Hello,
On Mon, May 09, 2022 at 04:34:00PM +0200, Alexander Bluhm wrote:
> On Sun, May 08, 2022 at 10:54:01PM +0200, Alexandr Nedvedicky wrote:
> > what bothers me is the situation where there are
> > more than one reader. The line 350 is executed by
> > the first reader which drops the lock. So the process
> > woken up by wakeup(rwl) are going to find out the
> > lock is still occupied by remaining readers.
>
> wakeup() activates all sleepers. They should check and sleep again.
> Maybe a little bit of wasted resources, but I don't see a severe
> problem.
I came to the same conclusion.
>
> I did a little digging in history. In rev 1.3 of kern_rwlock.c
> we had case RW_READ: need_wait = RWLOCK_WRLOCK | RWLOCK_WRWANT;
> and the commet "Let writers through before obtaining read lock."
>
> The comment
> * RW_READ RWLOCK_WRLOCK|RWLOCK_WRWANT may not be set. We increment
> * with RWLOCK_READ_INCR. RWLOCK_WAIT while waiting.
> is still there, just the RWLOCK_WRWANT got lost from the condition.
>
> So I think we should get back the original reader writer priority.
>
> Regarding the race in rw_do_exit() that sashan@ found I also found
> a comment in rev 1.7.
>
> /*
> * Potential MP race here. If the owner had WRWANT set we cleared
> * it and a reader can sneak in before a writer. Do we care?
> */
>
> I do not want to change anything to that behavior now. There is
> no easy fix and I did not see the problem during testing. But we
> can put the comment back to clarify the situation.
>
> ok?
this certainly works for me.
OK sashan
