Message-ID: <51A34250.6040601@canonical.com>
Date: Mon, 27 May 2013 13:24:00 +0200
From: Maarten Lankhorst <maarten.lankhorst@canonical.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org,
        dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
        robclark@gmail.com, rostedt@goodmis.org, tglx@linutronix.de,
        mingo@elte.hu, linux-media@vger.kernel.org,
        Dave Airlie <airlied@redhat.com>
Subject: Re: [PATCH v3 2/3] mutex: add support for wound/wait style locks,
 v3
References: <20130428165914.17075.57751.stgit@patser> <20130428170407.17075.80082.stgit@patser> <20130430191422.GA5763@phenom.ffwll.local> <519CA976.9000109@canonical.com> <20130522161831.GQ18810@twins.programming.kicks-ass.net> <519CFF56.90600@canonical.com> <20130527082149.GE2781@laptop> <51A32F0E.9000206@canonical.com> <20130527102457.GA4341@laptop> <51A33AD0.4030406@canonical.com> <20130527111557.GB4341@laptop>
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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
> :-)
>

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