Re: [Patch v4] rwsem: fix rwsem_is_locked() bugs

[Amerigo Wang]

David Howells wrote:
> Amerigo Wang <amwang@xxxxxxxxxx> wrote:
>
> > rwsem_is_locked() tests ->activity without locks, so we should always
> > keep ->activity consistent. However, the code in __rwsem_do_wake()
> > breaks this rule, it updates ->activity after _all_ readers waken up,
> > this may give some reader a wrong ->activity value, thus cause
> > rwsem_is_locked() behaves wrong.
> >
> > Quote from Andrew:
> >
> > "
> > - we have one or more processes sleeping in down_read(), waiting for access.
> >
> > - we wake one or more processes up without altering ->activity
> >
> > - they start to run and they do rwsem_is_locked().  This incorrectly
> >   returns "false", because the waker process is still crunching away in
> >   __rwsem_do_wake().
> >
> > - the waker now alters ->activity, but it was too late.
> > "
> >
> > So we need get a spinlock to protect this. And rwsem_is_locked()
> > should not block, thus we use spin_trylock.
> >
> > Reported-by: Brian Behlendorf <behlendorf1@xxxxxxxx>
> > Cc: Ben Woodard <bwoodard@xxxxxxxx>
> > Cc: David Howells <dhowells@xxxxxxxxxx>
> > Signed-off-by: WANG Cong <amwang@xxxxxxxxxx>
>
> I'd say the comment in __rwsem_do_wake() is unnecessary, but other than
> that...


The reason why I added it is to show that we have considered that
case already. :) If you have strong opinions to remove it, I can
update the patch.

> Acked-by: David Howells <dhowells@xxxxxxxxxx>


Thanks!
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