Re: [Xenomai-core] RFC: use a pool of pre-allocated buffers in rt_printf

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 06/23/2011 09:38 AM, Jan Kiszka wrote:
>> +#ifdef CONFIG_XENO_FASTSYNCH
>> +	if (xeno_get_current() != XN_NO_HANDLE &&
>> +	    !(xeno_get_current_mode() & XNRELAX) && buf_pool_get()) {
>> +		struct print_buffer *old;
>> +
>> +		old = buf_pool_get();
>> +		while (old != buffer) {
>> +			buffer = old;
>> +			old = buf_pool_cmpxchg(buffer, buffer->pool_next);
> 
> Though unlikely, it's still possible: The buffer obtained in the last
> round may have been dequeued meanwhile and then freed (in case a third
> buffer was released to the pool, filling it up to pool_capacity).

I do not get it: it seems to me that if the current head (that is
buf_pool_get()) is freed, then the cmpxchg will fail, so we will loop
and try again.

>> @@ -346,12 +396,36 @@ static void cleanup_buffer(struct print_buffer *buffer)
>>  {
>>  	struct print_buffer *prev, *next;
>>  
>> +	assert_nrt();
>> +
>>  	pthread_setspecific(buffer_key, NULL);
>>  
>>  	pthread_mutex_lock(&buffer_lock);
>>  
>>  	print_buffers();
> 
> You also need to unhook the buffer from the active list before returning
> it to the pool.

We can not do that: we want that when rt_print_init takes a buffer from
the pool, it does not need to do any operation which would need to
switch to secondary mode. This includes getting the active list mutex.
So, the buffer in the pool are also in the active list, but they remain
empty, so, nothing will be printed.

> This looks strange:
> 
> 	buffer->pool_next = buffer;
> 	...
> 	buf_pool_cmpxchg(buffer->pool_next, buffer);
> 
> Don't you want
> 
> 	buffer->pool_next = old;
> 	buf_pool_cmpxchg(old, buffer);

Yes, search and replace error.

-- 
					    Gilles.