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Date:   Sat, 29 Apr 2023 07:03:07 -0000
From:   "tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@linutronix.de>
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Subject: [tip: locking/core] locking/rwbase: Mitigate indefinite writer starvation
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
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The following commit has been merged into the locking/core branch of tip:

Commit-ID:     0abdb96fc6862681b926a0686012820229fab450
Gitweb:        https://git.kernel.org/tip/0abdb96fc6862681b926a0686012820229fab450
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Tue, 21 Mar 2023 17:11:40 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 29 Apr 2023 08:51:08 +02:00

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>
---
 kernel/locking/rwbase_rt.c |  9 ---------
 1 file changed, 9 deletions(-)

diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index c201aad..25ec023 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