linux-kernel - Re: [Linux kernel bug] WARNING in static_key_slow_inc

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Date: Mon, 10 Jun 2024 12:34:14 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
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 Mon, Jun 10 2024 at 08:46, Peter Zijlstra wrote:
> 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..)

Right, but all of this is too fragile. Let me send out those patches.