linux-kernel - Re: [PATCH] kdb: use ktime_get_seconds() instead of ktime_get

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Message-ID: <20180126140033.ix2n7j62k6iccmdh@oak.lan>
Date:   Fri, 26 Jan 2018 14:00:33 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Baolin Wang <baolin.wang@...aro.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kdb: use ktime_get_seconds() instead of ktime_get_ts()

On Fri, Jan 26, 2018 at 10:21:58AM +0100, Arnd Bergmann wrote:
> On Fri, Jan 26, 2018 at 4:03 AM, Baolin Wang <baolin.wang@...aro.org> wrote:
> > The kdb code will print the monotonic time by ktime_get_ts(), but
> > the ktime_get_ts() will be protected by a sequence lock, that will
> > introduce one deadlock risk if the lock was already held in the
> > context from which we entered the debugger.
> >
> > Since kdb is only interested in the second field, we can use the
> > ktime_get_seconds() to get the monotonic time without a lock,
> > moreover we can remove the 'struct timespec', which is not y2038
> > safe.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
> > ---
> >  kernel/debug/kdb/kdb_main.c |    4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
> > index 69e70f4..f0fc6f7 100644
> > --- a/kernel/debug/kdb/kdb_main.c
> > +++ b/kernel/debug/kdb/kdb_main.c
> > @@ -2486,10 +2486,8 @@ static int kdb_kill(int argc, const char **argv)
> >   */
> >  static void kdb_sysinfo(struct sysinfo *val)
> >  {
> > -       struct timespec uptime;
> > -       ktime_get_ts(&uptime);
> >         memset(val, 0, sizeof(*val));
> > -       val->uptime = uptime.tv_sec;
> > +       val->uptime = ktime_get_seconds();
> >         val->loads[0] = avenrun[0];
> >         val->loads[1] = avenrun[1];
> >         val->loads[2] = avenrun[2];
> 
> Using ktime_get_seconds() avoids locking problems, but I wonder what
> would happen if we trigger the 'WARN_ON(timekeeping_suspended)'
> from kdb. Is that a problem? If it is, we have to use ktime_get_mono_fast_ns()
> and div_u64() instead.

Normally a WARN_ON() doesn't triggered a kgdb_breakpoint() so (apart
from bugs) we can start executing the warning. Unfortunately
kdb_trap_printk isn't set when we call ktime_get_seconds() so printing
the warning isn't safe.

If we had no choice of time function we could work around by
enabling printk() trapping for the call but since ktime_get_mono_fast_ns()
already exists its probably best just to use that.


Daniel.