SCHED_WARN_ON() uses WARN_ONCE() and WARN_ONCE() uses printk(). Using
printk() from certain code paths in the process scheduler is not allowed
because printk() acquires the console_lock and this may cause printk()
to sleep. Sleeping is not allowed from many paths in the process
scheduler. Hence make sure that SCHED_WARN_ON() uses printk_deferred()
instead of printk().
Here is an example of a lockdep complaint fixed by this patch:
======================================================
WARNING: possible circular locking dependency detected
------------------------------------------------------
swapper/4/0 is trying to acquire lock:
ffffffc082179768 (console_owner){-...}-{0:0}, at: console_lock_spinning_enable+0x44/0x80
but task is already holding lock:
ffffff8b33d30a18 (&rq->__lock){-.-.}-{2:2}, at: __schedule+0x188/0x116c
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #5 (&rq->__lock){-.-.}-{2:2}:
_raw_spin_lock_nested+0x50/0x70
__task_rq_lock+0x74/0x154
wake_up_new_task+0x110/0x40c
kernel_clone+0x28c/0x508
user_mode_thread+0x60/0x8c
rest_init+0x28/0x1b8
start_kernel+0x390/0x4b0
-> #4 (&p->pi_lock){-.-.}-{2:2}:
_raw_spin_lock_irqsave+0x64/0xac
try_to_wake_up+0x6c/0xa18
default_wake_function+0x20/0x38
autoremove_wake_function+0x1c/0x64
__wake_up+0xac/0xfc
rpm_suspend+0x484/0x668
pm_runtime_work+0x8c/0xa8
process_one_work+0x26c/0x65c
worker_thread+0x33c/0x498
kthread+0x110/0x134
ret_from_fork+0x10/0x20
-> #3 (&dev->power.wait_queue){....}-{2:2}:
_raw_spin_lock_irqsave+0x64/0xac
__wake_up+0x34/0xfc
rpm_resume+0x4e8/0x5c8
__pm_runtime_resume+0x6c/0xb0
dt_idle_attach_cpu+0x70/0x9c
psci_cpuidle_probe+0x270/0x494
platform_probe+0xa0/0xe0
really_probe+0x114/0x454
__driver_probe_device+0xa4/0x160
driver_probe_device+0x44/0x23c
__device_attach_driver+0x15c/0x1f4
bus_for_each_drv+0x10c/0x168
__device_attach+0xc0/0x1a0
device_initial_probe+0x14/0x24
bus_probe_device+0x94/0x120
device_add+0x298/0x498
platform_device_add+0x80/0x24c
platform_device_register_full+0x144/0x154
psci_idle_init+0x74/0xc4
do_one_initcall+0x124/0x464
do_initcall_level+0xa8/0xd4
do_initcalls+0x74/0xf0
do_basic_setup+0x1c/0x28
kernel_init_freeable+0x154/0x204
kernel_init+0x20/0x1ac
-> #2 (&dev->power.lock){-.-.}-{2:2}:
_raw_spin_lock_irqsave+0x64/0xac
__pm_runtime_resume+0x5c/0xb0
__uart_start+0x4c/0x120
uart_write+0xe8/0x304
n_tty_write+0x310/0x4c8
file_tty_write+0x154/0x32c
redirected_tty_write+0x94/0xe0
vfs_write+0x2e8/0x3ac
ksys_write+0x78/0xe8
__arm64_sys_write+0x1c/0x2c
invoke_syscall+0x58/0x10c
el0_svc_common+0xa8/0xdc
do_el0_svc+0x1c/0x28
el0_svc+0x50/0xd4
-> #1 (&port_lock_key){-...}-{2:2}:
_raw_spin_lock_irqsave+0x64/0xac
google_serial8250_console_write+0xe0/0x8f0 [google_8250_base]
univ8250_console_write+0x2c/0x40 [google_8250]
console_flush_all+0x210/0x420
console_unlock+0x84/0x12c
vprintk_emit+0x12c/0x1b8
vprintk_default+0x44/0x70
vprintk+0xe4/0x164
_printk+0x54/0x80
register_console+0x428/0x524
serial_core_register_port+0x664/0x8b8
serial_ctrl_register_port+0x10/0x20
uart_add_one_port+0x10/0x20
google_serial8250_register_8250_port+0x834/0xa1c [google_8250]
dw8250_probe+0x550/0x5f0 [google_8250_dw]
platform_probe+0xa0/0xe0
really_probe+0x114/0x454
__driver_probe_device+0xa4/0x160
driver_probe_device+0x44/0x23c
__driver_attach+0x124/0x278
bus_for_each_dev+0x104/0x160
driver_attach+0x24/0x34
bus_add_driver+0x144/0x2d4
driver_register+0x68/0x104
__platform_driver_register+0x20/0x30
do_one_initcall+0x124/0x464
do_init_module+0x58/0x2f4
load_module+0x12c8/0x14b0
__arm64_sys_finit_module+0x238/0x33c
invoke_syscall+0x58/0x10c
el0_svc_common+0xa8/0xdc
do_el0_svc+0x1c/0x28
el0_svc+0x50/0xd4
-> #0 (console_owner){-...}-{0:0}:
__lock_acquire+0x150c/0x2e24
lock_acquire+0x13c/0x2f0
