Subject: Re: linux-next: boot warning from the final tree

From: Nathan Chancellor <[email-masked]>
Date: Sat, 22 Nov 2025 00:37:05 -0700

On Fri, Nov 21, 2025 at 02:33:14PM -0800, Boqun Feng wrote:
> On Fri, Nov 21, 2025 at 02:58:19PM -0700, Nathan Chancellor wrote:
> > On Mon, Nov 17, 2025 at 08:22:14PM +1100, Stephen Rothwell wrote:
> > > Hi all,
> > > 
> > > Today's linux-next qemu boot (powerpc pseries_le_defconfig) produced
> > > this warning:
> > > 
> > >   ftrace: allocating 48915 entries in 288 pages
> > >   ftrace: allocated 287 pages with 6 groups
> > >   ------------[ cut here ]------------
> > >   DEBUG_LOCKS_WARN_ON(lock->magic != lock)
> > >   WARNING: kernel/locking/mutex.c:156 at mutex_lock+0xcc/0x100, CPU#0: swapper/0/0
> > >   Modules linked in:
> > >   CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.18.0-rc6-09359-g921087e37218 #1 VOLUNTARY 
> > >   Hardware name: IBM pSeries (emulated by qemu) POWER9 (architected) 0x4e1202 0xf000005 of:SLOF,HEAD hv:linux,kvm pSeries
> > >   NIP:  c00000000148041c LR: c000000001480418 CTR: 0000000000000000
> > >   REGS: c000000002957a10 TRAP: 0700   Not tainted  (6.18.0-rc6-09359-g921087e37218)
> > >   MSR:  8000000002021033 <SF,VEC,ME,IR,DR,RI,LE>  CR: 24022240  XER: 00000000
> > >   CFAR: c00000000021123c IRQMASK: 3 
> > >   GPR00: c000000001480418 c000000002957cb0 c000000001a3a100 0000000000000028 
> > >   GPR04: 00000000ffffe04a c0000000026abe88 0000000000000001 000000000000004b 
> > >   GPR08: c0000000026abd28 0000000000000000 0000000000000000 0000000044022240 
> > >   GPR12: 0000000000000000 c000000002ae9000 0000000000000000 0000000001bff430 
> > >   GPR16: 000000007e68f070 c00000007f79c480 c000000002969160 c000000002a0f5d8 
> > >   GPR20: c0000000026a1138 c0000000026a1120 0000000000000000 c0000000019541b8 
> > >   GPR24: c00000000218a480 c00000000296e1d0 000000007d612000 c00000000380be10 
> > >   GPR28: c00000000380be20 c00000000380be00 c000000002640100 c00000000380be20 
> > >   NIP [c00000000148041c] mutex_lock+0xcc/0x100
> > >   LR [c000000001480418] mutex_lock+0xc8/0x100
> > >   Call Trace:
> > >   [c000000002957cb0] [c000000001480418] mutex_lock+0xc8/0x100 (unreliable)
> > >   [c000000002957d20] [c00000000024a60c] alloc_workqueue_noprof+0x38c/0x8ec
> > >   [c000000002957e00] [c00000000203018c] workqueue_init_early+0x4d8/0x6ec
> > >   [c000000002957f30] [c000000002004448] start_kernel+0x74c/0xa4c
> > >   [c000000002957fe0] [c00000000000e99c] start_here_common+0x1c/0x20
> > >   Code: 4182ffb4 3d2200f3 392971e4 81290000 2c090000 4082ffa0 3c82ffe0 3c62ffe0 3884bfe0 3863bf68 4ad90d45 60000000 <0fe00000> 4bffff80 60000000 60000000 
> > >   ---[ end trace 0000000000000000 ]---
> > >   rcu: Hierarchical RCU implementation.
> > > 
> > > I have no idea what caused this.
> > 
> > I noticed this warning in my QEMU boot tests as well and bisected it
> > down to commit 3572e2edc7b6 ("locking/mutex: Redo __mutex_init()").
> > 
> >   $ make -skj"$(nproc)" ARCH=powerpc CROSS_COMPILE=powerpc64-linux- clean ppc64le_guest_defconfig zImage.epapr
> > 
> >   $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20241120-044434/ppc64le-rootfs.cpio.zst | zstd -d >rootfs.cpio
> > 
> >   $ qemu-system-ppc64 \
> >       -display none \
> >       -nodefaults \
> >       -device ipmi-bmc-sim,id=bmc0 \
> >       -device isa-ipmi-bt,bmc=bmc0,irq=10 \
> >       -machine powernv \
> >       -kernel arch/powerpc/boot/zImage.epapr \
> >       -initrd rootfs.cpio \
> >       -m 2G \
> >       -serial mon:stdio
> >   ...
