Re: [PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)

From: Miklos Szeredi <miklos@szeredi.hu>
To: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Xie Yongji <xieyongji@bytedance.com>,
	fam.zheng@bytedance.com, Miklos Szeredi <mszeredi@redhat.com>
Subject: Re: [PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)
Date: Mon, 18 Jul 2022 10:18:18 +0200	[thread overview]
Message-ID: <CAJfpegu1uSMGrh==DS9+fbX+Gm8XaOyY3KTQ3xtZsbPEJo1M8A@mail.gmail.com> (raw)
In-Reply-To: <CAFQAk7jTJi_OcX=4nevbOquphcibtD=jG-jwwbC0KMJOfx9DeQ@mail.gmail.com>

On Mon, 18 Jul 2022 at 08:01, Jiachen Zhang
<zhangjiachen.jaycee@bytedance.com> wrote:
>
> On Fri, Jul 15, 2022 at 6:07 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Fri, 24 Jun 2022 at 07:58, Jiachen Zhang
> > <zhangjiachen.jaycee@bytedance.com> wrote:

> > > +       if (fc->writeback_cache_v2 && S_ISREG(inode->i_mode)) {
> > > +               inode_lock(inode);
> >
> > I don't think this can work.   fuse_change_attributes() might be
> > called from within inlode locked context.  E.g.
> >
> > lookup_slow -> __lookup_slow -> d_revalidate -> fuse_dentry_revalidate
> > -> fuse_change_attributes
> >
>
> Yes, this is a problem that should be fixed. As we can not check the
> inode lock state from the inode->i_rwsem structure, I think we can
> pass the inode lock state along the FUSE function call-path to
> fuse_change_attributes(), and only when we can certainly know whether
> the inode is locked or unlocked then we continue the
> writeback_cache_v2 logics. What do you think?

Not liking it very much.

Better create a new lock for this purpose that we do always know the state of.

Thanks,
Miklos