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From: Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
To: David Sterba <dsterba-AlSwsSmVLrQ@public.gmane.org>
Cc: dwarves-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: Brain fart alert
Date: Wed, 11 Nov 2015 19:13:40 -0300
Message-ID: <20151111221340.GB12579@kernel.org> (raw)
In-Reply-To: <20151111210519.GG6445-AlSwsSmVLrQ@public.gmane.org>

Em Wed, Nov 11, 2015 at 10:05:19PM +0100, David Sterba escreveu:
> On Wed, Nov 11, 2015 at 05:11:51PM -0300, Arnaldo Carvalho de Melo wrote:
> > > struct btrfs_path {
> ...
> > >         u8                         reada;                /*   128     1 */
> > >         u8                         lowest_level;         /*   129     1 */
> ...
> > > };
...
> > 	int                        reada;                /*   128     4 */
> > 	int                        lowest_level;         /*   132     4 */
> > 	unsigned int               search_for_split:1;   /*   136:31  4 */
> > 	unsigned int               keep_locks:1;         /*   136:30  4 */
...
> > Ok, but then 'reada' and 'lowest_level' are both int in my file, lemme try
> > converting it and rebuilding...
 
> Yes, they're both 'int' in current git versions, I'm trying to reduce
> size of the structure and using pahole to see the result. When both
> members were switched to u8 the warning appeared. Sorry for confusion.

So, I did the change and got the same BFA, with:

gcc version 4.9.2 20150212 (Red Hat 4.9.2-6) (GCC) 

I'm trying to figure out how this can make sense:

	u8                reada;                   /*   128     1 */
	u8                lowest_level;            /*   129     1 */

	/* Bitfield combined with previous fields */

	unsigned int      search_for_split:1;      /*   128:15  4 */
	unsigned int      keep_locks:1;            /*   128:14  4 */
	unsigned int      skip_locking:1;          /*   128:13  4 */
	unsigned int      leave_spinning:1;        /*   128:12  4 */
	unsigned int      search_commit_root:1;    /*   128:11  4 */
	unsigned int      need_commit_sem:1;       /*   128:10  4 */
	unsigned int      skip_release_on_error:1; /*   128: 9  4 */

I.e. it is like skip_release_on_error uses the first bit of 'lowest_level',
i.e. the ninth bit starting at byte offset 128... I was expecting it to be:

	u8                reada;                   /*   128     1 */
	u8                lowest_level;            /*   129     1 */

	/* Bitfield combined with previous fields */

	unsigned int      search_for_split:1;      /*   128:22  4 */
	unsigned int      keep_locks:1;            /*   128:21  4 */
	unsigned int      skip_locking:1;          /*   128:20  4 */
	unsigned int      leave_spinning:1;        /*   128:19  4 */
	unsigned int      search_commit_root:1;    /*   128:18  4 */
	unsigned int      need_commit_sem:1;       /*   128:17  4 */
	unsigned int      skip_release_on_error:1; /*   128:16  4 */

Because by combining the bitfield with the two u8 before it, it would read as:

	unsigned int      reada:8;                 /*   128:0   4 */
	unsigned int      lowest_level:8;          /*   128:8   4 */


:-\

Looking at the readelf output now...

- Arnaldo

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  parent reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-11-11 18:26 David Sterba
     [not found] ` <20151111182618.GF6445-AlSwsSmVLrQ@public.gmane.org>
2015-11-11 20:11   ` Arnaldo Carvalho de Melo
     [not found]     ` <20151111201151.GA6712-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2015-11-11 21:05       ` David Sterba
     [not found]         ` <20151111210519.GG6445-AlSwsSmVLrQ@public.gmane.org>
2015-11-11 22:13           ` Arnaldo Carvalho de Melo [this message]
     [not found]             ` <20151111221340.GB12579-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2015-11-11 22:17               ` Arnaldo Carvalho de Melo
     [not found]                 ` <20151111221747.GC12579-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2015-11-11 22:48                   ` Arnaldo Carvalho de Melo
  -- strict thread matches above, loose matches on Subject: below --
2012-07-21 18:22 Nicolas
     [not found] ` <CACHCVZ6oDyFwGbC4EwF_wsR9tCkoych5N6buLhRRLcyaL6iifw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2012-08-15 17:56   ` Arnaldo Carvalho de Melo
     [not found]     ` <20120815175647.GK2370-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2012-08-16 19:52       ` Arnaldo Carvalho de Melo
     [not found]         ` <20120816195224.GC10277-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2012-08-19 22:16           ` Mark Wielaard
2012-08-20 13:45         ` Dodji Seketeli
     [not found]           ` <m3obm5fzhs.fsf-fIDRvF7C5exg9hUCZPvPmw@public.gmane.org>
2012-08-20 17:35             ` Arnaldo Carvalho de Melo
2012-08-20 18:02   ` Arnaldo Carvalho de Melo
     [not found]     ` <20120820180216.GE3046-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2012-08-20 18:28       ` Arnaldo Carvalho de Melo
2012-08-30 16:35       ` Nicolas
     [not found]         ` <CACHCVZ5EJadxOTJ0T4phSfAcm1FK7wfeDTp9QcSa1V1JCpa8SA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2012-09-03 12:22           ` Mark Wielaard
     [not found]             ` <1346674948.3090.40.camel-OO0OHOuVXW89n/EXqpbZ8ns8WZQLr0HW@public.gmane.org>
2012-09-24 22:19               ` Nicolas
     [not found]                 ` <5060DC56.4080609-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2013-02-26  1:50                   ` Nicolas
     [not found]                     ` <CACHCVZ6uvDW1CpyxXio3P17EL6bE=FKGDNm11RNZTceR-dWLjg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2013-02-26 10:56                       ` Mark Wielaard
     [not found]                         ` <1361876163.4930.4.camel-Et2QWsRS+g7yukXOleVwRPp4M6BF03vq@public.gmane.org>
2013-03-20 13:37                           ` Arnaldo Carvalho de Melo

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