On Wed, 2007-05-09 at 11:57 +0200, Markus Osterried wrote: > Hello Gerum, > > actually, when the task's user function (the function which is given in > t_start()) is entered, the arguments have been copied to the new context > and are passed by value. > This is true for real pSOS and also for Xenomai. > But in Xenomai it's the trampoline which acts as a wrapper/prolog and is > created and ready to run in a concurrent context, while still holding a > reference to the arguments in the creator's context. > Real pSOS is a blackbox, so I don't no whether there is a moment in time, > where a new concurrent context is running while still holding a pointer to > old context.
To explain how the pSOS skin currently works: starter task: == syscall == t_start(targs by reference) kernel/skin: receives the genuine "targ" _pointer_, and copies it unmodified to the new task's trampoline code in user-space + wakes up the new task so that it really starts execution (i.e. it was locked into the trampoline code). new task (in trampoline code): ==syscall== wait on the start barrier kernel/skin: makes caller wait until t_start is issued receives "targs" pointer verbatim, copied back by the last syscall, and dereferences it to pass the 4 long words from the targs array to the callback entry routine of the new task. So, the issue that could happen, is the caller of the t_start() function in user-space returning to its own caller, with "targs" being laid into the stack space being unwound. _That_ would be a rainy day. The only issue otherwise, is the caller of t_start() modifying the arguments before or while the resumed task accesses them, but that would be more of a reasonable side-effect of passing arguments by reference, that a bug per se. Except of course, if pSOS does differently, and always copies the argument array in the t_start() syscall. > But it's the fact, we have sometimes seen problems with Xenomai, when our > pSOS application changes the arguments value after t_start(). > We have never seen this in pSOS. > It's worth to mention, I haven't seen this problem with the newest Xenomai > branch (there was some changes in scheduling?). > But I'm not sure the problem is 100% solved, because with older Xenomai we > have sometimes seen the problem and sometimes not. :-( Please be specific; which are the "older Xenomai" and "newest Xenomai branch"? i.e. which stable version number or SVN commit number for the trunk/? This said, I definitely fixed a scheduling issue due to primary/secondary mode switches involved in creating new user-space tasks which has been committed in v2.3.1 (i.e. a major rework of the so-called "RPI management", after Thomas sent me a test case to illustrate the issue). This bug, for sure, could have impacted your application in the situation I described earlier, regarding t_start arguments laid into the stack space. But I have not enough details to be 100% this was related to your problem though. > > > Markus > > > > > > > > Philippe Gerum > > <[EMAIL PROTECTED] An: Markus > Osterried <[EMAIL PROTECTED]> > g> Kopie: > firstname.lastname@example.org > > Gesendet von: Blindkopie: > > xenomai-core-bo Thema: Re: > [Xenomai-core] Antwort: Re: Arguments in psos t_start() > [EMAIL PROTECTED] > > > > > > 09.05.2007 > > 09:55 > > Bitte antworten > > an rpm > > > > > > > > > > On Wed, 2007-05-09 at 09:07 +0200, Markus Osterried wrote: > > Hello Gerum, > > > > sorry for not knowing the details. But there is another question. > > Is it okay to move the pointer to the arguments to the newly created > task? > > Isn't it possible that the task which created the new task is > rescheduled? > > The old task could change the value of the argument and because the new > > task only holds a reference to this argument, the new task is also > > affected. > > > > Yes, such side-effect would be possible, not necessarily for the > creator, but for the thread calling t_start() though. The worst case > would happen when calling t_start() with args laid into the caller's > stack space, and the caller unwinding the current frame immediately > since it has a higher priority than the started thread. > > This boils down to one question: does actually pSOS pass the argument > array by reference, or by value? In the latter case, the current > implementation would be wrong, otherwise, the side-effect would be > admitted. > > -- > Philippe. > > > > _______________________________________________ > Xenomai-core mailing list > Xenomaiemail@example.com > https://mail.gna.org/listinfo/xenomai-core > > > > > > -- Philippe. _______________________________________________ Xenomai-core mailing list Xenomaifirstname.lastname@example.org https://mail.gna.org/listinfo/xenomai-core