linux-kernel - Re: [PATCH] staging: exfat: add exfat filesystem code to staging

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Date:   Fri, 30 Aug 2019 14:26:12 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     devel@...verdev.osuosl.org,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Valdis Klētnieks <valdis.kletnieks@...edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [PATCH] staging: exfat: add exfat filesystem code to staging

On Fri, Aug 30, 2019 at 04:43:33PM +0800, Gao Xiang wrote:
> Hi Dan,
> 
> On Fri, Aug 30, 2019 at 11:34:45AM +0300, Dan Carpenter wrote:
> > On Fri, Aug 30, 2019 at 12:04:41AM +0800, Gao Xiang wrote:
> > > Anyway, I'm fine to delete them all if you like, but I think majority of these
> > > are meaningful.
> > > 
> > > data.c-		/* page is already locked */
> > > data.c-		DBG_BUGON(PageUptodate(page));
> > > data.c-
> > > data.c:		if (unlikely(err))
> > > data.c-			SetPageError(page);
> > > data.c-		else
> > > data.c-			SetPageUptodate(page);
> > 
> > If we cared about speed here then we would delete the DBG_BUGON() check
> > because that's going to be expensive.  The likely/unlikely annotations
> > should be used in places a reasonable person thinks it will make a
> > difference to benchmarks.
> 
> DBG_BUGON will be a no-op ((void)x) in non-debugging mode,

It expands to:

	((void)PageUptodate(page));

Calling PageUptodate() doesn't do anything, but it isn't free.  The
time it takes to do that function call completely negates any speed up
from using likely/unlikely.

I'm really not trying to be a jerk...

regards,
dan carpenter