[PATCH v4] hwrng: core - Stop/start hwrng_fillfn() kthread before/after suspend-resume
From: Thomas Richard (TI)
Date: Mon Jul 13 2026 - 07:26:19 EST
The hwrng_fillfn() kernel thread accesses the RNG device directly. During
suspend and resume sequences, hwrng_fillfn() may attempt to access the RNG
device while it is suspended. To address this, the hwrng_fillfn() kernel
thread is stopped before suspend, and restarted after resume. This is done
using the pm_notifier mechanism.
Issue was found while doing suspend-to-ram on J721S2 EVM board with
omap-rng driver.
echo mem > /sys/power/state
[ 27.922259] PM: suspend entry (deep)
[ 27.927191] Filesystems sync: 0.000 seconds
[ 27.933858] Freezing user space processes
[ 27.939119] Freezing user space processes completed (elapsed 0.001 seconds)
[ 27.946090] OOM killer disabled.
[ 27.949315] Freezing remaining freezable tasks
[ 27.954887] Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
[ 27.963337] GFP mask restricted
[ 27.967069] omap_rng 4e10000.rng: PM: calling platform_pm_suspend @ 195, parent: 4e00000.crypto
[ 27.967072] mmcblk mmc1:9fb0: PM: calling mmc_bus_suspend @ 122, parent: mmc1
[ 27.968636] mmcblk mmc1:9fb0: PM: mmc_bus_suspend returned 0 after 1546 usecs
[ 27.975778] omap_rng 4e10000.rng: PM: platform_pm_suspend returned 0 after 3 usecs
...
[ 33.510667] ti-sci 44083000.system-controller: PM: ti_sci_suspend_noirq returned 0 after 0 usecs
[ 33.510671] SError Interrupt on CPU0, code 0x00000000bf000000 -- SError
[ 33.510681] CPU: 0 UID: 0 PID: 132 Comm: hwrng Tainted: G M W 7.0.0-12695-g8923b7a6e11d #19 PREEMPT
[ 33.510690] Tainted: [M]=MACHINE_CHECK, [W]=WARN
[ 33.510693] Hardware name: Texas Instruments J721S2 EVM (DT)
[ 33.510697] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 33.510701] pc : omap_rng_do_read+0x3c/0xe0
[ 33.510709] lr : omap_rng_do_read+0x58/0xe0
[ 33.510712] sp : ffff80008942be00
[ 33.510713] x29: ffff80008942be00 x28: 0000000000000000 x27: 0000000000000000
[ 33.510719] x26: 0000000000000010 x25: 0000000000000010 x24: ffff0008065644e8
[ 33.510724] x23: ffff8000878b3370 x22: ffff00080148b2c0 x21: 0000000000000000
[ 33.510728] x20: ffff000806564480 x19: 0000000000000064 x18: 0000000000000000
[ 33.510732] x17: 6573752031207265 x16: 7466612030206465 x15: 6e72757465722071
[ 33.510737] x14: ffff0008062c8080 x13: 000031702bc0da42 x12: 0000000000000001
[ 33.510741] x11: 00000000000000c0 x10: 0000000000000b30 x9 : ffff80008942bc80
[ 33.510745] x8 : ffff0008062c8b90 x7 : ffff000b7dfa34c0 x6 : 0000000805ca16c1
[ 33.510749] x5 : 0000000000000000 x4 : ffff800080e17bfc x3 : ffff800087389c68
[ 33.510753] x2 : 0000000000000000 x1 : 0000000000000010 x0 : 000000000000a7c6
[ 33.510759] Kernel panic - not syncing: Asynchronous SError Interrupt
[ 33.510762] CPU: 0 UID: 0 PID: 132 Comm: hwrng Tainted: G M W 7.0.0-12695-g8923b7a6e11d #19 PREEMPT
[ 33.510767] Tainted: [M]=MACHINE_CHECK, [W]=WARN
[ 33.510768] Hardware name: Texas Instruments J721S2 EVM (DT)
[ 33.510770] Call trace:
[ 33.510772] show_stack+0x18/0x24 (C)
[ 33.510780] dump_stack_lvl+0x34/0x8c
[ 33.510788] dump_stack+0x18/0x24
[ 33.510792] vpanic+0x47c/0x4dc
[ 33.510799] do_panic_on_target_cpu+0x0/0x1c
[ 33.510803] add_taint+0x0/0xbc
[ 33.510807] arm64_serror_panic+0x70/0x80
[ 33.510812] do_serror+0x3c/0x70
[ 33.510815] el1h_64_error_handler+0x34/0x50
[ 33.510823] el1h_64_error+0x6c/0x70
[ 33.510827] omap_rng_do_read+0x3c/0xe0 (P)
[ 33.510831] hwrng_fillfn+0x98/0x330
[ 33.510834] kthread+0x130/0x13c
[ 33.510845] ret_from_fork+0x10/0x20
[ 33.510850] SMP: stopping secondary CPUs
[ 33.519442] Kernel Offset: disabled
[ 33.519444] CPU features: 0x04000000,800a0008,00040001,0400421b
[ 33.519448] Memory Limit: none
[ 33.732904] ---[ end Kernel panic - not syncing: Asynchronous SError Interrupt ]---
Signed-off-by: Thomas Richard (TI) <thomas.richard@xxxxxxxxxxx>
---
This is the fourth iteration, as requested by Herbert I replaced BUG_ON()
by lockdep_assert_held().
---
Changes in v4:
- use lockdep_assert_held() instead of BUG_ON()
- Link to v3: https://lore.kernel.org/r/20260703-hw-random-fix-hwrng-fillfn-crash-suspend-resume-v3-1-b7165cb9cf38@xxxxxxxxxxx
Changes in v3:
- use pm_notifier to stop/start the hwrng_fillfn() kthread before/after
suspend-resume.
