linux-kernel - Re: [PATCH][RESEND] vfs: serialize updates to file->f_sb_err with f

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Date:   Mon, 04 Jan 2021 14:00:18 -0500
From:   Jeff Layton <jlayton@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        sargun@...gun.me, amir73il@...il.com, vgoyal@...hat.com
Subject: Re: [PATCH][RESEND] vfs: serialize updates to file->f_sb_err with
 f_lock

On Mon, 2021-01-04 at 18:57 +0000, Al Viro wrote:
> On Mon, Jan 04, 2021 at 01:43:47PM -0500, Jeff Layton wrote:
> > @@ -172,7 +172,12 @@ SYSCALL_DEFINE1(syncfs, int, fd)
> >  	ret = sync_filesystem(sb);
> >  	up_read(&sb->s_umount);
> >  
> > 
> > -	ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
> > +	if (errseq_check(&sb->s_wb_err, f.file->f_sb_err)) {
> > +		/* Something changed, must use slow path */
> > +		spin_lock(&f.file->f_lock);
> > +		ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
> > +		spin_unlock(&f.file->f_lock);
> > +	}
> 
> 	Is there any point bothering with the fastpath here?
> I mean, look at the up_read() immediately prior to that thing...

It is a micro-optimization, but the vastly common case is that we will
avoid the spinlock there. That said, I'm fine with dropping the fastpath
if you prefer.

-- 
Jeff Layton <jlayton@...nel.org>