Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function v3

[Christian KÃnig]

Am 21.02.2018 um 11:54 schrieb Maarten Lankhorst:
> Op 21-02-18 om 00:56 schreef Daniel Vetter:
> > On Tue, Feb 20, 2018 at 04:21:58PM +0100, Peter Zijlstra wrote:
> > > On Tue, Feb 20, 2018 at 04:05:49PM +0100, Christian KÃnig wrote:
> > > > Am 20.02.2018 um 15:54 schrieb Peter Zijlstra:
> > > > > On Tue, Feb 20, 2018 at 03:34:07PM +0100, Christian KÃnig wrote:
> > > > > > > OK, but neither case would in fact need the !ctx case right? That's just
> > > > > > > there for completeness sake?
> > > > > > Unfortunately not. TTM uses trylock to lock BOs which are about to be
> > > > > > evicted to make room for all the BOs locked with a ctx.
> > > > > >
> > > > > > I need to be able to distinct between the BOs which are trylocked and those
> > > > > > which are locked with a ctx.
> > > > > >
> > > > > > Writing this I actually noticed the current version is buggy, cause even
> > > > > > when we check the mutex owner we still need to make sure that the ctx in the
> > > > > > lock is NULL.
> > > > > Hurm... I can't remember why trylocks behave like that, and it seems
> > > > > rather unfortunate / inconsistent.
> > > > Actually for me that is rather fortunate, cause I need to distinct between
> > > > the locks acquired through trylock and lock.
> > > I suppose that would always be possible using:
> > > ww_mutex_trylock(.ctx=NULL), and it could be that there simply weren't
> > > any immediate uses for a !NULL trylock and it was thus not implemented.
> > >
> > > But that is all very long ago..
> > I think we simple never had a use-case for interleaving ww_mutex_lock(ctx)
> > and ww_mutex_trylock(ctx). Nesting multiple trylocks in ctx-locks happens
> > plenty, but not further:
> >
> > The common use-case for that is locking a bunch of buffers you need (for
> > command submission or whatever), and then trylocking other buffers to make
> > space for the buffers you need to move into VRAM. I guess if only
> > trylocking buffers doesn't succeed in freeing up enough VRAM then we could
> > go into blocking ww_mutex_locks which need the ctx (and which would need
> > all the trylock-acquired buffers to be annotated with the ctx too). TTM
> > currently tries to be far enough away from that corner case (using a
> > defensive "never use more than 50% of all memory for gfx" approach) that
> > it doesn't seem to need that.
> >
> > Once we get there it should indeed be simply to add a ctx parameter to
> > ww_mutex_trylock to fix this case. The TTM side rework is definitely going
> > to be the much bigger issue here ...
> > -Daniel
> Yes, I think fixing trylock to take a ctx parameter would be a better fix than ww_mutex_is_owned_by..

Yeah, but as I noted now multiple times that won't work.

See I need to distinct between the BOs acquired with and without a 
context. Otherwise the whole approach doesn't make much sense.

Christian.