Re: [PATCH v3 2/3] mutex: add support for wound/wait style locks, v3

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

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