On 1.6.2023 19.05, Ricardo Ribalda wrote:
Hi Mathias
On Thu, 1 Jun 2023 at 16:13, Mathias Nyman
<mathias.nyman@xxxxxxxxxxxxxxx> wrote:
Do you still have the lockdep output showing the deadlock?
[ 459.731142] ======================================================
[ 459.731150] WARNING: possible circular locking dependency detected
[ 459.731161] 5.4.169-lockdep-17434-g505c8a10e6fe #1 Not tainted
[ 459.731168] ------------------------------------------------------
[ 459.731176] syz-executor.3/15308 is trying to acquire lock:
[ 459.731184] ffffff80c63e0ee0 (&queue->mutex){+.+.}, at:
uvc_queue_mmap+0x30/0xa0 [uvcvideo]
[ 459.731226]
but task is already holding lock:
[ 459.731232] ffffff80a748eea8 (&mm->mmap_sem){++++}, at:
vm_mmap_pgoff+0x10c/0x1f4
[ 459.731255]
which lock already depends on the new lock.
...
[ 459.732148] Chain exists of:
&queue->mutex --> &sb->s_type->i_mutex_key#4 --> &mm->mmap_sem
[ 459.732165] Possible unsafe locking scenario:
[ 459.732172] CPU0 CPU1
[ 459.732178] ---- ----
[ 459.732184] lock(&mm->mmap_sem);
[ 459.732193] lock(&sb->s_type->i_mutex_key#4);
[ 459.732204] lock(&mm->mmap_sem);
[ 459.732212] lock(&queue->mutex);
[ 459.732221]
*** DEADLOCK ***
I'm not sure how calling xhci_debugfs_create_endpoint() from
xhci_add_endpoint() instead of xhci_check_bandwidth() helps.
Both are called with hcd->bandwidth_mutex held:
usb_set_interface()
mutex_lock(hcd->bandwidth_mutex);
usb_hcd_alloc_bandwidth()
hcd->driver->add_endpoint() -> xhci_add_endpoint()
hcd->driver->check_bandwidth() -> xhci_check_bandwidth()
mutex_unlock(hcd->bandwidth_mutex);
Yep, I guess I was lucky not to be able to repro again :)
The locks involved are:
hcd->bandwidth_mutex
mm->mmap_sem
[uvc] queue->mutex
Ok, took a look at this.
I don't think the bandwidth mutex matters that much.
To my understanding this is caused by the following lock chains:
ucv_queue_mmap()
mmap_sem --> queue->mutex
uvc_ioctl_streamon() calling usb_set_interface() calling debugfs_create_dir()
queue->mutex --> i_mutex_key
Some debugfs error case:
i_mutex_key --> mmap_sem
So we could end up with this deadlock:
CPU0 CPU1 CPU2
mmap_sem queue->mutex i_mutex_key
waiting for waiting for waiting for
queue->mutex i_mutex_key mmap_sem
I have no idea if this can be triggered in real life.
Looks like that requires a some specific debugfs error
to trigger at the same time we are creating a debugfs directory
Thanks
Mathias
