From 6c9564384d5ae49a40293408d330f520f638de09 Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 22 Mar 2023 14:26:59 +0000
Subject: [PATCH] locking/rwbase: Mitigate indefinite writer starvation.
References: bsc#1189998 (PREEMPT_RT prerequisite backports), bsc#1206552
Patch-mainline: Queued in subsystem maintainer repository
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
Git-commit: 9d29d8e36cd6448966d83ab1acf946a2df3c2833
The rw_semaphore and rwlock_t locks are unfair to writers. Readers can
indefinitely acquire the lock unless the writer fully acquired the lock.
This can 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 reader and the 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 PREEMPT_RT point of view, there are no critical rw_semaphore or
rwlock_t locks left where the reader must be prefer.
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>
Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
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 6fd3162e4098..356b247521d8 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