63 lines
2.4 KiB
Diff
63 lines
2.4 KiB
Diff
From 23249660cee29a01660c4518a3046dfba2b7199c Mon Sep 17 00:00:00 2001
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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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Date: Tue, 21 Mar 2023 17:11:40 +0100
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Subject: [PATCH 54/62] locking/rwbase: Mitigate indefinite writer starvation
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On PREEMPT_RT, rw_semaphore and rwlock_t locks are unfair to writers.
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Readers can indefinitely acquire the lock unless the writer fully acquired
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the lock, which might never happen if there is always a reader in the
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critical section owning the lock.
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Mel Gorman reported that since LTP-20220121 the dio_truncate test case
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went from having 1 reader to having 16 readers and that number of readers
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is sufficient to prevent the down_write ever succeeding while readers
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exist. Eventually the test is killed after 30 minutes as a failure.
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Mel proposed a timeout to limit how long a writer can be blocked until
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the reader is forced into the slowpath.
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Thomas argued that there is no added value by providing this timeout. From
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a PREEMPT_RT point of view, there are no critical rw_semaphore or rwlock_t
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locks left where the reader must be preferred.
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Mitigate indefinite writer starvation by forcing the READER into the
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slowpath once the WRITER attempts to acquire the lock.
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Reported-by: Mel Gorman <mgorman@techsingularity.net>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Acked-by: Mel Gorman <mgorman@techsingularity.net>
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Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx
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Link: https://lore.kernel.org/r/20230321161140.HMcQEhHb@linutronix.de
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Cc: Linus Torvalds <torvalds@linux-foundation.org>
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(cherry picked from commit 286deb7ec03d941664ac3ffaff58814b454adf65)
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Signed-off-by: Clark Williams <clark.williams@gmail.com>
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---
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kernel/locking/rwbase_rt.c | 9 ---------
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1 file changed, 9 deletions(-)
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diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
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index c201aadb9301..25ec0239477c 100644
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--- a/kernel/locking/rwbase_rt.c
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+++ b/kernel/locking/rwbase_rt.c
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@@ -72,15 +72,6 @@ static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
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int ret;
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raw_spin_lock_irq(&rtm->wait_lock);
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- /*
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- * Allow readers, as long as the writer has not completely
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- * acquired the semaphore for write.
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- */
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- if (atomic_read(&rwb->readers) != WRITER_BIAS) {
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- atomic_inc(&rwb->readers);
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- raw_spin_unlock_irq(&rtm->wait_lock);
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- return 0;
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- }
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/*
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* Call into the slow lock path with the rtmutex->wait_lock
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--
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2.43.0
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