diff options
| author | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2023-03-21 17:11:40 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2023-04-29 09:08:52 +0200 |
| commit | 286deb7ec03d941664ac3ffaff58814b454adf65 (patch) | |
| tree | 1d01efdac97b9739ae0fb6e37f67e1ee9873790d /kernel | |
| parent | 068550631fbe0b7fb41625cea6fb204fdc8cb224 (diff) | |
locking/rwbase: Mitigate indefinite writer starvation
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>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/locking/rwbase_rt.c | 9 |
1 files changed, 0 insertions, 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 |