Jerome Souquieres wrote:
Hi,
I think I've spotted a bug in pthread_once (in ecos v3.0). The current
implementation does not enforce the part of the specification that
says: "On return from pthread_once(), it is guaranteed that
init_routine() has completed".
Here is a proposal for a patch, I did not post it to ecos-patches
because I currently don't have a working eCos target to test this.
[Patch at end]
Thanks. I agree there's a problem here, but the fix isn't right - you
can't leave pthread_mutex locked while running an arbitrary user routine.
You'll get a potential deadlock, or at least incorrect operation of the
pthread layer if anything uses pthread functions before the user function
terminates (which could be never).
The real solution probably involves linking a condition variable to
pthread_mutex and having an additional state for once_control to indicate
"once function being run but not yet complete". Any thread which sees the
once_control in that state will have to wait on the condvar (which is then
broadcast on when the once function returns).
Would you be able to make a fix along those lines? Or failing that if you
submit it to http://bugs.ecos.sourceware.org/ it won't get lost at least.
Jifl
--- pthread.cxx 2009-01-29 18:47:52.000000000 +0100
+++ pthread-fixonce.cxx 2009-11-01 21:07:42.639875000 +0100
@@ -1328,22 +1328,23 @@ externC int pthread_once (pthread_once_t
PTHREAD_ENTRY();
PTHREAD_CHECK( once_control );
PTHREAD_CHECK( init_routine );
- pthread_once_t old;
-
- // Do a test and set on the once_control object.
pthread_mutex.lock();
- old = *once_control;
- *once_control = 1;
+ // If the once_control is zero, call the init_routine().
+ // the mutex must stay locked during init_routine because
+ // concurrent threads must be blocked until init_routine has
+ // done its business
+ if ( !*once_control )
+ {
+ init_routine();
+ *once_control = 1;
+ }
pthread_mutex.unlock();
-
- // If the once_control was zero, call the init_routine().
- if( !old ) init_routine();
PTHREAD_RETURN(0);
}
--
--["No sense being pessimistic, it wouldn't work anyway"]-- Opinions==mine
