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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 10 Aug 2020 10:29:13 +0200
Message-ID: <CAJfpegs1CtPMaVmbDc__N0Fc3FkNEe=vQOkrr8RKiiS6NqTyHA@mail.gmail.com>
Subject: Re: [PATCH v2 18/20] fuse: Release file in process context
To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        virtio-fs-list <virtio-fs@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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On Fri, Aug 7, 2020 at 9:55 PM Vivek Goyal <vgoyal@redhat.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.

Thanks,
Miklos