linux-kernel - [PATCH RT 02/12] locking/rwbase: Mitigate indefinite writer starvation

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Message-Id: <20231018194833.651674-3-joseph.salisbury@canonical.com>
Date:   Wed, 18 Oct 2023 15:48:23 -0400
From:   Joseph Salisbury <joseph.salisbury@...onical.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <daniel.wagner@...e.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Clark Williams <williams@...hat.com>,
        Pavel Machek <pavel@...x.de>,
        Joseph Salisbury <joseph.salisbury@...onical.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH RT 02/12] locking/rwbase: Mitigate indefinite writer starvation

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

v5.15.133-rt70-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------

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@...hsingularity.net>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Mel Gorman <mgorman@...hsingularity.net>
Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx
Link: https://lore.kernel.org/r/20230321161140.HMcQEhHb@linutronix.de
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
(cherry picked from commit 286deb7ec03d941664ac3ffaff58814b454adf65)
Signed-off-by: Joseph Salisbury <joseph.salisbury@...onical.com>
---
 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 88191f6e252c..a28148a05383 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -73,15 +73,6 @@ 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
-- 
2.34.1