Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Gilles Chanteperdrix wrote:
>>> Jan Kiszka wrote:
>>>> Gilles Chanteperdrix wrote:
>>>>> Jan Kiszka wrote:
>>>>>> OTOH, we can safe some text size which is precious as well. So I'm
>>>>>> convinced to go your way (with a modification):
>>>>> My approach sucks: we get a silly atomic_cmpxchg if the mutex is already
>>>>> claimed, which is as least as much a common case as a currently
>>>>> unclaimed mutex. Need to think a bit. But I think a good solution is to
>>>>> re-read only if the mutex has been seen as already claimed.
>>>> That makes no difference as then we will go through the cmpxchg path
>>>> anyway.
>>>>
>>>> There is _no_ way around re-reading under nklock, all we can avoid is
>>>> atomic cmpxchg in the case of >1 waiters. But that would come at the
>>>> price of more complexity for all waiter.
>>>>
>>>> However, let's find some solution. I bet things will look different
>>>> again when we start fiddling with a generic lock + the additional bit to
>>>> replace XNWAKEN.
>>> What I meant is that if the claimed bit is already set, we can avoid the
>>> cmpxchg altogether, which was the intent of the original code. So I propose
>>> the following version:
>>>
>>> if(test_claimed(owner))
>>> owner = xnarch_atomic_intptr_get(mutex->owner);
>> This version lacks a test for owner == 0 here, otherwise you re-create
>> my old bug that bite me today.
>
> Ok, so jumping in the middle of the loop is the best thing to do then.
>
>>> while(!test_claimed(owner)) {
>>> old = xnarch_atomic_intptr_cmpxchg(mutex->owner,
>>> owner, set_claimed(owner,
>>> 1));
>>> if (likely(old == owner))
>>> break;
>>> if (old == NULL) {
>>> /* Owner called fast mutex_unlock
>>> (on another cpu) */
>>> xnlock_put_irqrestore(&nklock, s);
>>> goto retry_lock;
>>> }
>>> owner = old;
>>> }
>>>
>>> The compiler rearranges things correctly (at least on ARM), and avoids the
>>> redundant test.
>>>
>> My latest concern remains: Is all this additional complexity, are all
>> these conditional branches and the text size increase worth the effort?
>> How much cycles or micoseconds would be gain on the most suffering
>> architecture?
>
> Since the else and the while are folded together and the loop becomes
> post-tested, and with the help of conditional instructions, I do not
> think there is much more conditional branch (we need to to jump over
> the while loop anyway). However avoiding the cmpxchg is a real gain,
> since it is implemented as irq save/irq restore on an old arm.
>
> if(test_claimed(owner)) {
> old = xnarch_atomic_intptr_get(mutex->owner);
> goto test_old;
> }
> while(!test_claimed(owner)) {
> old = xnarch_atomic_intptr_cmpxchg(mutex->owner,
> owner, set_claimed(owner,
> 1));
> if (likely(old == owner))
> break;
> test_old:
> if (old == NULL) {
> /* Owner called fast mutex_unlock
> (on another cpu) */
> xnlock_put_irqrestore(&nklock, s);
> goto retry_lock;
> }
> owner = old;
> }
Should work now, but the beautifulness of the code suffers a bit...
What about making the expected compiler optimizations explicit?
if (test_claimed(owner)) {
old = xnarch_atomic_intptr_get(mutex->owner);
goto test_no_owner;
}
do {
old = xnarch_atomic_intptr_cmpxchg(mutex->owner, owner,
set_claimed(owner, 1));
if (likely(old == owner))
break;
test_no_owner:
if (old == NULL) {
/* Owner called fast mutex_unlock
(on another cpu) */
xnlock_put_irqrestore(&nklock, s);
goto retry_lock;
}
owner = old;
} while (!test_claimed(owner));
Jan
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