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Message-ID: <aL49oKS9v_sxYSee@tardis-2.local>
Date: Sun, 7 Sep 2025 19:21:20 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: "A. Sverdlin" <alexander.sverdlin@...mens.com>,
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>, 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 04:44:30AM -0700, Paul E. McKenney wrote:
> 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>
>
Thank you, I will queue this for future testing ans reviews.
Alexander, is there any link to a kcsan splat that we can use? Thanks!
Regards,
Boqun
> > ---
> > 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
> >
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