linux-kernel - Re: [re: PATCH v2 00/15 - 10/11] dyndbg: move lock,unlock into ddebug

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Message-ID: <CAK8ByeLaE1xumns+g6EMYK1weRwBhyEKR8YpYjx4uQqohvPTzg@mail.gmail.com>
Date:   Sat, 9 Dec 2023 01:32:37 +0100
From:   Łukasz Bartosik <lb@...ihalf.com>
To:     Jim Cromie <jim.cromie@...il.com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        bleung@...gle.com, contact@...rsion.fr, daniel@...ll.ch,
        dianders@...omium.org, groeck@...gle.com, jbaron@...mai.com,
        john.ogness@...utronix.de, keescook@...omium.org, pmladek@...e.com,
        ppaalanen@...il.com, rostedt@...dmis.org, seanpaul@...omium.org,
        sergey.senozhatsky@...il.com, upstream@...ihalf.com,
        vincent.whitchurch@...s.com, yanivt@...gle.com,
        gregkh@...uxfoundation.org
Subject: Re: [re: PATCH v2 00/15 - 10/11] dyndbg: move lock,unlock into
 ddebug_change, drop goto

pt., 8 gru 2023 o 01:15 Jim Cromie <jim.cromie@...il.com> napisał(a):
>
> Signed-off-by: Jim Cromie <jim.cromie@...il.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 *query,
>         struct ddebug_class_map *map = 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 *query,
>         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, const 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 =T:usb" > /proc/dynamic_debug/control   # from one console
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 = 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
>