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Date:   Tue, 22 Dec 2020 17:46:37 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-unionfs@vger.kernel.org, jlayton@kernel.org,
        amir73il@gmail.com, sargun@sargun.me, miklos@szeredi.hu,
        jack@suse.cz, neilb@suse.com, viro@zeniv.linux.org.uk, hch@lst.de
Subject: Re: [PATCH 3/3] overlayfs: Report writeback errors on upper
Message-ID: <20201222174637.GK874@casper.infradead.org>
References: <20201221195055.35295-1-vgoyal@redhat.com>
 <20201221195055.35295-4-vgoyal@redhat.com>
 <20201222162027.GJ874@casper.infradead.org>
 <20201222162925.GC3248@redhat.com>
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On Tue, Dec 22, 2020 at 11:29:25AM -0500, Vivek Goyal wrote:
> On Tue, Dec 22, 2020 at 04:20:27PM +0000, Matthew Wilcox wrote:
> > On Mon, Dec 21, 2020 at 02:50:55PM -0500, Vivek Goyal wrote:
> > > +static int ovl_errseq_check_advance(struct super_block *sb, struct file *file)
> > > +{
> > > +	struct ovl_fs *ofs = sb->s_fs_info;
> > > +	struct super_block *upper_sb;
> > > +	int ret;
> > > +
> > > +	if (!ovl_upper_mnt(ofs))
> > > +		return 0;
> > > +
> > > +	upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
> > > +
> > > +	if (!errseq_check(&upper_sb->s_wb_err, file->f_sb_err))
> > > +		return 0;
> > > +
> > > +	/* Something changed, must use slow path */
> > > +	spin_lock(&file->f_lock);
> > > +	ret = errseq_check_and_advance(&upper_sb->s_wb_err, &file->f_sb_err);
> > > +	spin_unlock(&file->f_lock);
> > 
> > Why are you microoptimising syncfs()?  Are there really applications which
> > call syncfs() in a massively parallel manner on the same file descriptor?
> 
> This is atleast theoritical race. I am not aware which application can
> trigger this race. So to me it makes sense to fix the race.
> 
> Jeff Layton also posted a fix for syncfs().
> 
> https://lore.kernel.org/linux-fsdevel/20201219134804.20034-1-jlayton@kernel.org/
> 
> To me it makes sense to fix the race irrespective of the fact if somebody
> hit it or not. People end up copying code in other parts of kernel and
> and they will atleast copy race free code.

Let me try again.  "Why are you trying to avoid taking the spinlock?"