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SubjectRe: [patch 05/15] Generic Mutex Subsystem, mutex-core.patch

* Steven Rostedt <rostedt@goodmis.org> wrote:

> How expensive is the xchg? Since __mutex_lock_common is called even
> when it's going to wake up. Maybe it might be more efficient to add
> something like:
>
> if (atomic_cmpxchg(&lock->count, 1, 0) {
> debug_set_owner(lock, ti __IP__);
> debug_unlock_irqrestore(&debug_lock, *flags, ti);
> return 1;
> }
>
> This way we save the overhead of grabbing another spinlock, adding the
> task to the wait_list and changing it's state.

in the first pass we definitely need to add ourselves to the list first
- hence have to grab the lock. Even after the schedule(), we have to
xchg it to -1, not 0. This is crutial to 'not drop the ball' property of
one-waiter-in-flight logic - we must not lose the -1 'there are more
waiters pending' property. Plus, we have the grab the lock because we
remove ourselves from the wait-list after the schedule(). So i'm not
sure your suggested optimization is possible.

Ingo
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