SoC: help with LISTs and killing procs

[Brooks Davis]

On Thu, Aug 10, 2006 at 05:35:43PM +0200, Divacky Roman wrote:
> On Thu, Aug 10, 2006 at 10:23:59AM -0500, Brooks Davis wrote:
> > On Thu, Aug 10, 2006 at 05:16:17PM +0200, Divacky Roman wrote:
> > > hi
> > > 
> > > I am doing this:
> > > 
> > > (pseudocode)
> > > LIST_FOREACH_SAFE(em, &td_em->shared->threads, threads, tmp_em) {
> > > 
> > > 	kill(em, SIGKILL);
> > > }
> > > 
> > > kill(SIGKILL) calls exit() which calls my exit_hook()
> > > 
> > > my exit_hook() does LIST_REMOVE(em, threads).
> > > 
> > > the problem is that this is not synchronous so I am getting a panic by INVARIANTS
> > > that "Bad link elm prev->next != elm". This is because I list 1st item in the list
> > > I call kill on it, then process 2nd list, then scheduler preempts my code and calls
> > > exit() on the first proc which removes the first entry and bad things happen. 
> > > 
> > > I see this possible solutions:
> > > 
> > > make this synchronous, it can be done by something like:
> > > 
> > >     ....
> > >     kill(em, SIGKILL);
> > >     wait_for_proc_to_vanish();
> > > 
> > > pls. tell me what do you think about this solution and if its correct what is the wait_for_proc_to_vanish()
> > > 
> > > maybe there's some better solution, pls tell me.
> > 
> > It sounds like you need a lock protecting the list.  If you held it over
> > the whole loop you could signal all processes before the exit_hook could
> > remove any.
> 
> I dont understand. I am protecting the lock by a rw_rlock();
> 
> the exit_hook() then acquires rw_wlock(); when removing the entry.
> what exactly do you suggest me to do? I dont get it.

This can't be the case.  If you're holding a read lock around the
loop (it must cover the entire loop), it should not be possible for the
exit_hook() to obtain a write lock while you are in the loop.  Just to
verify, is the lock for the list and not per element?

-- Brooks
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