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From:   =?UTF-8?Q?=C5=81ukasz_Bartosik?= <lb@semihalf.com>
Date:   Sat, 9 Dec 2023 01:32:37 +0100
Message-ID: <CAK8ByeLaE1xumns+g6EMYK1weRwBhyEKR8YpYjx4uQqohvPTzg@mail.gmail.com>
Subject: Re: [re: PATCH v2 00/15 - 10/11] dyndbg: move lock,unlock into
 ddebug_change, drop goto
To:     Jim Cromie <jim.cromie@gmail.com>
Cc:     linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        bleung@google.com, contact@emersion.fr, daniel@ffwll.ch,
        dianders@chromium.org, groeck@google.com, jbaron@akamai.com,
        john.ogness@linutronix.de, keescook@chromium.org, pmladek@suse.com,
        ppaalanen@gmail.com, rostedt@goodmis.org, seanpaul@chromium.org,
        sergey.senozhatsky@gmail.com, upstream@semihalf.com,
        vincent.whitchurch@axis.com, yanivt@google.com,
        gregkh@linuxfoundation.org
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pt., 8 gru 2023 o 01:15 Jim Cromie <jim.cromie@gmail.com> napisa=C5=82(a):
>
> Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
> ---
>  lib/dynamic_debug.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
> index fc903e90ea0d..b63429462d69 100644
> --- a/lib/dynamic_debug.c
> +++ b/lib/dynamic_debug.c
> @@ -537,6 +537,8 @@ static int ddebug_change(const struct ddebug_query *q=
uery,
>         struct ddebug_class_map *map =3D NULL;
>         int __outvar valid_class;
>
> +       mutex_lock(&ddebug_lock);
> +
>         /* search for matching ddebugs */
>         list_for_each_entry(dt, &ddebug_tables, link) {
>
> @@ -625,6 +627,7 @@ static int ddebug_change(const struct ddebug_query *q=
uery,
>         if (nfound)
>                 update_tr_default_dst(modifiers);
>
> +       mutex_unlock(&ddebug_lock);
>         return nfound;
>  }
>
> @@ -932,23 +935,17 @@ static int ddebug_exec_query(char *query_string, co=
nst char *modname)
>                 return -EINVAL;
>         }
>
> -       mutex_lock(&ddebug_lock);
> -

I deliberately moved locking here from ddebug_change because
ddebug_parse_flags calls read_T_args (when T flag is present) and this
in turn calls find_tr_instance which uses global bitmap (tr.bmap).
If we add that ddebug_proc_show can be triggered concurrently then it
can lead to trouble.

For example
echo "open usb" > /proc/dynamic_debug/control
echo "module usbcore =3DT:usb" > /proc/dynamic_debug/control   # from one c=
onsole
echo "close usb" > / /proc/dynamic_debug/control
# from another console

and we can end up with an attempt to use closed trace instance"usb"


>         /* check flags 1st (last arg) so query is pairs of spec,val */
>         if (ddebug_parse_flags(words[nwords-1], &modifiers)) {
>                 pr_err("flags parse failed\n");
> -               goto err;
> +               return -EINVAL;
>         }
>
>         /* actually go and implement the change */
>         nfound =3D ddebug_change(&query, &modifiers);
>
> -       mutex_unlock(&ddebug_lock);
>         vpr_info_dq(&query, nfound ? "applied" : "no-match");
>         return nfound;
> -err:
> -       mutex_unlock(&ddebug_lock);
> -       return -EINVAL;
>  }
>
>  /* handle multiple queries in query string, continue on error, return
> --
> 2.43.0
>