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SubjectRe: SMP syncronization on AMD processors (broken?)
Thanks a lot for the interesting idea provided below.
I will try to implement it.

Kirill

>>The whole story started when we wrote the following code:
>>
>>void XXX(void)
>>{
>> /* ints disabled */
>>restart:
>> spin_lock(&lock);
>> do_something();
>> if (!flag)
>> need_restart = 1;
>> spin_unlock(&lock);
>> if (need_restart)
>> goto restart; <<<< LOOPS 4EVER ON AMD!!!
>>}
>>
>>void YYY(void)
>>{
>> spin_lock(&lock); <<<< SPINS 4EVER ON AMD!!!
>> flag = 1;
>> spin_unlock(&lock);
>>}
>>
>>function XXX() starts on CPU0 and begins to loop since flag is not set,
>>then CPU1 calls function YYY() and it turns out that it can't take the
>>lock any arbitrary time.
>
>
> The right thing to do here is to wait for the flag to be set *outside*
> the lock, and then re-validate inside the lock:
>
> void XXX(void)
> {
> /* ints disabled */
> restart:
> spin_lock(&lock);
> do_something();
> if (!flag)
> need_restart = 1;
> spin_unlock(&lock);
> if (need_restart) {
> while (!flag)
> cpu_relax();
> goto restart;
> }
> }
>
> This way, XXX() keeps the lock dropped for as long as it takes for
> YYY() to notice and grab it.
>
>
> However, I realize that this is of course a simplified case of some real
> code, where even *finding* the flag requires the spin lock.
>
> The generic solution is to have a global "progress" counter, which
> records "I made progress toward setting flag", that XXX() can
> busy-loop on:
>
> int progress;
>
> void XXX(void)
> {
> int old_progress;
> /* ints disabled */
> restart:
> spin_lock(&lock);
> do_something();
> if (!flag) {
> old_progress = progress;
> need_restart = 1;
> }
> spin_unlock(&lock);
> if (need_restart) {
> while (progress == old_progress)
> cpu_relax();
> goto restart;
> }
> }
>
> void YYY(void)
> {
> spin_lock(&lock);
> flag = 1;
> progress++;
> spin_unlock(&lock);
> }
>
> It may be that in your data structure, there is one or a series of
> fields that already exist that you can use for the purpose. The goal
> is to merely detect *change*, so you can reacquire the lock and test
> definitively. It's okay to read freed memory while doing this, as long as
> you can be sure that:
> - The memory read won't oops the kernel, and
> - You don't end up depending on the value of the freed memory to
> get you out of the stall.
>


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