linux-kernel - Re: [PATCH 4/4] staging: lustre: obdclass: change object lookup to no wait mode

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Date:   Wed, 16 May 2018 08:44:06 -0700
From:   Joe Perches <joe@...ches.com>
To:     "Dilger, Andreas" <andreas.dilger@...el.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Cc:     James Simmons <jsimmons@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "Drokin, Oleg" <oleg.drokin@...el.com>,
        "Siyao, Lai" <lai.siyao@...el.com>, NeilBrown <neilb@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lustre Development List <lustre-devel@...ts.lustre.org>
Subject: Re: [PATCH 4/4] staging: lustre: obdclass: change object lookup to
 no wait mode

On Wed, 2018-05-16 at 09:12 +0000, Dilger, Andreas wrote:
> On May 16, 2018, at 02:00, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > 
> > On Tue, May 15, 2018 at 04:02:55PM +0100, James Simmons wrote:
> > > 
> > > > > 	/*
> > > > > 	 * Allocate new object. This may result in rather complicated
> > > > > 	 * operations, including fld queries, inode loading, etc.
> > > > > 	 */
> > > > > 	o = lu_object_alloc(env, dev, f, conf);
> > > > > -	if (IS_ERR(o))
> > > > > +	if (unlikely(IS_ERR(o)))
> > > > > 		return o;
> > > > > 
> > > > 
> > > > This is an unrelated and totally pointless.  likely/unlikely annotations
> > > > hurt readability, and they should only be added if it's something which
> > > > is going to show up in benchmarking.  lu_object_alloc() is already too
> > > > slow for the unlikely() to make a difference and anyway IS_ERR() has an
> > > > unlikely built in so it's duplicative...
> > > 
> > > Sounds like a good checkpatch case to test for :-)
> > 
> > The likely/unlikely annotations have their place in fast paths so a
> > checkpatch warning would get annoying...
> 
> I think James was suggesting a check for unlikely(IS_ERR()),

Probably so.

$ git grep -P 'likely\s*\(\s*\!?\s*IS_ERR' | wc -l
42

Are there other known likely/unlikely duplications?

>  or possibly
> a check for unlikely() on something that is already unlikely() after CPP
> expansion.

checkpatch isn't the tool for that type of test
as it is a collection of trivial regex tests and
it is not a c90 preprocessor.

Anyway, here's a possible checkpatch patch.
---
 scripts/checkpatch.pl | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index baddac9379f0..20c0973f1c39 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -6299,6 +6299,12 @@ sub process {
 			      "#define of '$1' is wrong - use Kconfig variables or standard guards instead\n" . $herecurr);
 		}
 
+# likely/unlikely tests with IS_ERR (already unlikely)"
+		if ($line =~ /\b((?:un)?likely)\s*\(\s*\!?\s*(IS_ERR[A-Z_]*)\s*\(/) {
+			WARN("DUPLICATE_LIKELY",
+			     "Unnecessary use of $1 with $2 as it already has an unlikely\n" . $herecurr);
+		}
+
 # likely/unlikely comparisons similar to "(likely(foo) > 0)"
 		if ($^V && $^V ge 5.10.0 &&
 		    $line =~ /\b((?:un)?likely)\s*\(\s*$FuncArg\s*\)\s*$Compare/) {