Date: Tue, 22 Feb 2005 15:34:41 -0500 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-hackers@FreeBSD.org Cc: Ashwin Chandra <ashcs@ucla.edu> Subject: Re: Kernel monitor, the return Message-ID: <200502221534.41979.jhb@FreeBSD.org> In-Reply-To: <20050215132020.GA376@pm514-9.comsys.ntu-kpi.kiev.ua> References: <000801c51327$0e2bf020$58e243a4@ash> <20050215132020.GA376@pm514-9.comsys.ntu-kpi.kiev.ua>
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On Tuesday 15 February 2005 08:20 am, Andrey Simonenko wrote:
> On Mon, Feb 14, 2005 at 10:24:50PM -0800, Ashwin Chandra wrote:
> > In trying to create a simple kernel thread that prints out all
> > the processes data and stack size, i still get a panic fault
> > (vm_fault on no entry) at the printf statement...ive narrowed
> > it down to the ru_idrss variable that is causing the problem,
>
> Definitely ru_idrss cannot cause any error, may be you made
> such desition, because arguments are pushed to the stack
> in the reverse order. p->p_stats pointer causes the error.
>
> > im not sure why
>
> (I think that) If some process is not running, then you cannot
> use p->p_stats without additional checks for memory p->p_stats
> points to, since p->p_stats points to u_stats in struct user{},
> which can be swapped out if a process is not running.
>
> Actually you can read this in the comment before struct user{}
> in /sys/sys/user.h.
>
> >. I thought maybe I was not locking properly or
> > obseriving correct mutexes, but I have tried everything.
>
> You'll get an error at some time, because of incorrect usage
> (really not usage) of locks.
>
> > If anyone
> > knows why the fault is occurring at the printf, please let me know. =)
>
> Following code works on my system:
>
> ----
>
> sx_slock(&allproc_lock);
>
> FOREACH_PROC_IN_SYSTEM(p) {
> mtx_lock_spin(&sched_lock);
> PROC_LOCK(p);
> printf("proc %ld:", (long)p->p_pid);
> if ((p->p_sflag & PS_INMEM) && p->p_stats != NULL)
> printf(" ru_isrss %ld, rui_idrss %ld\n", p->p_stats->p_ru.ru_isrss,
> p->p_stats->p_ru.ru_idrss); else {
> if (!(p->p_sflag & PS_INMEM))
> printf(" !PS_INMEM");
> if (p->p_stats == NULL)
> printf(" p_stats == NULL");
> }
> printf("\n");
> PROC_UNLOCK(p);
> mtx_unlock(&sched_lock);
> }
>
> sx_sunlock(&allproc_lock);
You don't need sched_lock to check PS_INMEM, proc lock is sufficient (PS_INMEM
is magic this way). If you did though, you would lock proc first, then
sched_lock. You can _not_ lock a regular mutex (proc lock) after a spin
mutex (sched_lock) or you can deadlock.
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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