[PATCH v2 ] devcoredump : Serialize devcd_del work

Mukesh Ojha
2022-04-25T13:10:40Z

In following scenario(diagram), when one thread X running dev_coredumpm() adds devcd
device to the framework which sends uevent notification to userspace
and another thread Y reads this uevent and call to devcd_data_write()
which eventually try to delete the queued timer that is not initialized/queued yet.

So, debug object reports some warning and in the meantime, timer is initialized
and queued from X path. and from Y path, it gets reinitialized again and
timer->entry.pprev=NULL and try_to_grab_pending() stucks.

To fix this, introduce mutex to serialize the behaviour.

cpu0(X) cpu1(Y)

dev_coredump() uevent sent to userspace
device_add() =========================> userspace process Y reads the uevents
writes to devcd fd which
results into writes to

devcd_data_write()
mod_delayed_work()
try_to_grab_pending()
del_timer()
debug_assert_init()
INIT_DELAYED_WORK
schedule_delayed_work
debug_object_fixup()
timer_fixup_assert_init()
timer_setup()
do_init_timer()   ==> reinitialized the
timer to
timer->entry.pprev=NULL

timer_pending()
!hlist_unhashed_lockless(&timer->entry)
!h->pprev ==> del_timer checks
and finds it to be NULL
try_to_grab_pending() stucks.

Link: https://lore.kernel.org/lkml/2e1f81e2-428c-f11f-ce92-eb11048cb271@quicinc.com/
Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
---
v1->v2:
- Added del_wk_queued to serialize the race between devcd_data_write()
and disabled_store().

drivers/base/devcoredump.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/base/devcoredump.c b/drivers/base/devcoredump.c
index f4d794d..3e6fd6b 100644
--- a/drivers/base/devcoredump.c
+++ b/drivers/base/devcoredump.c
@@ -25,6 +25,8 @@ struct devcd_entry {
struct device devcd_dev;
void *data;
size_t datalen;
+ struct mutex mutex;
+ bool del_wk_queued;
struct module *owner;
ssize_t (*read)(char *buffer, loff_t offset, size_t count,
void *data, size_t datalen);
@@ -84,7 +86,12 @@ 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);

- mod_delayed_work(system_wq, &devcd->del_wk, 0);
+ mutex_lock(&devcd->mutex);
+ if (!devcd->del_wk_queued) {
+ devcd->del_wk_queued = true;
+ mod_delayed_work(system_wq, &devcd->del_wk, 0);
+ }
+ mutex_unlock(&devcd->mutex);

return count;
}
@@ -112,7 +119,12 @@ static int devcd_free(struct device *dev, void *data)
{
struct devcd_entry *devcd = dev_to_devcd(dev);

+ mutex_lock(&devcd->mutex);
+ if (!devcd->del_wk_queued)
+ devcd->del_wk_queued = true;
+
flush_delayed_work(&devcd->del_wk);
+ mutex_unlock(&devcd->mutex);
return 0;
}

@@ -278,13 +290,15 @@ void dev_coredumpm(struct device *dev, struct module *owner,
devcd->read = read;
devcd->free = free;
devcd->failing_dev = get_device(dev);
-
+ mutex_init(&devcd->mutex);
device_initialize(&devcd->devcd_dev);

dev_set_name(&devcd->devcd_dev, "devcd%d",
atomic_inc_return(&devcd_count));
devcd->devcd_dev.class = &devcd_class;

+ mutex_lock(&devcd->mutex);
+ devcd->del_wk_queued = false;
if (device_add(&devcd->devcd_dev))
goto put_device;

@@ -301,10 +315,11 @@ void dev_coredumpm(struct device *dev, struct module *owner,

INIT_DELAYED_WORK(&devcd->del_wk, devcd_del);
schedule_delayed_work(&devcd->del_wk, DEVCD_TIMEOUT);
-
+ mutex_unlock(&devcd->mutex);
return;
put_device:
put_device(&devcd->devcd_dev);
+ mutex_unlock(&devcd->mutex);
put_module:
module_put(owner);
free:
--
2.7.4