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

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Message-ID: <CAK8P3a2eHfepGXrWAVSwYQZX-vFS84SiKXCRuGdw+QTbproZhg@mail.gmail.com>
Date:   Fri, 26 Jan 2018 10:21:58 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        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 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.

       Arnd