> >   [    0.000000][    T0] Linux version 6.18.0-rc2-00016-g3572e2edc7b6 (nathan@ax162) (powerpc64-linux-gcc (GCC) 15.2.0, GNU ld (GNU Binutils) 2.45) #1 SMP Fri Nov 21 13:55:26 MST 2025
> >   ...
> >   [    0.000000][    T0] ------------[ cut here ]------------
> >   [    0.000000][    T0] DEBUG_LOCKS_WARN_ON(lock->magic != lock)
> >   [    0.000000][    T0] WARNING: CPU: 0 PID: 0 at kernel/locking/mutex.c:156 mutex_lock+0xd4/0x100
> >   [    0.000000][    T0] Modules linked in:
> >   [    0.000000][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.18.0-rc2-00016-g3572e2edc7b6 #1 VOLUNTARY
> >   [    0.000000][    T0] Hardware name: IBM PowerNV (emulated by qemu) POWER10 0x801200 opal:v7.1-106-g785a5e307 PowerNV
> >   [    0.000000][    T0] NIP:  c0000000014b2974 LR: c0000000014b2970 CTR: 0000000000000000
> >   [    0.000000][    T0] REGS: c0000000029979f0 TRAP: 0700   Not tainted  (6.18.0-rc2-00016-g3572e2edc7b6)
> >   [    0.000000][    T0] MSR:  9000000002021033 <SF,HV,VEC,ME,IR,DR,RI,LE>  CR: 24000220  XER: 00000000
> >   [    0.000000][    T0] CFAR: c00000000021ed7c IRQMASK: 3
> >   [    0.000000][    T0] GPR00: c0000000014b2970 c000000002997c90 c000000001a78100 0000000000000028
> >   [    0.000000][    T0] GPR04: 00000000ffffe04a c0000000026ed958 0000000000000001 000000000000004b
> >   [    0.000000][    T0] GPR08: c0000000026ed7f0 0000000000000000 0000000000000000 0000000044000220
> >   [    0.000000][    T0] GPR12: c0000000026ed880 c000000002ba0000 0000000000000018 0000000000000000
> >   [    0.000000][    T0] GPR16: 0000000000000000 c0000000026e2b88 c0000000026e2ba0 c00000007be5a400
> >   [    0.000000][    T0] GPR20: c0000000029ed0e0 c000000002aaf7e0 0000000000000000 c0000000019911b8
> >   [    0.000000][    T0] GPR24: c0000000021ca400 c0000000029f2150 0000000079c90000 c000000003081410
> >   [    0.000000][    T0] GPR28: c000000003081420 c000000003081400 c0000000021cce98 c000000003081420
> >   [    0.000000][    T0] NIP [c0000000014b2974] mutex_lock+0xd4/0x100
> >   [    0.000000][    T0] LR [c0000000014b2970] mutex_lock+0xd0/0x100
> >   [    0.000000][    T0] Call Trace:
> >   [    0.000000][    T0] [c000000002997c90] [c0000000014b2970] mutex_lock+0xd0/0x100 (unreliable)
> >   [    0.000000][    T0] [c000000002997d10] [c000000000258ddc] alloc_workqueue_noprof+0x44c/0x8c8
> >   [    0.000000][    T0] [c000000002997df0] [c00000000203080c] workqueue_init_early+0x4e4/0x700
> >   [    0.000000][    T0] [c000000002997f30] [c000000002004388] start_kernel+0x638/0x938
> >   [    0.000000][    T0] [c000000002997fe0] [c00000000000e99c] start_here_common+0x1c/0x20
> >   [    0.000000][    T0] Code: 4182ffa8 3d2200f8 3929d134 81290000 2c090000 4082ff94 3c82ffde 3c62ffde 38846d98 38636d20 4ad6c32d 60000000 <0fe00000> e9410068 4bffff70 38210080
> >   [    0.000000][    T0] ---[ end trace 0000000000000000 ]---
> >   ...
> > 
> > At the parent change, there is no warning.
> > 
> 
> Thank you both, seems we missed the case where LOCKDEP=n but
> DEBUG_MUTEXES=y, I feel like the following should be the correct fix.

Yeah, that appears to resolve it for me and I do not see any other
warnings in my tests.

