This is the mail archive of the
ecos-discuss@sources.redhat.com
mailing list for the eCos project.
Re: pthread_mutex_unlock when mutex is not locked
Nick Garnett wrote:
RubÃn PÃrez de Aranda Alonso <rperez@sidsa.es> writes:
Ok,
I understand you explain me. The standard also defines that a mutex
locked by a thread must be unlocked by the same thread. However,
many implementations of Pthreads allow use the mutex to signal events
carring out lock and unlock of the mutex from different threads.
Such implementation are broken in that case. It is not even clear how
such an implementation could handle priority inheritance correctly.
But, ok, the DSR is not a thread :-).
An example is the next code, that works with the POSIX compat layer
of eCOS.
N threads are created and executed in differents priority
levels each one of them in SCHED_FIFO policy. The thread th(K)
takes the priority equal to K and locks the pre-locked mutex
m((K-1)%N).
The th(K) also unlocks the mutex m(K) where the th(K+1) is
waiting. So, because the Prio(th(K+1)) > Prio(th(K)) then the
th(K+1) is dispatched at the same time that the th(K) unlocks
the mutex m(K).
Sorry, but this program totally violates the intended and documented
way in which mutexes should be used.
I know it. In my firmware design I don't use things like this. It only
is an
example that works fine in my platform.
Anothe cuestion. Can I use the next code to guaratee the mutual
exclusion in access to critical data from a thread and a DSR?
DSR ()
{
if (pthread_mutex_trylock(&mutex) == 0)
{
// Access to data
// Calculate .....
// Unlock
pthread_mutex_unlock(&mutex);
}
}
That is not guaranteed to work. Posix threads have additional data
attached to them, which is absent in non-Posix threads. Any
Posix functions called in a DSR may try to access this extra data. If
it is not a Posix thread then the access will fail, or attempt to
access invalid memory.
ISRs and DSRs are not part of the Posix secification, any attempt to
use Posix calls in these contexts in undefined. Instead you should be
using the eCos API, specifically the cyg_drv_* routines defined in the
driver API.
Ok, Posix doesn't define any thing about interrupts behaviour and handling,
but I prefer use Posix API to make easy the port of code in the future,
from eCOS
to other RTOS. This scheme of trylock-unlock in DSRs works fine in my
firmware, although I will think about you explain me to find another
solution
based on Posix API but using a little part of eCOS API to allow the mutual
exclusion between the threads and the DSRs.
Than you
RubÃn
--
______________________________________________________________
RubÃn Israel PÃrez de Aranda Alonso
SIDSA - Semiconductores InvestigaciÃn y DiseÃo S.A.
Parque TecnolÃgico de Madrid Phone : +34 91 803 5052
C/ Torres Quevedo, n 1 Fax: +34 91 803 9557
28760 TRES CANTOS (Madrid) (SPAIN)
e-mail: rperez@sidsa.com www.sidsa.com
______________________________________________________________
--
Before posting, please read the FAQ: http://ecos.sourceware.org/fom/ecos
and search the list archive: http://ecos.sourceware.org/ml/ecos-discuss