LinuxLists.cc - please pull from the trivial tree

2006-12-01 11:37:37

Subject: please pull from the trivial tree

2006-12-01 12:39:44

by Tim Schmielau

[permalink] [raw]

Subject: Re: please pull from the trivial tree

> Chase Venters (1):
> Fix jiffies.h comment

This one actually obscures the comment rather than fixing it.

>From jiffies.h:
> 76 /*
> 77 * The 64-bit value is not volatile - you MUST NOT read it
> 78 * without sampling the sequence number in xtime_lock.
> 79 * get_jiffies_64() will do this for you as appropriate.
> 80 */
> 81 extern u64 __jiffy_data jiffies_64;
> 82 extern unsigned long volatile __jiffy_data jiffies;

Note that jiffies is volatile, while jiffies_64 is not; the comment
currently explains that. The proposed patch

> Fix jiffies.h comment
> jiffies.h includes a comment informing that jiffies_64 must be read with the
> assistance of the xtime_lock seqlock. The comment text, however, calls
> jiffies_64 "not volatile", which should probably read "not atomic".
>
> --- a/include/linux/jiffies.h
> +++ b/include/linux/jiffies.h
> @@ -74,7 +74,7 @@
> #define __jiffy_data __attribute__((section(".data")))
> /*
> - * The 64-bit value is not volatile - you MUST NOT read it
> + * The 64-bit value is not atomic - you MUST NOT read it
> * without sampling the sequence number in xtime_lock.
> * get_jiffies_64() will do this for you as appropriate.
> */

would leave a comment that is correct, but less useful (I'd expect any
kernel hacker to know that u64 is non-atomic on many platforms).

Tim

2006-12-01 12:55:00

by Adrian Bunk

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Subject: Re: please pull from the trivial tree

On Fri, Dec 01, 2006 at 01:41:18PM +0100, Tim Schmielau wrote:
> > Chase Venters (1):
> > Fix jiffies.h comment
>
> This one actually obscures the comment rather than fixing it.
>
> >From jiffies.h:
> > 76 /*
> > 77 * The 64-bit value is not volatile - you MUST NOT read it
> > 78 * without sampling the sequence number in xtime_lock.
> > 79 * get_jiffies_64() will do this for you as appropriate.
> > 80 */
> > 81 extern u64 __jiffy_data jiffies_64;
> > 82 extern unsigned long volatile __jiffy_data jiffies;
>
> Note that jiffies is volatile, while jiffies_64 is not; the comment
> currently explains that. The proposed patch
>
> > Fix jiffies.h comment
> > jiffies.h includes a comment informing that jiffies_64 must be read with the
> > assistance of the xtime_lock seqlock. The comment text, however, calls
> > jiffies_64 "not volatile", which should probably read "not atomic".
> >
> > --- a/include/linux/jiffies.h
> > +++ b/include/linux/jiffies.h
> > @@ -74,7 +74,7 @@
> > #define __jiffy_data __attribute__((section(".data")))
> > /*
> > - * The 64-bit value is not volatile - you MUST NOT read it
> > + * The 64-bit value is not atomic - you MUST NOT read it
> > * without sampling the sequence number in xtime_lock.
> > * get_jiffies_64() will do this for you as appropriate.
> > */
>
> would leave a comment that is correct, but less useful (I'd expect any
> kernel hacker to know that u64 is non-atomic on many platforms).

If kernel hackers are expected to already know it's non-atomic we could
remove the whole comment.

The comment regarding "volatile" was bogus since "volatile" wouldn't
help against getting garbage when reading an u64 variable.

> Tim

cu
Adrian

--

"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed

2006-12-01 13:08:21

by Tim Schmielau

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Subject: Re: please pull from the trivial tree

On Fri, 1 Dec 2006, Adrian Bunk wrote:

> On Fri, Dec 01, 2006 at 01:41:18PM +0100, Tim Schmielau wrote:
[...]
> > would leave a comment that is correct, but less useful (I'd expect any
> > kernel hacker to know that u64 is non-atomic on many platforms).
>
>
> If kernel hackers are expected to already know it's non-atomic we could
> remove the whole comment.
>
> The comment regarding "volatile" was bogus since "volatile" wouldn't
> help against getting garbage when reading an u64 variable.

Well, it's probably too trivial to argue about anyways - I don't mind much
in either direction.

Tim