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authorLinus Torvalds <torvalds@linux-foundation.org>2021-09-19 13:11:19 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2021-09-19 13:11:19 -0700
commitf5e29a26c42b2c1b149118319a03bf937f168298 (patch)
treec24e0accae1cfeca27c8e03ca480c63e7055b5cf
parent62453a460a000156b100bf20ab78cf77232284d0 (diff)
parent81121524f1c798c9481bd7900450b72ee7ac2eef (diff)
Merge tag 'locking-urgent-2021-09-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking fixes from Thomas Gleixner: "A set of updates for the RT specific reader/writer locking base code: - Make the fast path reader ordering guarantees correct. - Code reshuffling to make the fix simpler" [ This plays ugly games with atomic_add_return_release() because we don't have a plain atomic_add_release(), and should really be cleaned up, I think - Linus ] * tag 'locking-urgent-2021-09-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: locking/rwbase: Take care of ordering guarantee for fastpath reader locking/rwbase: Extract __rwbase_write_trylock() locking/rwbase: Properly match set_and_save_state() to restore_state()
-rw-r--r--kernel/locking/rwbase_rt.c65
1 files changed, 45 insertions, 20 deletions
diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 4ba15088e640..88191f6e252c 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -41,6 +41,12 @@
* The risk of writer starvation is there, but the pathological use cases
* which trigger it are not necessarily the typical RT workloads.
*
+ * Fast-path orderings:
+ * The lock/unlock of readers can run in fast paths: lock and unlock are only
+ * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
+ * semantics of rwbase_rt. Atomic ops should thus provide _acquire()
+ * and _release() (or stronger).
+ *
* Common code shared between RT rw_semaphore and rwlock
*/
@@ -53,6 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
* set.
*/
for (r = atomic_read(&rwb->readers); r < 0;) {
+ /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
return 1;
}
@@ -162,6 +169,8 @@ static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
/*
* rwb->readers can only hit 0 when a writer is waiting for the
* active readers to leave the critical section.
+ *
+ * dec_and_test() is fully ordered, provides RELEASE.
*/
if (unlikely(atomic_dec_and_test(&rwb->readers)))
__rwbase_read_unlock(rwb, state);
@@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
- atomic_add(READER_BIAS - bias, &rwb->readers);
+ /*
+ * _release() is needed in case that reader is in fast path, pairing
+ * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
+ */
+ (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
rwbase_rtmutex_unlock(rtm);
}
@@ -196,6 +209,23 @@ static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
}
+static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
+{
+ /* Can do without CAS because we're serialized by wait_lock. */
+ lockdep_assert_held(&rwb->rtmutex.wait_lock);
+
+ /*
+ * _acquire is needed in case the reader is in the fast path, pairing
+ * with rwbase_read_unlock(), provides ACQUIRE.
+ */
+ if (!atomic_read_acquire(&rwb->readers)) {
+ atomic_set(&rwb->readers, WRITER_BIAS);
+ return 1;
+ }
+
+ return 0;
+}
+
static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
unsigned int state)
{
@@ -210,34 +240,30 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
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);
+ if (__rwbase_write_trylock(rwb))
+ goto out_unlock;
- /* Block until all readers have left the critical section. */
- for (; atomic_read(&rwb->readers);) {
+ rwbase_set_and_save_current_state(state);
+ for (;;) {
/* Optimized out for rwlocks */
if (rwbase_signal_pending_state(state, current)) {
- __set_current_state(TASK_RUNNING);
+ rwbase_restore_current_state();
__rwbase_write_unlock(rwb, 0, flags);
return -EINTR;
}
+
+ if (__rwbase_write_trylock(rwb))
+ break;
+
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+ rwbase_schedule();
+ raw_spin_lock_irqsave(&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();
+
+out_unlock:
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
return 0;
}
@@ -253,8 +279,7 @@ static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
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);
+ if (__rwbase_write_trylock(rwb)) {
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
return 1;
}