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To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>, Peter Zijlstra
 <peterz@infradead.org>
Cc: oe-lkp@lists.linux.dev, lkp@intel.com, kernel test robot
 <oliver.sang@intel.com>
Subject: Re: [bigeasy-staging:futex_local_v6] [futex]  865221325b:
 WARNING:at_lib/rcuref.c:#rcuref_put_slowpath
In-Reply-To: <20250116233137.v8s-FDdH@linutronix.de>
References: <202412311453.9d7636a2-lkp@intel.com>
 <20250116233137.v8s-FDdH@linutronix.de>
Date: Fri, 17 Jan 2025 09:49:35 +0100
Message-ID: <87r051akps.ffs@tglx>
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On Fri, Jan 17 2025 at 00:31, Sebastian Andrzej Siewior wrote:
> On 2024-12-31 16:41:41 [+0800], kernel test robot wrote:
>> [  184.841511][T52159] rcuref - imbalanced put()
>> [  184.841536][T52159] WARNING: CPU: 64 PID: 52159 at lib/rcuref.c:267 rcuref_put_slowpath+0x66/0x70
>
> after hours of yelling and cursing:
>
>       ref  = 0 (via rcuref_init(ref, 1))
>  T1                                      T2
>  rcuref_put(ref)
>  -> atomic_add_negative_release(-1, ref)                                         # ref -> 0xffffffff
>  -> rcuref_put_slowpath(ref)
>                                          rcuref_get(ref)
>                                          -> atomic_add_negative_relaxed(1, &ref->refcnt)
>                                            -> return true;                       # ref -> 0
>
>                                          rcuref_put(ref)
>                                          -> atomic_add_negative_release(-1, ref) # ref -> 0xffffffff
>                                          -> rcuref_put_slowpath()
>
>     -> cnt = atomic_read(&ref->refcnt);                                          # cnt -> 0xffffffff / RCUREF_NOREF
>     -> atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD))              # ref -> 0xe0000000 / RCUREF_DEAD
>        -> return true
>                                            -> cnt = atomic_read(&ref->refcnt);   # cnt -> 0xe0000000 / RCUREF_DEAD
>                                            -> if (cnt > RCUREF_RELEASED)         # 0xe0000000 > 0xc0000000
>                                              -> WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()")

Duh. Indeed.

That's a nasty one. Let me think about it.