- Link to v2: https://lore.kernel.org/r/20260601-hw-random-fix-hwrng-fillfn-crash-suspend-resume-v2-1-667ce5da32ee@xxxxxxxxxxx
Changes in v2:
- Initialize rc variable.
- Link to v1: https://lore.kernel.org/r/20260513-hw-random-fix-hwrng-fillfn-crash-suspend-resume-v1-1-b12551c1c7dd@xxxxxxxxxxx
---
drivers/char/hw_random/core.c | 90 +++++++++++++++++++++++++++++++++++--------
1 file changed, 75 insertions(+), 15 deletions(-)
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 6931657ad2ca..82cc8f825494 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -17,14 +17,17 @@
#include <linux/hw_random.h>
#include <linux/kernel.h>
#include <linux/kthread.h>
+#include <linux/lockdep.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
+#include <linux/notifier.h>
#include <linux/random.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/sched/signal.h>
#include <linux/slab.h>
#include <linux/string.h>
+#include <linux/suspend.h>
#include <linux/sysfs.h>
#include <linux/uaccess.h>
#include <linux/workqueue.h>
@@ -57,6 +60,8 @@ MODULE_PARM_DESC(default_quality,
static int hwrng_init(struct hwrng *rng);
static int hwrng_fillfn(void *unused);
+static void hwrng_start_hwrng_fillfn(void);
+static void hwrng_stop_hwrng_fillfn(void);
static size_t rng_buffer_size(void)
{
@@ -114,13 +119,7 @@ static int set_current_rng(struct hwrng *rng)
}
/* if necessary, start hwrng thread */
- if (!hwrng_fill) {
- hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
- if (IS_ERR(hwrng_fill)) {
- pr_err("hwrng_fill thread creation failed\n");
- hwrng_fill = NULL;
- }
- }
+ hwrng_start_hwrng_fillfn();
return 0;
}
@@ -137,10 +136,7 @@ static void drop_current_rng(void)
RCU_INIT_POINTER(current_rng, NULL);
synchronize_rcu();
- if (hwrng_fill) {
- kthread_stop(hwrng_fill);
- hwrng_fill = NULL;
- }
+ hwrng_stop_hwrng_fillfn();
/* decrease last reference for triggering the cleanup */
kref_put(&rng->ref, cleanup_rng);
@@ -506,6 +502,29 @@ static struct attribute *rng_dev_attrs[] = {
ATTRIBUTE_GROUPS(rng_dev);
+static void hwrng_start_hwrng_fillfn(void)
+{
+ lockdep_assert_held(&rng_mutex);
+
+ if (!hwrng_fill) {
+ hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
+ if (IS_ERR(hwrng_fill)) {
+ pr_err("hwrng_fill thread creation failed\n");
+ hwrng_fill = NULL;
+ }
+ }
+}
+
+static void hwrng_stop_hwrng_fillfn(void)
+{
+ lockdep_assert_held(&rng_mutex);
+
+ if (hwrng_fill) {
+ kthread_stop(hwrng_fill);
+ hwrng_fill = NULL;
+ }
+}
+
static int hwrng_fillfn(void *unused)
{
size_t entropy, entropy_credit = 0; /* in 1/1024 of a bit */
@@ -559,6 +578,35 @@ static int hwrng_fillfn(void *unused)
return 0;
}
+static int hwrng_pm_notifier(struct notifier_block *nb, unsigned long action,
+ void *data)
+{
+ switch (action) {
+ case PM_SUSPEND_PREPARE:
+ case PM_HIBERNATION_PREPARE:
+ case PM_RESTORE_PREPARE:
+ mutex_lock(&rng_mutex);
+ hwrng_stop_hwrng_fillfn();
+ mutex_unlock(&rng_mutex);
+ break;
+
+ case PM_POST_SUSPEND:
+ case PM_POST_HIBERNATION:
+ case PM_POST_RESTORE:
+ mutex_lock(&rng_mutex);
+ if (rcu_access_pointer(current_rng))
+ hwrng_start_hwrng_fillfn();
+ mutex_unlock(&rng_mutex);
+ break;
+ }
+
+ return NOTIFY_DONE;
+}
+
+static struct notifier_block hwrng_pm_nb = {
+ .notifier_call = hwrng_pm_notifier,
+};
+
int hwrng_register(struct hwrng *rng)
{
int err = -EINVAL;
@@ -705,10 +753,20 @@ static int __init hwrng_modinit(void)
}
ret = misc_register(&rng_miscdev);
- if (ret) {
- kfree(rng_fillbuf);
- kfree(rng_buffer);
- }
+ if (ret)
+ goto misc_err;
+
+ ret = register_pm_notifier(&hwrng_pm_nb);
+ if (ret)
+ goto pm_err;
+
+ return 0;
+
+pm_err:
+ misc_deregister(&rng_miscdev);
+misc_err:
+ kfree(rng_fillbuf);
+ kfree(rng_buffer);
return ret;
}
@@ -721,6 +779,8 @@ static void __exit hwrng_modexit(void)
kfree(rng_fillbuf);
mutex_unlock(&rng_mutex);
+ unregister_pm_notifier(&hwrng_pm_nb);
+
misc_deregister(&rng_miscdev);
}
---
base-commit: f4b80d91193e2453f7569c0249e59d6f2805d248
change-id: 20260513-hw-random-fix-hwrng-fillfn-crash-suspend-resume-29fdb0638f59
Best regards,
--
Thomas Richard (TI) <thomas.richard@xxxxxxxxxxx>