Date: Thu, 10 Aug 2006 13:31:50 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Divacky Roman <xdivac02@stud.fit.vutbr.cz>, hackers@freebsd.org Subject: Re: SoC: help with LISTs and killing procs Message-ID: <200608101331.51473.jhb@freebsd.org> In-Reply-To: <20060810161705.GB19047@stud.fit.vutbr.cz> References: <20060810151616.GA17109@stud.fit.vutbr.cz> <20060810154305.GA21483@lor.one-eyed-alien.net> <20060810161705.GB19047@stud.fit.vutbr.cz>
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On Thursday 10 August 2006 12:17, Divacky Roman wrote:
> On Thu, Aug 10, 2006 at 10:43:05AM -0500, Brooks Davis wrote:
> > 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?
>
> oh.. I see whats going on.. in the exit_hook I am doing this:
>
>
> em = em_find(p->p_pid, EMUL_UNLOCKED); // this performs
EMUL_RLOCK(&emul_lock);
> ...
> EMUL_RUNLOCK(&emul_lock);
>
> EMUL_WLOCK(&emul_lock);
> LIST_REMOVE(em, threads);
> SLIST_REMOVE(&emuldata_head, em, linux_emuldata, emuldatas);
> EMUL_WUNLOCK(&emul_lock);
>
> the EMUL_RUNLOCK() unlocks it so it doesnt wait. This should be turned into
rw_try_upgrade()
> but I dont understand how ;(
You could make em_find() take an argument to specify if it should acquire
a WLOCK instead of an RLOCK. Really, unless you have measured a lot of
contention on this lock, you should just make it a mtx, and only go back and
make it a rwlock if it really needs it. Also, you currently can't do an
interlocked msleep() or cv_wait() with a rwlock, so you really need to use a
mutex anyway.
> anyway, I still dont understand how should I use the lock to achieve the
synchronization.
>
> my code looks like:
>
> EMUL_RLOCK(&emul_lock);
> LIST_FOREACH_SAFE(em, &td_em->shared->threads, threads, tmp_em) {
> }
> EMUL_RUNLOCK(&emul_lock);
>
> what do you suggest? I need to process the list first and then let the
exit_hook in the various processes run.
This is not safe anyway. kill() is way too big of a function to call with a
lock held. You also pass the wrong thread to it IIRC (you should always pass
curthread as the td argument to a syscall). You probably need to use
psignal, and you probably should be doing something like this:
EMUL_LOCK();
LIST_FOREACH(td, &td_em->shared->threads, threads) {
p = td->td_proc;
PROC_LOCK(p);
psignal(p, SIGKILL);
PROC_UNLOCK(p);
}
while (THREADS_STILL_AROUND(&td->em))
msleep(td_em, &emul_lock, PWAIT, "foo", 0);
EMUL_UNLOCK();
Then in your exit_hook you should do this:
em = em_find(p->p_pid, EMUL_UNLOCKED);
LIST_REMOVE(...);
SLIST_REMOVE(...);
wakeup(em);
EMUL_UNLOCK();
--
John Baldwin
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