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Message-ID: <20250723142416.1020423-1-dev@lankhorst.se>
Date: Wed, 23 Jul 2025 16:24:16 +0200
From: Maarten Lankhorst <dev@...khorst.se>
To: linux-kernel@...r.kernel.org
Cc: intel-xe@...ts.freedesktop.org,
Maarten Lankhorst <dev@...khorst.se>,
Mukesh Ojha <quic_mojha@...cinc.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Johannes Berg <johannes@...solutions.net>,
"Rafael J. Wysocki" <rafael@...nel.org>,
Danilo Krummrich <dakr@...nel.org>,
stable@...r.kernel.org,
Matthew Brost <matthew.brost@...el.com>
Subject: [PATCH] devcoredump: Fix circular locking dependency with devcd->mutex.
The original code causes a circular locking dependency found by lockdep.
======================================================
WARNING: possible circular locking dependency detected
6.16.0-rc6-lgci-xe-xe-pw-151626v3+ #1 Tainted: G S U
------------------------------------------------------
xe_fault_inject/5091 is trying to acquire lock:
ffff888156815688 ((work_completion)(&(&devcd->del_wk)->work)){+.+.}-{0:0}, at: __flush_work+0x25d/0x660
but task is already holding lock:
ffff888156815620 (&devcd->mutex){+.+.}-{3:3}, at: dev_coredump_put+0x3f/0xa0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&devcd->mutex){+.+.}-{3:3}:
mutex_lock_nested+0x4e/0xc0
devcd_data_write+0x27/0x90
sysfs_kf_bin_write+0x80/0xf0
kernfs_fop_write_iter+0x169/0x220
vfs_write+0x293/0x560
ksys_write+0x72/0xf0
__x64_sys_write+0x19/0x30
x64_sys_call+0x2bf/0x2660
do_syscall_64+0x93/0xb60
entry_SYSCALL_64_after_hwframe+0x76/0x7e
-> #1 (kn->active#236){++++}-{0:0}:
kernfs_drain+0x1e2/0x200
__kernfs_remove+0xae/0x400
kernfs_remove_by_name_ns+0x5d/0xc0
remove_files+0x54/0x70
sysfs_remove_group+0x3d/0xa0
sysfs_remove_groups+0x2e/0x60
device_remove_attrs+0xc7/0x100
device_del+0x15d/0x3b0
devcd_del+0x19/0x30
process_one_work+0x22b/0x6f0
worker_thread+0x1e8/0x3d0
kthread+0x11c/0x250
ret_from_fork+0x26c/0x2e0
ret_from_fork_asm+0x1a/0x30
-> #0 ((work_completion)(&(&devcd->del_wk)->work)){+.+.}-{0:0}:
__lock_acquire+0x1661/0x2860
lock_acquire+0xc4/0x2f0
__flush_work+0x27a/0x660
flush_delayed_work+0x5d/0xa0
dev_coredump_put+0x63/0xa0
xe_driver_devcoredump_fini+0x12/0x20 [xe]
devm_action_release+0x12/0x30
release_nodes+0x3a/0x120
devres_release_all+0x8a/0xd0
device_unbind_cleanup+0x12/0x80
device_release_driver_internal+0x23a/0x280
device_driver_detach+0x14/0x20
unbind_store+0xaf/0xc0
drv_attr_store+0x21/0x50
sysfs_kf_write+0x4a/0x80
kernfs_fop_write_iter+0x169/0x220
vfs_write+0x293/0x560
ksys_write+0x72/0xf0
__x64_sys_write+0x19/0x30
x64_sys_call+0x2bf/0x2660
do_syscall_64+0x93/0xb60
entry_SYSCALL_64_after_hwframe+0x76/0x7e
other info that might help us debug this:
Chain exists of: (work_completion)(&(&devcd->del_wk)->work) --> kn->active#236 --> &devcd->mutex
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&devcd->mutex);
lock(kn->active#236);
lock(&devcd->mutex);
lock((work_completion)(&(&devcd->del_wk)->work));
*** DEADLOCK ***
5 locks held by xe_fault_inject/5091:
#0: ffff8881129f9488 (sb_writers#5){.+.+}-{0:0}, at: ksys_write+0x72/0xf0
#1: ffff88810c755078 (&of->mutex#2){+.+.}-{3:3}, at: kernfs_fop_write_iter+0x123/0x220
#2: ffff8881054811a0 (&dev->mutex){....}-{3:3}, at: device_release_driver_internal+0x55/0x280
#3: ffff888156815620 (&devcd->mutex){+.+.}-{3:3}, at: dev_coredump_put+0x3f/0xa0