> ------------->8
> Subject: [PATCH] locking/mutex: Initialize mutex::magic even when LOCKDEP=n
> 
> When DEBUG_MUTEXES=y and LOCKDEP=n, mutex_lock() still checks on
> ->magic, hence debug_mutex_init() should be called in
> mutex_init_generic() as well. While we are at it, decouple LOCKDEP
> logic from debug_mutex_init(), because in this way debug_mutex_init()
> only needs one parameter, and we now have mutex_init_lockep() for
> LOCKDEP=y scenarios.
> 
> Reported-by: Stephen Rothwell <[email-masked]>
> Closes: https://lore.kernel.org/lkml/[email-masked]/
> Reported-by: Nathan Chancellor <[email-masked]>
> Closes: https://lore.kernel.org/lkml/20251121215819.GA1374726@ax162/
> Fixes: 3572e2edc7b6 ("locking/mutex: Redo __mutex_init()")
> Signed-off-by: Boqun Feng <[email-masked]>
> ---
>  kernel/locking/mutex-debug.c | 10 +---------
>  kernel/locking/mutex.c       |  8 +++++++-
>  kernel/locking/mutex.h       |  5 ++---
>  3 files changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
> index 949103fd8e9b..2c6b02d4699b 100644
> --- a/kernel/locking/mutex-debug.c
> +++ b/kernel/locking/mutex-debug.c
> @@ -78,16 +78,8 @@ void debug_mutex_unlock(struct mutex *lock)
>  	}
>  }
>  
> -void debug_mutex_init(struct mutex *lock, const char *name,
> -		      struct lock_class_key *key)
> +void debug_mutex_init(struct mutex *lock)
>  {
> -#ifdef CONFIG_DEBUG_LOCK_ALLOC
> -	/*
> -	 * Make sure we are not reinitializing a held lock:
> -	 */
> -	debug_check_no_locks_freed((void *)lock, sizeof(*lock));
> -	lockdep_init_map_wait(&lock->dep_map, name, key, 0, LD_WAIT_SLEEP);
> -#endif
>  	lock->magic = lock;
>  }
>  
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index f3bb352a368d..2a1d165b3167 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -51,6 +51,7 @@ static void __mutex_init_generic(struct mutex *lock)
>  #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
>  	osq_lock_init(&lock->osq);
>  #endif
> +	debug_mutex_init(lock);
>  }
>  
>  static inline struct task_struct *__owner_task(unsigned long owner)
> @@ -173,7 +174,12 @@ static __always_inline bool __mutex_unlock_fast(struct mutex *lock)
>  void mutex_init_lockep(struct mutex *lock, const char *name, struct lock_class_key *key)
>  {
>  	__mutex_init_generic(lock);
> -	debug_mutex_init(lock, name, key);
> +
> +	/*
> +	 * Make sure we are not reinitializing a held lock:
> +	 */
> +	debug_check_no_locks_freed((void *)lock, sizeof(*lock));
> +	lockdep_init_map_wait(&lock->dep_map, name, key, 0, LD_WAIT_SLEEP);
>  }
>  EXPORT_SYMBOL(mutex_init_lockep);
>  #endif /* !CONFIG_DEBUG_LOCK_ALLOC */
> diff --git a/kernel/locking/mutex.h b/kernel/locking/mutex.h
> index 2e8080a9bee3..9ad4da8cea00 100644
> --- a/kernel/locking/mutex.h
> +++ b/kernel/locking/mutex.h
> @@ -59,8 +59,7 @@ extern void debug_mutex_add_waiter(struct mutex *lock,
>  extern void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
>  				      struct task_struct *task);
>  extern void debug_mutex_unlock(struct mutex *lock);
> -extern void debug_mutex_init(struct mutex *lock, const char *name,
> -			     struct lock_class_key *key);
> +extern void debug_mutex_init(struct mutex *lock);
>  #else /* CONFIG_DEBUG_MUTEXES */
>  # define debug_mutex_lock_common(lock, waiter)		do { } while (0)
>  # define debug_mutex_wake_waiter(lock, waiter)		do { } while (0)
> @@ -68,6 +67,6 @@ extern void debug_mutex_init(struct mutex *lock, const char *name,
>  # define debug_mutex_add_waiter(lock, waiter, ti)	do { } while (0)
>  # define debug_mutex_remove_waiter(lock, waiter, ti)	do { } while (0)
>  # define debug_mutex_unlock(lock)			do { } while (0)
> -# define debug_mutex_init(lock, name, key)		do { } while (0)
> +# define debug_mutex_init(lock)				do { } while (0)
>  #endif /* !CONFIG_DEBUG_MUTEXES */
>  #endif /* CONFIG_PREEMPT_RT */
> -- 
> 2.50.1 (Apple Git-155)
>