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authorSebastian Andrzej Siewior2023-03-21 17:11:40 +0100
committerIngo Molnar2023-04-29 09:08:52 +0200
commit286deb7ec03d941664ac3ffaff58814b454adf65 (patch)
tree1d01efdac97b9739ae0fb6e37f67e1ee9873790d /kernel
parent068550631fbe0b7fb41625cea6fb204fdc8cb224 (diff)
locking/rwbase: Mitigate indefinite writer starvation
On PREEMPT_RT, rw_semaphore and rwlock_t locks are unfair to writers. Readers can indefinitely acquire the lock unless the writer fully acquired the lock, which might never happen if there is always a reader in the critical section owning the lock. Mel Gorman reported that since LTP-20220121 the dio_truncate test case went from having 1 reader to having 16 readers and that number of readers is sufficient to prevent the down_write ever succeeding while readers exist. Eventually the test is killed after 30 minutes as a failure. Mel proposed a timeout to limit how long a writer can be blocked until the reader is forced into the slowpath. Thomas argued that there is no added value by providing this timeout. From a PREEMPT_RT point of view, there are no critical rw_semaphore or rwlock_t locks left where the reader must be preferred. Mitigate indefinite writer starvation by forcing the READER into the slowpath once the WRITER attempts to acquire the lock. Reported-by: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Ingo Molnar <mingo@kernel.org> Acked-by: Mel Gorman <mgorman@techsingularity.net> Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx Link: https://lore.kernel.org/r/20230321161140.HMcQEhHb@linutronix.de Cc: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/locking/rwbase_rt.c9
1 files changed, 0 insertions, 9 deletions
diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index c201aadb9301..25ec0239477c 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -72,15 +72,6 @@ static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
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