Re: [PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER

[Thomas Hellstrom]

On 10/03/2012 10:53 AM, Daniel Vetter wrote:
> On Wed, Oct 3, 2012 at 10:37 AM, Thomas Hellstrom <thellstrom@xxxxxxxxxx> wrote:
> > > > So if I understand you correctly, the reservation changes in TTM are
> > > > motivated by the
> > > > fact that otherwise, in the generic reservation code, lockdep can only be
> > > > annotated for a trylock and not a waiting lock, when it *is* in fact a
> > > > waiting lock.
> > > >
> > > > I'm completely unfamiliar with setting up lockdep annotations, but the
> > > > only
> > > > place a
> > > > deadlock might occur is if the trylock fails and we do a
> > > > wait_for_unreserve().
> > > > Isn't it possible to annotate the call to wait_for_unreserve() just like
> > > > an
> > > > interruptible waiting lock
> > > > (that is always interrupted, but at least any deadlock will be catched?).
> > > Hm, I have to admit that idea hasn't crossed my mind, but it's indeed
> > > a hole in our current reservation lockdep annotations - since we're
> > > blocking for the unreserve, other threads could potential block
> > > waiting on us to release a lock we're holding already, resulting in a
> > > deadlock.
> > >
> > > Since no other locking primitive that I know of has this
> > > wait_for_unlocked interface, I don't know how we could map this in
> > > lockdep. One idea is to grab the lock and release it again immediately
> > > (only in the annotations, not the real lock ofc). But I need to check
> > > the lockdep code to see whether that doesn't trip it up.
> >
> > I imagine doing the same as mutex_lock_interruptible() does in the
> > interrupted path should work...
> It simply calls the unlock lockdep annotation function if it breaks
> out. So doing a lock/unlock cycle in wait_unreserve should do what we
> want.
>
> And to properly annotate the ttm reserve paths we could just add an
> unconditional wait_unreserve call at the beginning like you suggested
> (maybe with #ifdef CONFIG_PROVE_LOCKING in case ppl freak out about
> the added atomic read in the uncontended case).
> -Daniel

I think atomic_read()s are cheap, at least on intel as IIRC they don't 
require bus locking,
still I think we should keep it within CONFIG_PROVE_LOCKING

which btw reminds me there's an optimization that can be done in that 
one should really only
call atomic_cmpxchg() if a preceding atomic_read() hints that it will 
succeed.

Now, does this mean TTM can keep the atomic reserve <-> lru list removal?

Thanks,
Thomas


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