaphores and rw
+ * locks on RT are not writer fair, but writers, which should be avoided in
+ * RT tasks (think mmap_sem), are subject to the rtmutex priority/DL
+ * inheritance mechanism.
+ *
+ * It's possible to make the rw primitives writer fair by keeping a list of
+ * active readers. A blocked writer would force all newly incoming readers
+ * to block on the rtmutex, but the rtmutex would have to be proxy locked
+ * for one reader after the other. We can't use multi-reader inheritance
+ * because there is no way to support that with SCHED_DEADLINE.
+ * Implementing the one by one reader boosting/handover mechanism is a
+ * major surgery for a very dubious value.
+ *
+ * The risk of writer starvation is there, but the pathological use cases
+ * which trigger it are not necessarily the typical RT workloads.
+ *
+ * Common code shared between RT rw_semaphore and rwlock
+ */
+
+static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
+{
+ int r;
+
+ /*
+ * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
+ * set.
+ */
+ for (r = atomic_read(&rwb->readers); r < 0;) {
+ if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
+ return 1;
+ }
+ return 0;
+}
+
+static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
+ unsigned int state)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+ int ret;
+
+ raw_spin_lock_irq(&rtm->wait_lock);
+ /*
+ * Allow readers, as long as the writer has not completely
+ * acquired the semaphore for write.
+ */
+ if (atomic_read(&rwb->readers) != WRITER_BIAS) {
+ atomic_inc(&rwb->readers);
+ raw_spin_unlock_irq(&rtm->wait_lock);
+ return 0;
+ }
+
+ /*
+ * Call into the slow lock path with the rtmutex->wait_lock
+ * held, so this can't result in the following race:
+ *
+ * Reader1 Reader2 Writer
+ * down_read()
+ * down_write()
+ * rtmutex_lock(m)
+ * wait()
+ * down_read()
+ * unlock(m->wait_lock)
+ * up_read()
+ * wake(Writer)
+ * lock(m->wait_lock)
+ * sem->writelocked=true
+ * unlock(m->wait_lock)
+ *
+ * up_write()
+ * sem->writelocked=false
+ * rtmutex_unlock(m)
+ * down_read()
+ * down_write()
+ * rtmutex_lock(m)
+ * wait()
+ * rtmutex_lock(m)
+ *
+ * That would put Reader1 behind the writer waiting on
+ * Reader2 to call up_read(), which might be unbound.
+ */
+
+ /*
+ * For rwlocks this returns 0 unconditionally, so the below
+ * !ret conditionals are optimized out.
+ */
+ ret = rwbase_rtmutex_slowlock_locked(rtm, state);
+
+ /*
+ * On success the rtmutex is held, so there can't be a writer
+ * active. Increment the reader count and immediately drop the
+ * rtmutex again.
+ *
+ * rtmutex->wait_lock has to be unlocked in any case of course.
+ */
+ if (!ret)
+ atomic_inc(&rwb->readers);
+ raw_spin_unlock_irq(&rtm->wait_lock);
+ if (!ret)
+ rwbase_rtmutex_unlock(rtm);
+ return ret;
+}
+
+static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
+ unsigned int state)
+{
+ if (rwbase_read_trylock(rwb))
+ return 0;
+
+ return __rwbase_read_lock(rwb, state);
+}
+
+static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
+ unsigned int state)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+ struct task_struct *owner;
+
+ raw_spin_lock_irq(&rtm->wait_lock);
+ /*
+ * Wake the writer, i.e. the rtmutex owner. It might release the
+ * rtmutex concurrently in the fast path (due to a signal), but to
+ * clean up rwb->readers it needs to acquire rtm->wait_lock. The
+ * worst case which can happen is a spurious wakeup.
+ */
+ owner = rt_mutex_owner(rtm);
+ if (owner)
+ wake_up_state(owner, state);
+
+ raw_spin_unlock_irq(&rtm->wait_lock);
+}
+
+static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
+ unsigned int state)
+{
+ /*
+ * rwb->readers can only hit 0 when a writer is waiting for the
+ * active readers to leave the critical section.
+ */
+ if (unlikely(atomic_dec_and_test(&rwb->readers)))
+ __rwbase_read_unlock(rwb, state);
+}
+
+static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
+ unsigned long flags)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+
+ atomic_add(READER_BIAS - bias, &rwb->readers);
+ raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+ rwbase_rtmutex_unlock(rtm);
+}
+
+static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&rtm->wait_lock, flags);
+ __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
+}
+
+static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&rtm->wait_lock, flags);
+ /* Release it and account current as reader */
+ __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
+}
+
+static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
+ unsigned int state)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+ unsigned long flags;
+
+ /* Take the rtmutex as a first step */
+ if (rwbase_rtmutex_lock_state(rtm, state))
+ return -EINTR;
+
+ /* Force readers into slow path */
+ atomic_sub(READER_BIAS, &rwb->readers);
+
+ raw_spin_lock_irqsave(&rtm->wait_lock, flags);
+ /*
+ * set_current_state() for rw_semaphore
+ * current_save_and_set_rtlock_wait_state() for rwlock
+ */
+ rwbase_set_and_save_current_state(state);
+
+ /* Block until all readers have left the critical section. */
+ for (; atomic_read(&rwb->readers);) {
+ /* Optimized out for rwlocks */
+ if (rwbase_signal_pending_state(state, current)) {
+ __set_current_state(TASK_RUNNING);
+ __rwbase_write_unlock(rwb, 0, flags);
+ return -EINTR;
+ }
+ raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+
+ /*
+ * Schedule and wait for the readers to leave the critical
+ * section. The last reader leaving it wakes the waiter.
+ */
+ if (atomic_read(&rwb->readers) != 0)
+ rwbase_schedule();
+ set_current_state(state);
+ raw_spin_lock_irqsave(&rtm->wait_lock, flags);
+ }
+
+ atomic_set(&rwb->readers, WRITER_BIAS);
+ rwbase_restore_current_state();
+ raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+ return 0;
+}
+
+static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
+{
+ struct rt_mutex_base *rtm = &rwb->rtmutex;
+ unsigned long flags;
+
+ if (!rwbase_rtmutex_trylock(rtm))
+ return 0;
+
+ atomic_sub(READER_BIAS, &rwb->readers);
+
+ raw_spin_lock_irqsave(&rtm->wait_lock, flags);
+ if (!atomic_read(&rwb->readers)) {
+ atomic_set(&rwb->readers, WRITER_BIAS);
+ raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+ return 1;
+ }
+ __rwbase_write_unlock(rwb, 0, flags);
+ return 0;
+}
--
2.26.2