linux-kernel - Re: [PATCH RESEND] locking/spinlock/debug: Fix data-race in do_raw_write

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Message-ID: <7325588b-189c-4825-a87f-5494b1230d7a@paulmck-laptop>
Date: Tue, 26 Aug 2025 04:44:30 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "A. Sverdlin" <alexander.sverdlin@...mens.com>
Cc: Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	stable@...r.kernel.org,
	Adrian Freihofer <adrian.freihofer@...mens.com>
Subject: Re: [PATCH RESEND] locking/spinlock/debug: Fix data-race in
 do_raw_write_lock

On Tue, Aug 26, 2025 at 12:27:27PM +0200, A. Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@...mens.com>
> 
> KCSAN reports:
> 
> BUG: KCSAN: data-race in do_raw_write_lock / do_raw_write_lock
> 
> write (marked) to 0xffff800009cf504c of 4 bytes by task 1102 on cpu 1:
>  do_raw_write_lock+0x120/0x204
>  _raw_write_lock_irq
>  do_exit
>  call_usermodehelper_exec_async
>  ret_from_fork
> 
> read to 0xffff800009cf504c of 4 bytes by task 1103 on cpu 0:
>  do_raw_write_lock+0x88/0x204
>  _raw_write_lock_irq
>  do_exit
>  call_usermodehelper_exec_async
>  ret_from_fork
> 
> value changed: 0xffffffff -> 0x00000001
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 1103 Comm: kworker/u4:1 6.1.111
> 
> Commit 1a365e822372 ("locking/spinlock/debug: Fix various data races") has
> adressed most of these races, but seems to be not consistent/not complete.
> 
> >From do_raw_write_lock() only debug_write_lock_after() part has been
> converted to WRITE_ONCE(), but not debug_write_lock_before() part.
> Do it now.
> 
> Cc: stable@...r.kernel.org
> Fixes: 1a365e822372 ("locking/spinlock/debug: Fix various data races")
> Reported-by: Adrian Freihofer <adrian.freihofer@...mens.com>
> Acked-by: Waiman Long <longman@...hat.com>
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
> There are still some inconsistencies remaining IMO:
> - lock->magic is sometimes accessed with READ_ONCE() even though it's only
> being plain-written;
> - debug_spin_unlock() and debug_write_unlock() both do WRITE_ONCE() on
> lock->owner and lock->owner_cpu, but examine them with plain read accesses.
> 
>  kernel/locking/spinlock_debug.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index 87b03d2e41dbb..2338b3adfb55f 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -184,8 +184,8 @@ void do_raw_read_unlock(rwlock_t *lock)
>  static inline void debug_write_lock_before(rwlock_t *lock)
>  {
>  	RWLOCK_BUG_ON(lock->magic != RWLOCK_MAGIC, lock, "bad magic");
> -	RWLOCK_BUG_ON(lock->owner == current, lock, "recursion");
> -	RWLOCK_BUG_ON(lock->owner_cpu == raw_smp_processor_id(),
> +	RWLOCK_BUG_ON(READ_ONCE(lock->owner) == current, lock, "recursion");
> +	RWLOCK_BUG_ON(READ_ONCE(lock->owner_cpu) == raw_smp_processor_id(),
>  							lock, "cpu recursion");
>  }
>  
> -- 
> 2.47.1
>