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Message-ID: <799a4e7e-076d-4a9d-888d-69f4e2e89520@lankhorst.se>
Date: Fri, 3 Oct 2025 15:29:22 +0200
From: Maarten Lankhorst <dev@...khorst.se>
To: linux-kernel@...r.kernel.org
Cc: intel-xe@...ts.freedesktop.org, 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: Re: [PATCH] devcoredump: Fix circular locking dependency with
devcd->mutex.
Ping?
Den 2025-07-23 kl. 16:24, skrev Maarten Lankhorst:
> 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:
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