Subject: locking/rwbase: Mitigate indefinite writer starvation.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Tue, 21 Mar 2023 17:11:40 +0100

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.

Cc: [email-masked]
Reported-by: Mel Gorman <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
Acked-by: Mel Gorman <[email-masked]>
Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx
Link: https://lore.kernel.org/r/[email-masked]
---
 kernel/locking/rwbase_rt.c |    9 ---------
 1 file changed, 9 deletions(-)

Index: linux-6.3.0-rt11/kernel/locking/rwbase_rt.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/locking/rwbase_rt.c
+++ linux-6.3.0-rt11/kernel/locking/rwbase_rt.c
@@ -72,15 +72,6 @@ static int __sched __rwbase_read_lock(st
 	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