#4: ffffffff8359e020 (rcu_read_lock){....}-{1:2}, at: __flush_work+0x72/0x660
stack backtrace:
CPU: 14 UID: 0 PID: 5091 Comm: xe_fault_inject Tainted: G S U 6.16.0-rc6-lgci-xe-xe-pw-151626v3+ #1 PREEMPT_{RT,(lazy)}
Tainted: [S]=CPU_OUT_OF_SPEC, [U]=USER
Hardware name: Micro-Star International Co., Ltd. MS-7D25/PRO Z690-A DDR4(MS-7D25), BIOS 1.10 12/13/2021
Call Trace:
<TASK>
dump_stack_lvl+0x91/0xf0
dump_stack+0x10/0x20
print_circular_bug+0x285/0x360
check_noncircular+0x135/0x150
? register_lock_class+0x48/0x4a0
__lock_acquire+0x1661/0x2860
lock_acquire+0xc4/0x2f0
? __flush_work+0x25d/0x660
? mark_held_locks+0x46/0x90
? __flush_work+0x25d/0x660
__flush_work+0x27a/0x660
? __flush_work+0x25d/0x660
? trace_hardirqs_on+0x1e/0xd0
? __pfx_wq_barrier_func+0x10/0x10
flush_delayed_work+0x5d/0xa0
dev_coredump_put+0x63/0xa0
xe_driver_devcoredump_fini+0x12/0x20 [xe]
devm_action_release+0x12/0x30
release_nodes+0x3a/0x120
devres_release_all+0x8a/0xd0
device_unbind_cleanup+0x12/0x80
device_release_driver_internal+0x23a/0x280
? bus_find_device+0xa8/0xe0
device_driver_detach+0x14/0x20
unbind_store+0xaf/0xc0
drv_attr_store+0x21/0x50
sysfs_kf_write+0x4a/0x80
kernfs_fop_write_iter+0x169/0x220
vfs_write+0x293/0x560
ksys_write+0x72/0xf0
__x64_sys_write+0x19/0x30
x64_sys_call+0x2bf/0x2660
do_syscall_64+0x93/0xb60
? __f_unlock_pos+0x15/0x20
? __x64_sys_getdents64+0x9b/0x130
? __pfx_filldir64+0x10/0x10
? do_syscall_64+0x1a2/0xb60
? clear_bhb_loop+0x30/0x80
? clear_bhb_loop+0x30/0x80
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x76e292edd574
Code: c7 00 16 00 00 00 b8 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 80 3d d5 ea 0e 00 00 74 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 55 48 89 e5 48 83 ec 20 48 89
RSP: 002b:00007fffe247a828 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000076e292edd574
RDX: 000000000000000c RSI: 00006267f6306063 RDI: 000000000000000b
RBP: 000000000000000c R08: 000076e292fc4b20 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 00006267f6306063
R13: 000000000000000b R14: 00006267e6859c00 R15: 000076e29322a000
</TASK>
xe 0000:03:00.0: [drm] Xe device coredump has been deleted.
Fixes: 01daccf74832 ("devcoredump : Serialize devcd_del work")
Cc: Mukesh Ojha <quic_mojha@...cinc.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Johannes Berg <johannes@...solutions.net>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Danilo Krummrich <dakr@...nel.org>
Cc: linux-kernel@...r.kernel.org
Cc: <stable@...r.kernel.org> # v6.1+
Signed-off-by: Maarten Lankhorst <dev@...khorst.se>
Cc: Matthew Brost <matthew.brost@...el.com>
---
drivers/base/devcoredump.c | 136 ++++++++++++++++++++++---------------
1 file changed, 83 insertions(+), 53 deletions(-)
diff --git a/drivers/base/devcoredump.c b/drivers/base/devcoredump.c
index 03a39c417dc41..ad4bddde12ccb 100644
--- a/drivers/base/devcoredump.c
+++ b/drivers/base/devcoredump.c
@@ -23,50 +23,46 @@ struct devcd_entry {
void *data;
size_t datalen;
/*
- * Here, mutex is required to serialize the calls to del_wk work between
- * user/kernel space which happens when devcd is added with device_add()
- * and that sends uevent to user space. User space reads the uevents,
- * and calls to devcd_data_write() which try to modify the work which is
- * not even initialized/queued from devcoredump.
