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From: Thomas Gleixner <tglx@linutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sam Sun <samsun1006219@gmail.com>, linux-kernel@vger.kernel.org,
 x86@kernel.org, syzkaller-bugs@googlegroups.com, peterz@infradead.org,
 jpoimboe@kernel.org, jbaron@akamai.com, ardb@kernel.org, mingo@redhat.com,
 Borislav Petkov <bp@alien8.de>, dave.hansen@linux.intel.com,
 hpa@zytor.com, xrivendell7@gmail.com, Greg Kroah-Hartman
 <gregkh@linuxfoundation.org>, Tejun Heo <tj@kernel.org>
Subject: Re: [Linux kernel bug] WARNING in static_key_slow_inc_cpuslocked
In-Reply-To: <87le3exfx2.ffs@tglx>
References: <20240609090431.3af238bc@gandalf.local.home>
 <87o78axlbm.ffs@tglx> <20240609102530.0a292b07@rorschach.local.home>
 <87le3exfx2.ffs@tglx>
Date: Sun, 09 Jun 2024 18:56:14 +0200
Message-ID: <87h6e2xdg1.ffs@tglx>
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On Sun, Jun 09 2024 at 18:02, Thomas Gleixner wrote:
> On Sun, Jun 09 2024 at 10:25, Steven Rostedt wrote:
> Well the bug is there to detect inconsistency and that clearly works :)
>
> But I clearly can't read, because the jump label operations are
> serialized via jump_label_mutex. Hrm...

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.

I'll send a patch after testing it.

Thanks,

        tglx