Re: [PATCHv3 6/6] tty/ldsem: Decrement wait_readers on timeouted down_read()

Dmitry Safonov
2018-09-11T13:33:28Z

On Tue, 2018-09-11 at 14:01 +0100, Dmitry Safonov wrote:
> On Tue, 2018-09-11 at 14:02 +0200, Peter Zijlstra wrote:
> > On Tue, Sep 11, 2018 at 02:48:21AM +0100, Dmitry Safonov wrote:
> > > It seems like when ldsem_down_read() fails with timeout, it
> > > misses
> > > update for sem->wait_readers. By that reason, when writer finally
> > > releases write end of the semaphore __ldsem_wake_readers() does
> > > adjust
> > > sem->count with wrong value:
> > > sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS)
> > >
> > > I.e, if update comes with 1 missed wait_readers decrement, sem-
> > > > count
> > >
> > > will be 0x100000001 which means that there is active reader and
> > > it'll
> > > make any further writer to fail in acquiring the semaphore.
> > >
> > > It looks like, this is a dead-code, because ldsem_down_read() is
> > > never
> > > called with timeout different than MAX_SCHEDULE_TIMEOUT, so it
> > > might be
> > > worth to delete timeout parameter and error path fall-back..
> >
> > You might want to think about ditching that ldsem thing entirely,
> > and
> > use a regular rwsem ?
>
> Yeah, but AFAICS, regular rwsem will need to have a timeout then (for
> write). So, I thought fixing this pile would be simpler than adding
> timeout and probably writer-priority to generic rwsem?
>
> And I guess, we still will need fixes for stable for the bugs here..
>
> I expect that timeouts are ABI, while the gain of adding priority may
> be measured. I'll give it a shot (adding timeout/priority for linux-
> next) to rwsem if you say it's acceptable.

Actually, priority looks quite simple: we can add writers in the head
of wait_list and it probably may work.
Timeout looks also not a rocket science.
So, I can try to do that if you say it's acceptable (with the gain
measures).

After this can of worms that I need to fix regardless.

--
Thanks,
Dmitry