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

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

Op 27-05-13 13:15, Peter Zijlstra schreef:
> 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.
And this sounds like something lockdep is designed to complain about.

Nothing stops you from doing try A then doing try B, which would be the correct way to deal with this situation.
Why would you trylock one, and then not do the same for another?

> OTOH, anybody doing creative things like that might well deserve
> whatever they get ;-)
Indeed!

>>> 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
> :-)
>