+ * There are 2 races for which mutex is required.
*
+ * The first race is between device creation and userspace writing to
+ * schedule immediately destruction.
*
+ * This race is handled by arming the timer before device creation, but
+ * when device creation fails the timer still exists.
*
- * cpu0(X) cpu1(Y)
+ * To solve this, hold the mutex during device_add(), and set
+ * init_completed on success before releasing the mutex.
*
- * dev_coredump() uevent sent to user space
- * device_add() ======================> user space process Y reads the
- * uevents writes to devcd fd
- * which results into writes to
+ * That way the timer will never fire until device_add() is called,
+ * it will do nothing if init_completed is not set. The timer is also
+ * cancelled in that case.
*
- * devcd_data_write()
- * mod_delayed_work()
- * try_to_grab_pending()
- * timer_delete()
- * debug_assert_init()
- * INIT_DELAYED_WORK()
- * schedule_delayed_work()
- *
- *
- * Also, mutex alone would not be enough to avoid scheduling of
- * del_wk work after it get flush from a call to devcd_free()
- * mentioned as below.
- *
- * disabled_store()
- * devcd_free()
- * mutex_lock() devcd_data_write()
- * flush_delayed_work()
- * mutex_unlock()
- * mutex_lock()
- * mod_delayed_work()
- * mutex_unlock()
- * So, delete_work flag is required.
+ * The second race involves multiple parallel invocations of devcd_free(),
+ * add a deleted flag so only 1 can call the destructor.
*/
struct mutex mutex;
- bool delete_work;
+ bool init_completed, deleted;
struct module *owner;
ssize_t (*read)(char *buffer, loff_t offset, size_t count,
void *data, size_t datalen);
void (*free)(void *data);
+ /*
+ * If nothing interferes and device_add() was returns success,
+ * del_wk will destroy the device after the timer fires.
+ *
+ * Multiple userspace processes can interfere in the working of the timer:
+ * - Writing to the coredump will reschedule the timer to run immediately,
+ * if still armed.
+ *
+ * This is handled by using "if (cancel_delayed_work()) {
+ * schedule_delayed_work() }", to prevent re-arming after having
+ * been previously fired.
+ * - Writing to /sys/class/devcoredump/disabled will destroy the
+ * coredump synchronously.
+ * This is handled by using disable_delayed_work_sync(), and then
+ * checking if deleted flag is set with &devcd->mutex held.
+ */
struct delayed_work del_wk;
struct device *failing_dev;
};
@@ -95,14 +91,27 @@ static void devcd_dev_release(struct device *dev)
kfree(devcd);
}
+static void __devcd_del(struct devcd_entry *devcd)
+{
+ devcd->deleted = true;
+ device_del(&devcd->devcd_dev);
+ put_device(&devcd->devcd_dev);
+}
+
static void devcd_del(struct work_struct *wk)
{
struct devcd_entry *devcd;
+ bool init_completed;
devcd = container_of(wk, struct devcd_entry, del_wk.work);
- device_del(&devcd->devcd_dev);
- put_device(&devcd->devcd_dev);
+ /* devcd->mutex serializes against dev_coredumpm_timeout */
+ mutex_lock(&devcd->mutex);
+ init_completed = devcd->init_completed;
+ mutex_unlock(&devcd->mutex);
+
+ if (init_completed)
+ __devcd_del(devcd);
}
static ssize_t devcd_data_read(struct file *filp, struct kobject *kobj,
@@ -122,12 +131,12 @@ static ssize_t devcd_data_write(struct file *filp, struct kobject *kobj,
struct device *dev = kobj_to_dev(kobj);
struct devcd_entry *devcd = dev_to_devcd(dev);
- mutex_lock(&devcd->mutex);
- if (!devcd->delete_work) {
- devcd->delete_work = true;
- mod_delayed_work(system_wq, &devcd->del_wk, 0);
- }
- mutex_unlock(&devcd->mutex);
+ /*
+ * Although it's tempting to use mod_delayed work here,
+ * that will cause a reschedule if the timer already fired.
