Re: [PATCH] fence: Use smp_mb__before_atomic()

[Sumit Semwal]

Hi Greg,

On 5 June 2014 21:18, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jun 05, 2014 at 07:51:10AM -0400, Rob Clark wrote:
>> On Wed, Jun 4, 2014 at 1:49 PM, Greg Kroah-Hartman
>> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Wed, Jun 04, 2014 at 03:28:33PM +0200, Thierry Reding wrote:
>> >> On Wed, Jun 04, 2014 at 04:57:07PM +0530, Sumit Semwal wrote:
>> >> > Hi Greg,
>> >> >
>> >> >
>> >> > On 30 May 2014 21:38, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> >> > > On Fri, May 30, 2014 at 10:15:05AM +0200, Thierry Reding wrote:
>> >> > >> On Wed, May 28, 2014 at 01:51:45PM -0700, Greg Kroah-Hartman wrote:
>> >> > >> > On Wed, May 28, 2014 at 04:26:32PM +0200, Thierry Reding wrote:
>> >> > >> > > From: Thierry Reding <treding@xxxxxxxxxx>
>> >> > >> > >
>> >> > >> > > Commit febdbfe8a91c (arch: Prepare for smp_mb__{before,after}_atomic())
>> >> > >> > > deprecated the smp_mb__{before,after}_{atomic,clear}_{dec,inc,bit}*()
>> >> > >> > > functions in favour of the unified smp_mb__{before,after}_atomic().
>> >> > >> > >
>> >> > >> > > Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
>> >> > >> > > ---
>> >> > >> > >  drivers/base/fence.c | 4 ++--
>> >> > >> >
>> >> > >> > Where does this file come from?  I've not seen it before, and it's not
>> >> > >> > in my tree.
>> >> > >>
>> >> > >> I think it came in through Sumit's tree and it's only in linux-next I
>> >> > >> believe.
>> >> > >
>> >> > > Odd, linux-next is for merging things in Linus's next release.
>> >> > >
>> >> > > And as I have never seen this code that will end up being my
>> >> > > responsibility to maintain, it seems strange that it will be merged in
>> >> > > the next kernel development cycle.
>> >> > >
>> >> > > What broke down here with our review process that required something to
>> >> > > be merged without at least a cc: to me?
>> >> >
>> >> > This is a new file added by Maarten's patches [1], that got reviewed
>> >> > on dri-devel and other mailing lists. Since it was quite closely
>> >> > associated with dma-buf, I figured I should take it through the
>> >> > dma-buf tree.
>> >> >
>> >> > I am sorry I didn't notice that you weren't CC'ed on these patches -
>> >> > Sincere apologies, since I should've noticed that during the patch
>> >> > review process - I would take part of the blame here as well  :(
>> >> >
>> >> > I do realize now that atleast on my part, I should've asked you before
>> >> > taking it through the dma-buf tree - I will make sure things like this
>> >> > don't happen again through me.
>> >> >
>> >> > May I request you to help us handle this - would it help if we add
>> >> > Maarten as the maintainer for this file? Any other suggestions?
>> >>
>> >> Perhaps something like the following would help?
>> >>
>> >> diff --git a/MAINTAINERS b/MAINTAINERS
>> >> index fb39c9c3f0c1..d582f54adec8 100644
>> >> --- a/MAINTAINERS
>> >> +++ b/MAINTAINERS
>> >> @@ -2867,7 +2867,9 @@ L:        linux-media@xxxxxxxxxxxxxxx
>> >>  L:   dri-devel@xxxxxxxxxxxxxxxxxxxxx
>> >>  L:   linaro-mm-sig@xxxxxxxxxxxxxxxx
>> >>  F:   drivers/base/dma-buf*
>> >> +F:   drivers/base/fence.c
>> >>  F:   include/linux/dma-buf*
>> >> +F:   include/linux/fence.h
>> >>  F:   Documentation/dma-buf-sharing.txt
>> >>  T:   git git://git.linaro.org/people/sumitsemwal/linux-dma-buf.git
>> >> @@ -2936,6 +2938,8 @@ T:        git git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git
>> >>  S:   Supported
>> >>  F:   Documentation/kobject.txt
>> >>  F:   drivers/base/
>> >> +X:   drivers/base/dma-buf*
>> >> +X:   drivers/base/fence.c
>> >>  F:   fs/sysfs/
>> >>  F:   fs/debugfs/
>> >>  F:   include/linux/kobj*
>> >>
>> >> That removes Greg from the list generated by get_maintainer.pl for
>> >> anything that touches the DMA-BUF files.
>> >
>> > That doesn't really work for most people, I'll still be "responsible"
>> > for the code.
>> >
>> >> Thinking about it, perhaps moving DMA-BUF into its own subdirectory
>> >> would be an option too, to make the separation more obvious.
>> >
>> > That might be best for some of this.
>> >
>> > But again, why is the fence.c code needed at all anyway?  I'm not sold
>> > on that.
>>
>> Fence serves as a way to synchronize between (for example) multiple
>> asynchronous gpu's.  There is definitely a need for this.  Otherwise
>> performance for optimus/prime type setups is going to suck.
>
> What's wrong with the 'sync' code in the drivers/staging/android/
> directory?  I thought that is what that codebase was supposed to be
> doing.
>
(Just a quick recap on sync v/s fences):
Well, there was a discussion on Sync v/s Fences at LPC 2013 [1], and
the agreement was that if (explicit) sync could be implemented on top
of (implicitly synchronized) fences, fences might provide more
flexibility.
Maarten implemented that (it is a part of this patch series), and
Android guys confirmed that it worked the same way as expected in the
Android system. That is a major reason for acceptance of fence as the
solution.

>> I thought we had added something under Documentation/ about it, but I
>> can't find it now (although possibly looking at the wrong trees)..
>> there is at least a bit of a description in the commit msg:
>>
>>   https://lkml.org/lkml/2014/2/24/602
>
> Ah, so no documenation, no discussion, and you want to just throw it in
> a directory I am responsible for?  That sounds like a major rush job...
>
While I totally agree that it was an honest mistake to not have you
notified in CC for the patch series, for your other observations,
please allow me to answer one-by-one:
 (1) No documentation: there is in-code documentation which is also
added to Documentation/DocBook/device-drivers.tmpl; the commit message
has a lot of description about it. Of course, it could do with its own
documentation as well.
 (2) No discussion: the patch series is into v17 - v2 was posted in
July 2012, so it does seem like it has gone through a series of
reviews - I would admit that it has been mostly limited to the people
it seemed to matter the most for - the dri-devel and v4l communities.

>> I don't think the question about whether we need something like fence
>> to augment dma-buf is really in doubt.  Maybe it should live somewhere
>> else, I'm not sure.  But it makes sense for it to live wherever
>> dma-buf does, as they are intended to work together.
>
> Ok, then let's give it a proper review cycle, and notify everyone
> involved.  It doesn't look like this happened at all.  We don't add core
> primitives to the kernel without a lot of discussion and agreement.  And
> we sure don't add them without telling the person who owns the directory
> (again, my pet peeve, I know...)
>
And again, I do apologize that we all missed the fact that you weren't
CC'ed onto the patch series.

Still, even in the wake of above information, if you feel we are not
ready to take it for 3.16, I would drop it from my queue for 3.16
(though there are quite a few people who've waited long for this to
land in).

Thanks, and best regards,
~Sumit.

> greg k-h

[1]: http://lwn.net/Articles/569704/
[2]: https://lkml.org/lkml/2012/7/13/270
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/