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From f4d3bd6d8f68cd12e5fd96173c999fa0e7a7554d Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Tue, 21 Mar 2023 17:11:40 +0100
Subject: [PATCH 54/64] locking/rwbase: Mitigate indefinite writer starvation
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.82-rt27.tar.xz
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>
(cherry picked from commit 286deb7ec03d941664ac3ffaff58814b454adf65)
Signed-off-by: Clark Williams <clark.williams@gmail.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 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
--
2.44.0
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