Re: A problem regarding SMP

[Philippe Gerum <rpm@xenomai.org>]

Philippe Gerum <rpm@xenomai.org> writes:

> JiajunDu <wonderboy512@163.com> writes:
>
>> Thanks for your reply!
>>
>> By using a barrier to synchronize all evl kthreads, I did achieve the
>> effect of multiple processes running in parallel. My solution is
>> in the next patch, and the branch I tried on is the latest
>> `v5.13-evl-rebase`.
>>
>> But I still ran into two other problems while trying this solution.
>>
>> The first problem is that `evl_flag` cann't solve the problem. The wait
>> condition in `evl_wait_flag_timeout` is `evl_read_flag`, in which the
>> flag is set to false as soon as the flag is found to be raised in
>> `evl_raise_flag`, so `evl_raise_flag` can only wake up one waiting process
>> at a time. And it's not possible to wake up more processes by calling
>> `evl_raise_flag` multiple times, because it's not possible to determine
>> the time when processes are woken up in the SMP environment. If we did, it's
>> possible to have a situation where two processes are woken up at the same
>> time after the flag was raised twice, and then only one of them can be
>> woken up at the same time. So in light of this, I defined two new functions,
>> `evl_wait_flag_same` and `evl_wait_flag_timeout_same`, to read flags using
>> `evl_peek_flag`.
>
> The semantics you are looking for are available from evl_wait_queue,
> which basically implements a condition variable.

This said, I agree that the current implementation of evl_wait_flag()
prevents broadcast, which is wrong.

-- 
Philippe.