linux-kernel - Re: [PATCH] fs: assert on ->i_count in iput

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Message-ID: <CAGudoHHZL0g9=eRqjUOS2sez8Mew7r1TRWaR+uX-7YuYomd3WA@mail.gmail.com>
Date: Wed, 1 Oct 2025 14:12:13 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: assert on ->i_count in iput_final()

On Wed, Oct 1, 2025 at 2:07 PM Jan Kara <jack@...e.cz> wrote:
> > diff --git a/fs/inode.c b/fs/inode.c
> > index ec9339024ac3..fa82cb810af4 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -1879,6 +1879,7 @@ static void iput_final(struct inode *inode)
> >       int drop;
> >
> >       WARN_ON(inode->i_state & I_NEW);
> > +     VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
>
> This seems pointless given when iput_final() is called...
>

This and the other check explicitly "wrap" the ->drop_inode call.

> >       if (op->drop_inode)
> >               drop = op->drop_inode(inode);
> > @@ -1893,6 +1894,12 @@ static void iput_final(struct inode *inode)
> >               return;
> >       }
> >
> > +     /*
> > +      * Re-check ->i_count in case the ->drop_inode() hooks played games.
> > +      * Note we only execute this if the verdict was to drop the inode.
> > +      */
> > +     VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
> > +
>
> I'm not sure this can catch much but OK...
>

It can catch drop routines which bumped the ref but did not release
it, or which indicated to continue with drop while someone else
snatched the reference.

Preferaby the APIs would prevent that in the first place, but there is
quite a bit of shit-shoveling before that happens.