Op 28-06-13 03:32, Davidlohr Bueso schreef:
> On Thu, 2013-06-27 at 11:00 +0200, Ingo Molnar wrote:
> [...]
>> So I tried this out yesterday, but it interacted with the Wait/Wound
>> patches in tip:core/mutexes.
>>
>> Maarten Lankhorst pointed out that if this patch is applied on top of the
>> WW patches as-is, then we get this semantic merge conflict:
>>
>>>> @@ -340,6 +339,14 @@ slowpath:
>>>> #endif
>>>> spin_lock_mutex(&lock->wait_lock, flags);
>>>>
>>>> + /* once more, can we acquire the lock? */
>>>> + if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1))
>>>> {
>>>> + lock_acquired(&lock->dep_map, ip);
>>>> + mutex_set_owner(lock);
>>>> + spin_unlock_mutex(&lock->wait_lock, flags);
>>>> + goto done;
>>>> + }
>>>>
>>> ^^^^^^^^^^^^^^
>>>
>>> This part skips the whole if (!__builtin_constant_p(ww_ctx == NULL)) {
>>> section with the wait_lock held.
> I see what you mean, I hadn't really looked at the W/W patches. BTW
> those __builtin_constant_p() calls are pretty ugly/annoying to read,
> plus why the negation of the NULL check? Couldn't we just do something
> like:
It's to kill overhead.. ww_ctx == NULL is a constant only when the function is
called with null as explicit parameter.
So !__builtin_constant_p(ww_ctx == NULL) means that the function was called
with a variable ww_ctx.
> #define is_ww_ctx(x) (__builtin_constant_p(x))
> ...
> if (is_ww_ctxt(ww_ctx)) { ... }
>
>
> Anyway, so going back to the actual patch, we need a few cleanups in
> __mutex_lock_common() before we can rebase this patch - otherwise we're
> going to end up duplicating a lot of code (and the function is already
> big enough):
>
> How about a new ww_mutex_set_context_slowpath() function that does the
> w/w lock acquiring and wakes up any sleeping processes. We'd use this
> function whenever we acquire the lock in the slowpath (with the
> ->wait_lock taken):
>
> static __always_inline void
> ww_mutex_set_context_slowpath(struct mutex *lock,
> struct ww_acquire_ctx *ww_ctx, bool debug)
> {
> if (!__builtin_constant_p(ww_ctx == NULL)) {
> struct mutex_waiter *cur;
> struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
>
> /*
> * This branch gets optimized out for the common case,
> * and is only important for ww_mutex_lock.
> */
> ww_mutex_lock_acquired(ww, ww_ctx);
> ww->ctx = ww_ctx;
>
> /*
> * Give any possible sleeping processes the chance to wake up,
> * so they can recheck if they have to back off.
> */
> list_for_each_entry(cur, &lock->wait_list, list) {
> if (debug)
> debug_mutex_wake_waiter(lock, cur);
> wake_up_process(cur->task);
> }
> }
> }
>
> In ww_mutex_set_context_fastpath() I'm a little confused with the
> debug_mutex_wake_waiter() calls since we don't deal with debug in the
> fast path (->wait_lock isn't held). So are these calls
> correct/necessary?
Well spotted, but in that case the !debug case mutex_wake_waiter gets optimized
out anyway,
so please don't add a conditional like that.
> For ww_mutex_set_context_slowpath(), the 'debug' parameter would be
> necessary since with this patch we avoid doing the debug_mutex on a
> quick attempt to grab the lock, otherwise we do the slowpath debug,
> waiters, etc. For instance:
>
> ...
> slowpath:
> #endif
> spin_lock_mutex(&lock->wait_lock, flags);
> /* once more, can we acquire the lock? */
> if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) {
> lock_acquired(&lock->dep_map, ip);
> mutex_set_owner(lock);
> ww_mutex_set_context_fastpath(lock, ww_ctx, false);
> spin_unlock_mutex(&lock->wait_lock, flags);
> goto done;
> }
> ...
>
> lock_acquired(&lock->dep_map, ip);
> /* got the lock - rejoice! */
> mutex_remove_waiter(lock, &waiter, current_thread_info());
> mutex_set_owner(lock);
> ww_mutex_set_context_slowpath(lock, ww_ctx, true);
> ...
I used the power of goto's in my own fixed up version below, and reshuffled
some calls a bit.
Maybe you could verify if it's correct, and if it is use it as base?
8<---------
diff --git a/kernel/mutex.c b/kernel/mutex.c
index e581ada..f93be1d 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -486,8 +486,7 @@ __mutex_lock_common(struct mutex *lock, long state,
unsigned int subclass,
mutex_set_owner(lock);
mspin_unlock(MLOCK(lock), &node);
- preempt_enable();
- return 0;
+ goto done;
}
mspin_unlock(MLOCK(lock), &node);
@@ -512,6 +511,10 @@ slowpath:
#endif
spin_lock_mutex(&lock->wait_lock, flags);
+ /* once more, can we acquire the lock? */
+ if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1))
+ goto skip_wait;
+
debug_mutex_lock_common(lock, &waiter);
debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
@@ -519,9 +522,6 @@ slowpath:
list_add_tail(&waiter.list, &lock->wait_list);
waiter.task = task;
- if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
- goto done;
-
lock_contended(&lock->dep_map, ip);
for (;;) {
@@ -535,7 +535,7 @@ slowpath:
* other waiters:
*/
if (MUTEX_SHOW_NO_WAITER(lock) &&
- (atomic_xchg(&lock->count, -1) == 1))
+ (atomic_xchg(&lock->count, -1) == 1))
break;
/*
@@ -560,11 +560,15 @@ slowpath:
schedule_preempt_disabled();
spin_lock_mutex(&lock->wait_lock, flags);
}
+ mutex_remove_waiter(lock, &waiter, current_thread_info());
+ /* set it to 0 if there are no waiters left: */
+ if (likely(list_empty(&lock->wait_list)))
+ atomic_set(&lock->count, 0);
+ debug_mutex_free_waiter(&waiter);
-done:
+skip_wait:
+ /* got the lock - cleanup and rejoice! */
lock_acquired(&lock->dep_map, ip);
- /* got the lock - rejoice! */
- mutex_remove_waiter(lock, &waiter, current_thread_info());
mutex_set_owner(lock);
if (!__builtin_constant_p(ww_ctx == NULL)) {
@@ -591,15 +595,9 @@ done:
}
}
- /* set it to 0 if there are no waiters left: */
- if (likely(list_empty(&lock->wait_list)))
- atomic_set(&lock->count, 0);
-
spin_unlock_mutex(&lock->wait_lock, flags);
-
- debug_mutex_free_waiter(&waiter);
+done:
preempt_enable();
-
return 0;
err:
--
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