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

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

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();"

~Maarten