+ */
+ if (cancel_delayed_work(&devcd->del_wk))
+ schedule_delayed_work(&devcd->del_wk, 0);
return count;
}
@@ -151,11 +160,21 @@ static int devcd_free(struct device *dev, void *data)
{
struct devcd_entry *devcd = dev_to_devcd(dev);
+ /*
+ * To prevent a race with devcd_data_write(), disable work and
+ * complete manually instead.
+ *
+ * We cannot rely on the return value of
+ * disable_delayed_work_sync() here, because it might be in the
+ * middle of a cancel_delayed_work + schedule_delayed_work pair.
+ *
+ * devcd->mutex here guards against multiple parallel invocations
+ * of devcd_free().
+ */
+ disable_delayed_work_sync(&devcd->del_wk);
mutex_lock(&devcd->mutex);
- if (!devcd->delete_work)
- devcd->delete_work = true;
-
- flush_delayed_work(&devcd->del_wk);
+ if (!devcd->deleted)
+ __devcd_del(devcd);
mutex_unlock(&devcd->mutex);
return 0;
}
@@ -179,12 +198,10 @@ static ssize_t disabled_show(const struct class *class, const struct class_attri
* put_device() <- last reference
* error = fn(dev, data) devcd_dev_release()
* devcd_free(dev, data) kfree(devcd)
- * mutex_lock(&devcd->mutex);
*
*
* In the above diagram, it looks like disabled_store() would be racing with parallelly
- * running devcd_del() and result in memory abort while acquiring devcd->mutex which
- * is called after kfree of devcd memory after dropping its last reference with
+ * running devcd_del() and result in memory abort after dropping its last reference with
* put_device(). However, this will not happens as fn(dev, data) runs
* with its own reference to device via klist_node so it is not its last reference.
* so, above situation would not occur.
@@ -374,7 +391,7 @@ void dev_coredumpm_timeout(struct device *dev, struct module *owner,
devcd->read = read;
devcd->free = free;
devcd->failing_dev = get_device(dev);
- devcd->delete_work = false;
+ devcd->deleted = false;
mutex_init(&devcd->mutex);
device_initialize(&devcd->devcd_dev);
@@ -383,8 +400,14 @@ void dev_coredumpm_timeout(struct device *dev, struct module *owner,
atomic_inc_return(&devcd_count));
devcd->devcd_dev.class = &devcd_class;
- mutex_lock(&devcd->mutex);
dev_set_uevent_suppress(&devcd->devcd_dev, true);
+
+ /* devcd->mutex prevents devcd_del() completing until init finishes */
+ mutex_lock(&devcd->mutex);
+ devcd->init_completed = false;
+ INIT_DELAYED_WORK(&devcd->del_wk, devcd_del);
+ schedule_delayed_work(&devcd->del_wk, timeout);
+
if (device_add(&devcd->devcd_dev))
goto put_device;
@@ -401,13 +424,20 @@ void dev_coredumpm_timeout(struct device *dev, struct module *owner,
dev_set_uevent_suppress(&devcd->devcd_dev, false);
kobject_uevent(&devcd->devcd_dev.kobj, KOBJ_ADD);
- INIT_DELAYED_WORK(&devcd->del_wk, devcd_del);
- schedule_delayed_work(&devcd->del_wk, timeout);
+
+ /*
+ * Safe to run devcd_del() now that we are done with devcd_dev.
+ * Alternatively we could have taken a ref on devcd_dev before
+ * dropping the lock.
+ */
+ devcd->init_completed = true;
mutex_unlock(&devcd->mutex);
return;
put_device:
- put_device(&devcd->devcd_dev);
mutex_unlock(&devcd->mutex);
+ cancel_delayed_work_sync(&devcd->del_wk);
+ put_device(&devcd->devcd_dev);
+
put_module:
module_put(owner);
free:
--
2.45.2
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