console_lock_spinning_enable+0x6c/0x80
console_flush_all+0x1dc/0x420
console_unlock+0x84/0x12c
vprintk_emit+0x12c/0x1b8
vprintk_default+0x44/0x70
vprintk+0xe4/0x164
_printk+0x54/0x80
__warn_printk+0x10c/0x1d4
sub_running_bw+0x164/0x1a8
task_non_contending+0x29c/0x4b0
dequeue_dl_entity+0x1f0/0x260
pick_task_dl+0x7c/0x1c4
__schedule+0x554/0x116c
schedule_idle+0x24/0x48
do_idle+0x2f0/0x338
cpu_startup_entry+0x34/0x3c
other info that might help us debug this:
Chain exists of:
console_owner --> &p->pi_lock --> &rq->__lock
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&rq->__lock);
lock(&p->pi_lock);
lock(&rq->__lock);
lock(console_owner);
*** DEADLOCK ***
3 locks held by swapper/4/0:
#0: ffffff8b33d30a18 (&rq->__lock){-.-.}-{2:2}, at: __schedule+0x188/0x116c
#1: ffffffc082179790 (console_lock){+.+.}-{0:0}, at: vprintk_emit+0x124/0x1b8
#2: ffffffc082109238 (console_srcu){....}-{0:0}, at: rcu_try_lock_acquire+0x0/0x40
stack backtrace:
Call trace:
dump_backtrace+0xfc/0x17c
show_stack+0x18/0x28
dump_stack_lvl+0x40/0x104
dump_stack+0x18/0x3c
print_circular_bug+0x32c/0x334
check_noncircular+0x158/0x178
__lock_acquire+0x150c/0x2e24
lock_acquire+0x13c/0x2f0
console_lock_spinning_enable+0x6c/0x80
console_flush_all+0x1dc/0x420
console_unlock+0x84/0x12c
vprintk_emit+0x12c/0x1b8
vprintk_default+0x44/0x70
vprintk+0xe4/0x164
_printk+0x54/0x80
__warn_printk+0x10c/0x1d4
sub_running_bw+0x164/0x1a8
task_non_contending+0x29c/0x4b0
dequeue_dl_entity+0x1f0/0x260
pick_task_dl+0x7c/0x1c4
__schedule+0x554/0x116c
schedule_idle+0x24/0x48
do_idle+0x2f0/0x338
cpu_startup_entry+0x34/0x3c
secondary_start_kernel+0x138/0x1a8
__secondary_switched+0xc0/0xc4
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
kernel/sched/sched.h | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e289d364459e..34a9c473104a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -91,7 +91,21 @@ struct cpuidle_state;
#include "cpupri.h"
#include "cpudeadline.h"
-#define SCHED_WARN_ON(x) WARN_ONCE(x, #x)
+#define SCHED_WARN_ON(x) \
+ ({ \
+ bool __ret = (x); \
+ \
+ if (unlikely(__ret)) { \
+ unsigned long __flags; \
+ \
+ local_irq_save(__flags); \
+ printk_deferred_enter(); \
+ WARN_ONCE(__ret, #x); \
+ printk_deferred_exit(); \
+ local_irq_restore(__flags); \
+ } \
+ unlikely(__ret); \
+ })
/* task_struct::on_rq states: */
#define TASK_ON_RQ_QUEUED 1On Tue, Oct 28, 2025 at 09:50:58AM -0700, Bart Van Assche wrote:
> +#define SCHED_WARN_ON(x) \
> + ({ \
> + bool __ret = (x); \
> + \
> + if (unlikely(__ret)) { \
> + unsigned long __flags; \
> + \
> + local_irq_save(__flags); \
> + printk_deferred_enter(); \
> + WARN_ONCE(__ret, #x); \
> + printk_deferred_exit(); \
> + local_irq_restore(__flags); \
> + } \
> + unlikely(__ret); \
> + })
Also, this will generate absolutely atrocious code.
On Tue, Oct 28, 2025 at 09:50:58AM -0700, Bart Van Assche wrote: > SCHED_WARN_ON() uses WARN_ONCE() and WARN_ONCE() uses printk(). Using > printk() from certain code paths in the process scheduler is not allowed > because printk() acquires the console_lock and this may cause printk() > to sleep. Sleeping is not allowed from many paths in the process > scheduler. Hence make sure that SCHED_WARN_ON() uses printk_deferred() > instead of printk(). Yeah, so no. I hate that deferred thing with a passion. It means your warning will never make it out if something fatal happens. The printk() people have been working on fixing this for a long while and I think they're close. All the crap consoles will get threaded printing while the useful consoles (serial) will get atomic printing. Also, you should never hit these WARNs to begin with, and if you do, that lockdep splat is the least of your problems.
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