Commit 952041a8 authored Jul 18, 2019 by Peter Zijlstra Committed by Ingo Molnar Jul 25, 2019

tty/ldsem, locking/rwsem: Add missing ACQUIRE to read_failed sleep loop



While reviewing rwsem down_slowpath, Will noticed ldsem had a copy of
a bug we just found for rwsem.

  X = 0;

  CPU0			CPU1

  rwsem_down_read()
    for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);

                        X = 1;
                        rwsem_up_write();
                          rwsem_mark_wake()
                            atomic_long_add(adjustment, &sem->count);
                            smp_store_release(&waiter->task, NULL);

      if (!waiter.task)
        break;

      ...
    }

  r = X;

Allows 'r == 0'.

Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 4898e640 ("tty: Add timed, writer-prioritized rw semaphore")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 99143f82

drivers/tty/tty_ldsem.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -93,8 +93,7 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)

		list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
		tsk = waiter->task;
		smp_mb();
		waiter->task = NULL;
		smp_store_release(&waiter->task, NULL);
		wake_up_process(tsk);
		put_task_struct(tsk);
		}
		@@ -194,7 +193,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
		for (;;) {
		set_current_state(TASK_UNINTERRUPTIBLE);

		if (!waiter.task)
		if (!smp_load_acquire(&waiter.task))
		break;
		if (!timeout)
		break;

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