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
:-)