Op 27-05-13 12:24, Peter Zijlstra schreef: > On Mon, May 27, 2013 at 12:01:50PM +0200, Maarten Lankhorst wrote: >>> Again, early.. monday.. would a trylock, even if successful still need >>> the ctx? >> No ctx for trylock is supported. You can still do a trylock while >> holding a context, but the mutex won't be a part of the context. >> Normal lockdep rules apply. lib/locking-selftest.c: >> >> context + ww_mutex_lock first, then a trylock: >> dotest(ww_test_context_try, SUCCESS, LOCKTYPE_WW); >> >> trylock first, then context + ww_mutex_lock: >> dotest(ww_test_try_context, FAILURE, LOCKTYPE_WW); >> >> For now I don't want to add support for a trylock with context, I'm >> very glad I managed to fix ttm locking to not require this any more, >> and it was needed there only because it was a workaround for the >> locking being wrong. There was no annotation for the buffer locking >> it was using, so the real problem wasn't easy to spot. > Ah, ok. > > My question really was whether there even was sense for a trylock with > context. I couldn't come up with a case for it; but I think I see one > now. 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. > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
