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

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

On 06/23/2011 11:09 AM, Jan Kiszka wrote:
> On 2011-06-23 11:05, Gilles Chanteperdrix wrote:
>> 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.
> 
> Problematic is the dereferencing of the stale buffer pointer obtained
> during the last cmpxchg or via buf_pool_get. This happens before the new
> cmpxchg.

Ok. Got it, that would be a problem only if the stale pointer pointed to
an unmapped area, but ok, better avoid freeing the buffers. I guess it
would not be a problem as applications tend to have a fixed number of
threads anyway.

-- 
                                                                Gilles.