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Message-ID: <20251028165059.682306-3-bvanassche@acm.org>
Date: Tue, 28 Oct 2025 09:50:58 -0700
From: Bart Van Assche <bvanassche@....org>
To: Ingo Molnar <mingo@...hat.com>
Cc: linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Bart Van Assche <bvanassche@....org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 2/2] Fix SCHED_WARN_ON()
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@....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 1
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