On Mon, May 27, 2013 at 12:52:00PM +0200, Maarten Lankhorst wrote:
> The reason ttm needed it was because there was another lock that interacted
> with the ctx lock in a weird way. The ww lock it was using was inverted with
> another
> lock, so it had to grab that lock first, perform a trylock on the ww lock,
> and if that failed
> unlock the lock, wait for it to be unlocked, then retry the same thing again.
> I'm so glad I managed to fix that mess, if you really need ww_mutex_trylock
> with a ctx,
> it's an indication your locking is wrong.
>
> For ww_mutex_trylock with a context to be of any use you would also need to
> return
> 0 or a -errno, (-EDEADLK, -EBUSY (already locked by someone else), or
> -EALREADY).
> This would make the trylock very different from other trylocks, and very
> confusing because
> if (ww_mutex_trylock(lock, ctx)) would not do what you would think it would
> do.
Yuck ;-)
Anyway, what I was thinking of is something like:
T0 T1
try A
lock B
lock B
lock A
Now, if for some reason T1 won the lottery such that T0 would have to be
wounded, T0's context would indicate its the first entry and not return
-EDEADLK.
OTOH, anybody doing creative things like that might well deserve
whatever they get ;-)
> > The thing is; if there could exist something like:
> >
> > ww_mutex_trylock(struct ww_mutex *, struct ww_acquire_ctx *ctx);
> >
> > Then we should not now take away that name and make it mean something
> > else; namely: ww_mutex_trylock_single().
> >
> > Unless we want to allow .ctx=NULL to mean _single.
> >
> > As to why I proposed that (.ctx=NULL meaning _single); I suppose because
> > I'm a minimalist at heart.
> Minimalism isn't bad, it's just knowing when to sto
:-)
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