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Date:   Wed, 9 Dec 2020 20:35:00 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-fsdevel@vger.kernel.org
Subject: Re: fs/namei.c: Make status likely to be ECHILD in lookup_fast()
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On Wed, Dec 09, 2020 at 03:24:03PM -0500, Steven Rostedt wrote:
> From:  Steven Rostedt (VMware) <rostedt@goodmis.org>
> 
> Running my yearly branch profiling code, it detected a 100% wrong branch
> condition in name.c for lookup_fast(). The code in question has:
> 
> 		status = d_revalidate(dentry, nd->flags);
> 		if (likely(status > 0))
> 			return dentry;
> 		if (unlazy_child(nd, dentry, seq))
> 			return ERR_PTR(-ECHILD);
> 		if (unlikely(status == -ECHILD))
> 			/* we'd been told to redo it in non-rcu mode */
> 			status = d_revalidate(dentry, nd->flags);
> 
> If the status of the d_revalidate() is greater than zero, then the function
> finishes. Otherwise, if it is an "unlazy_child" it returns with -ECHILD.
> After the above two checks, the status is compared to -ECHILD, as that is
> what is returned if the original d_revalidate() needed to be done in a
> non-rcu mode.
> 
> Especially this path is called in a condition of:
> 
> 	if (nd->flags & LOOKUP_RCU) {
> 
> And most of the d_revalidate() functions have:
> 
> 	if (flags & LOOKUP_RCU)
> 		return -ECHILD;

Umm...  That depends upon the filesystem mix involved; said that, I'd rather
drop that "unlikely"...