[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20240610064651.GS8774@noisy.programming.kicks-ass.net>
Date: Mon, 10 Jun 2024 08:46:51 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>, Sam Sun <samsun1006219@...il.com>,
linux-kernel@...r.kernel.org, x86@...nel.org,
syzkaller-bugs@...glegroups.com, jpoimboe@...nel.org,
jbaron@...mai.com, ardb@...nel.org, mingo@...hat.com,
Borislav Petkov <bp@...en8.de>, dave.hansen@...ux.intel.com,
hpa@...or.com, xrivendell7@...il.com,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Tejun Heo <tj@...nel.org>
Subject: Re: [Linux kernel bug] WARNING in static_key_slow_inc_cpuslocked
On Sun, Jun 09, 2024 at 06:56:14PM +0200, Thomas Gleixner wrote:
> Ok. Now I found if for real. It's in the jump label core:
>
> CPU0 CPU1
>
> static_key_slow_dec()
> static_key_slow_try_dec()
>
> key->enabled == 1
> val = atomic_fetch_add_unless(&key->enabled, -1, 1);
> if (val == 1)
> return false;
>
> jump_label_lock();
> if (atomic_dec_and_test(&key->enabled)) {
> --> key->enabled == 0
> __jump_label_update()
>
> static_key_slow_dec()
> static_key_slow_try_dec()
>
> key->enabled == 0
> val = atomic_fetch_add_unless(&key->enabled, -1, 1);
>
> --> key->enabled == -1 <- FAIL
>
> static_key_slow_try_dec() is buggy. It needs similar logic as
> static_key_slow_try_inc() to work correctly.
>
> It's not only the 0, key->enabled can be -1 when the other CPU is in the
> slow path of enabling it.
Well, the -1 thing is in the 0->1 path, that is, the very first enabler.
That *should* not race with a disabler. If it does, there is external
confusion. (As I think the follow up email shows..)
Powered by blists - more mailing lists