From 3afb71880ea7159662c149a9a049b03039000115 Mon Sep 17 00:00:00 2001
From: Mel Gorman <mgorman@suse.de>
Date: Apr 26 2023 09:40:38 +0000
Subject: locking/rwbase: Mitigate indefinite writer starvation

(bsc#1189998 (PREEMPT_RT prerequisite backports), bsc#1206552).

---

diff --git a/patches.suse/locking-rwbase-Mitigate-indefinite-writer-starvation.patch b/patches.suse/locking-rwbase-Mitigate-indefinite-writer-starvation.patch
new file mode 100644
index 0000000..e3c5456
--- /dev/null
+++ b/patches.suse/locking-rwbase-Mitigate-indefinite-writer-starvation.patch
@@ -0,0 +1,58 @@
+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
diff --git a/series.conf b/series.conf
index f713ec3..2b45e13 100644
--- a/series.conf
+++ b/series.conf
@@ -2601,6 +2601,9 @@
 	patches.suse/rt-Add-documentation-describing-what-kernel-debug-options-to-add-for-testing.patch
 	patches.suse/rt-Add-helper-script-to-refresh-RT-configs-based-on-the-parent.patch
 
+	# Pending merge to ALP-current
+	patches.suse/locking-rwbase-Mitigate-indefinite-writer-starvation.patch
+
 	########################################################
 	# You'd better have a good reason for adding a patch
 	# below here.