Re: Kernel deadlock in 6.7.5 + hacks, maybe debugfs related.

[Johannes Berg <johannes@sipsolutions.net>]

On Tue, 2024-02-27 at 06:29 -0800, Ben Greear wrote:
> > --- a/fs/debugfs/inode.c
> > +++ b/fs/debugfs/inode.c
> > @@ -751,13 +751,19 @@ static void __debugfs_file_removed(struct dentry *dentry)
> >   	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
> >   		return;
> >   
> > -	/* if we hit zero, just wait for all to finish */
> > -	if (!refcount_dec_and_test(&fsd->active_users)) {
> > -		wait_for_completion(&fsd->active_users_drained);
> > -		return;
> > -	}
> > +	/*
> > +	 * Now that debugfs_file_get() no longer sees a valid entry,
> > +	 * decrement the refcount to remove the initial reference.
> > +	 */
> > +	refcount_dec(&fsd->active_users);

> 
> [   94.576688] ------------[ cut here ]------------
> [   94.576699] refcount_t: decrement hit 0; leaking memory.
> 

Ah ... right, refcount_dec() doesn't like to hit 0, it's not meant for
this path.

I guess we can

if (refcount_dec_and_test(...))
   return;

while (refcount_read(...)) { ... }

johannes