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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 27, 2013 at 10:26:39AM +0200, Maarten Lankhorst wrote:
> Op 27-05-13 10:00, Peter Zijlstra schreef:
> > On Wed, May 22, 2013 at 07:24:38PM +0200, Maarten Lankhorst wrote:
> >>>> +- Functions to only acquire a single w/w mutex, which results in the exact same
> >>>> +  semantics as a normal mutex. These functions have the _single postfix.
> >>> This is missing rationale.
> >> trylock_single is useful when iterating over a list, and you want to evict a bo, but only the first one that can be acquired.
> >> lock_single is useful when only a single bo needs to be acquired, for example to lock a buffer during mmap.
> > OK, so given that its still early, monday and I haven't actually spend
> > much time thinking on this; would it be possible to make:
> > ww_mutex_lock(.ctx=NULL) act like ww_mutex_lock_single()?
> >
> > The idea is that if we don't provide a ctx, we'll get a different
> > lockdep annotation; mutex_lock() vs mutex_lock_nest_lock(). So if we
> > then go and make a mistake, lockdep should warn us.
> >
> > Would that work or should I stock up on morning juice?
> >
> It's easy to merge unlock_single and unlock, which I did in the next version I'll post.
> Lockdep will already warn if ww_mutex_lock and ww_mutex_lock_single are both
> used. ww_test_block_context and ww_test_context_block in lib/locking-selftest.c
> are the testcases for this.
> 
> The locking paths are too different, it will end up with doing "if (ctx == NULL) mutex_lock(); else ww_mutex_lock();"

I was more thinking like:

int __sched ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
	might_sleep();
	return __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE, 0,
				   ctx ? ctx->dep_map : NULL, _RET_IP_,
				   ctx, 0);
}

That should make ww_mutex_lock(.ctx=NULL) equivalent to
mutex_lock(&lock->base), no?

Anyway, implementation aside, it would again reduce the interface some.