Op 27-05-13 11:13, Peter Zijlstra schreef:
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.
It doesn't work like that. __builtin_constant_p(ctx == NULL) will evaluate to false in __mutex_lock_common, even if you call ww_mutex_lock(lock, NULL); gcc cannot prove at compile time whether ctx == NULL is true or false for the __mutex_lock_common inlining here, so __builtin_constant_p() will return false.
And again, that's just saying
ww_mutex_lock() { if (ctx) original ww_mutex_lock's slowpath(lock, ctx); else mutex_lock's slowpath(lock->base); }
And the next version will already remove unlock_single, and this is the implementation for lock_single currently: static inline void ww_mutex_lock_single(struct ww_mutex *lock) { mutex_lock(&lock->base); }
So why do you want to merge it?
~Maarten