Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()

Peter Zijlstra
2023-09-12T14:23:14Z

On Tue, Sep 12, 2023 at 02:58:32PM +0100, Matthew Wilcox wrote:
> On Tue, Sep 12, 2023 at 03:52:13PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 12, 2023 at 01:28:13PM +0100, Matthew Wilcox wrote:
> > > On Tue, Sep 12, 2023 at 11:03:42AM +0200, Peter Zijlstra wrote:
> > > > If not, then sure we can do this; it's not like I managed to get rid of
> > > > muteX_is_locked() -- and I actually tried at some point :/
> > > >
> > > > And just now I grepped for it, and look what I find:
> > > >
> > > > drivers/hid/hid-nintendo.c: if (unlikely(mutex_is_locked(&ctlr->output_mutex))) {
> > > > drivers/nvdimm/btt.c: if (mutex_is_locked(&arena->err_lock)
> > > >
> > > > And there's more :-(
> > >
> > > Are these actually abuse? I looked at these two, and they both seem to
> > > be asking "Does somebody else currently have this mutex?" rather than
> > > "Do I have this mutex?".
> >
> > It's effectively a random number generator in that capacity. Someone
> > might have it or might have had it when you looked and no longer have
> > it, or might have it now but not when you asked.
>
> Well, no.
>
> if (mutex_is_locked(&arena->err_lock)
> || arena->freelist[lane].has_err) {
> nd_region_release_lane(btt->nd_region, lane);
>
> ret = arena_clear_freelist_error(arena, lane);
>
> So that's "Is somebody currently processing an error, or have they
> already finished setting an error". Sure, it's somewhat racy, but
> it looks like a performance optimisation, not something that needs
> 100% accuracy.

We're arguing past one another I think. Yes mutex_is_locked() is a
random number generator when asked for something you don't own. But it
might not be a bug because the code is ok with races.

It is still fully dodgy IMO, such usage is pretty close to UB.