Re: [PATCH] netdevsim: fix deadlock in nsim_bus_dev_max_vfs_write()

[Slawomir Stepien]

On sie 04, 2026 16:33, Paolo Abeni wrote:
> On 7/31/26 11:55 AM, syzbot wrote:
> > From: Slawomir Stepien <sst@xxxxxxxxx>
> > 
> > There is an ABBA deadlock between the devlink instance lock and the debugfs
> > active file reference mechanism.
> > 
> > When a device is being removed (e.g., via nsim_drv_remove()), the driver
> > acquires the devlink instance lock and then calls
> > debugfs_remove_recursive(). This function blocks waiting for all active
> > file operations on the debugfs files to complete.
> > 
> > Concurrently, if a user writes to the "max_vfs" debugfs file, the VFS layer
> > acquires an active reference to the file. The write handler,
> > nsim_bus_dev_max_vfs_write(), then attempts to acquire the devlink instance
> > lock, which is already held by the removal task.
> > 
> > This creates a circular dependency resulting in a deadlock:
> > 
> > INFO: task blocked for more than 143 seconds.
> > Call Trace:
> >  wait_for_completion+0x2ca/0x5e0 kernel/sched/completion.c:153
> >  __debugfs_file_removed fs/debugfs/inode.c:751 [inline]
> >  remove_one+0x2df/0x3b0 fs/debugfs/inode.c:758
> >  __simple_recursive_removal+0x215/0x520 fs/libfs.c:623
> >  debugfs_remove+0x5b/0x70 fs/debugfs/inode.c:781
> >  nsim_dev_debugfs_exit drivers/net/netdevsim/dev.c:372 [inline]
> >  nsim_drv_remove+0xc0/0x170 drivers/net/netdevsim/dev.c:1803
> > 
> > INFO: task blocked for more than 143 seconds.
> > Call Trace:
> >  __mutex_lock+0x7bf/0x1550 kernel/locking/mutex.c:821
> >  nsim_bus_dev_max_vfs_write+0x229/0x3d0 drivers/net/netdevsim/dev.c:276
> >  full_proxy_write+0x127/0x1f0 fs/debugfs/file.c:388
> >  vfs_write+0x296/0xba0 fs/read_write.c:685
> > 
> > To fix this, use devl_trylock() in nsim_bus_dev_max_vfs_write() instead of
> > devl_lock(). If the lock cannot be acquired, return -EBUSY. This aborts the
> > write operation, releases the debugfs active file reference, and allows the
> > pending debugfs_remove_recursive() to proceed.
> > 
> > Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
> > Reported-by: syzbot+3147c5de186107ffc7a1@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=3147c5de186107ffc7a1
> > Link: https://syzkaller.appspot.com/ai_job?id=fcdda182-bebf-49ab-ada0-d4d2e814ebf7
> > Signed-off-by: Slawomir Stepien <sst@xxxxxxxxx>
> > 
> > ---
> > diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
> > index aed9ad5f1..421cd7327 100644
> > --- a/drivers/net/netdevsim/dev.c
> > +++ b/drivers/net/netdevsim/dev.c
> > @@ -273,7 +273,11 @@ static ssize_t nsim_bus_dev_max_vfs_write(struct file *file,
> >  		return -ENOMEM;
> >  
> >  	nsim_dev = file->private_data;
> > -	devl_lock(priv_to_devlink(nsim_dev));
> > +	if (!devl_trylock(priv_to_devlink(nsim_dev))) {
> > +		ret = -EBUSY;
> 
> I think this should return restart_syscall(). This kind of schema is
> calling for trouble, but for netdevsim should be okish.

Thanks Paolo for your comment!

Can you explain a bit more? What is your reasoning here? Is it, that we expect that we could have
the lock in e.g. 1ns, so it is worth restarting without bothering userspace[1]?

I see a lot of:

if (!rtnl_trylock())
	return restart_syscall();

Why this pattern is so popular? Is it for the same reason as above?

[1] https://kernel-internals.org/syscalls/restart-block/

-- 
Slawomir Stepien