From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> v5.15.133-rt70-rc1 stable review patch. If anyone has any objections, please let me know. ----------- 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@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Link: https://lore.kernel.org/877cwbq4cq.ffs@tglx Link: https://lore.kernel.org/r/20230321161140.HMcQEhHb@xxxxxxxxxxxxx Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> (cherry picked from commit 286deb7ec03d941664ac3ffaff58814b454adf65) Signed-off-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx> --- 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 88191f6e252c..a28148a05383 100644 --- a/kernel/locking/rwbase_rt.c +++ b/kernel/locking/rwbase_rt.c @@ -73,15 +73,6 @@ 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 -- 2.34.1
