linux-kernel - Re: [PATCH v2 18/20] fuse: Release file in process context

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Date:   Mon, 10 Aug 2020 21:39:46 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v2 18/20] fuse: Release file in process context

On Mon, Aug 10, 2020 at 9:37 PM Vivek Goyal <vgoyal@...hat.com> wrote:
>
> On Mon, Aug 10, 2020 at 10:29:13AM +0200, Miklos Szeredi wrote:
> > On Fri, Aug 7, 2020 at 9:55 PM Vivek Goyal <vgoyal@...hat.com> wrote:
> > >
> > > fuse_file_put(sync) can be called with sync=true/false. If sync=true,
> > > it waits for release request response and then calls iput() in the
> > > caller's context. If sync=false, it does not wait for release request
> > > response, frees the fuse_file struct immediately and req->end function
> > > does the iput().
> > >
> > > iput() can be a problem with DAX if called in req->end context. If this
> > > is last reference to inode (VFS has let go its reference already), then
> > > iput() will clean DAX mappings as well and send REMOVEMAPPING requests
> > > and wait for completion. (All the the worker thread context which is
> > > processing fuse replies from daemon on the host).
> > >
> > > That means it blocks worker thread and it stops processing further
> > > replies and system deadlocks.
> >
> > Is this reasoning specific to DAX?  Seems to me this is a virtio-fs
> > specific issue.
>
> Hi Miklos,
>
> I am looking at this patch closely and maybe we don't need it, given
> current code.
>
> Reason being that looks like for virtiofs now we set ctx->destroy=true
> and that sets fc->destroy. And if that's set, we don't schedule async req
> in fuse_release_common();
>
> fuse_release_common() {
>   fuse_file_put(ff, ff->fc->destroy, isdir);
> }
>
> And if we don't schedule async file put, we don't run the risk of
> blocking worker thread in evict_inode() with DAX enabled.
>
> I ran blogbench multiple times with this patch reverted and I can't
> reproduce the deadlock.
>
> So we can drop this patch for now. If need be I will fix it as
> post merge patch.

Okay.  Yeah, I was thinking along the lines of no-async-release-ever
for virtiofs, but apparently that's already done.

Thanks,
Miklos