Re: [RFCv7 PATCH 0/4] Add poll_requested_events() function.

[Enke Chen]

Hi, folks:

I would like to voice my support for Hans' patch.

1) The functionality provided by this patch is needed.  I have been 
involved in an app that implements a use-land sockets (using FUSE). 
Passing the accurate poll events is essential in the app.  We have been 
using a local patch that is similar (but not identical) for more than a 
year.  IMO the functionality of passing accurate and consistent events 
to a driver is basic, and should be provided by Linux.

2) The patch is safe as far as I can tell.  Without the patch, unwanted 
events may be passed to a driver. However once the poll returns to the 
kernel, the unwanted events would be masked out by the kernel anyway and 
would not be passed to an app.  Thus a driver that relies on the 
unwanted events would not work anyway.

Thanks.   -- Enke

On 2/2/12 2:26 AM, Hans Verkuil wrote:
> Hi all,
>
> This is the seventh version of this patch series (the fifth and sixth where
> never posted and where internal iterations only).
>
> Al Viro had concerns about silent API changes. I have made an extensive
> analysis of that in my comments in patch 2/4.
>
> This patch series is rebased to v3.3-rc2. The changes compared to the
> previously posted version are:
>
> - I have renamed the qproc field to pq_proc to prevent any driver that tries
>    to access that directly to fail. No kernel driver does this, BTW.
>
> - I added a new poll_does_not_wait() inline that returns true if it is known
>    that poll() will not wait on return. This removes the last reason for
>    looking inside the poll_table struct. include/net/sock.h has been adapted
>    to use this new inline (and it is the only place inside the kernel that
>    need this).
>
> I hope that the analysis I made answers any remaining concerns about possible
> silent API changes.
>
> This patch series is also available here:
>
> http://git.linuxtv.org/hverkuil/media_tree.git/shortlog/refs/heads/pollv7
>
> It was suggested to me that creating a new poll system call might be an option
> as well. I've attempted that as well and code implementing that can be found
> here:
>
> http://git.linuxtv.org/hverkuil/media_tree.git/shortlog/refs/heads/pollwithkey
>
> However, I think this turned out to be very messy. And because some drivers
> call the poll fop directly or through some framework I could not be certain I
> was not introducing any errors.
>
> If it is really required to change the API in some way, then I would suggest
> changing this:
>
> typedef struct poll_table_struct {
>          poll_queue_proc pq_proc;
>          unsigned long key;
> } poll_table;
>
> to this:
>
> struct poll_table {
>          poll_queue_proc pq_proc;
>          unsigned long key;
> };
>
> and adapting all users.
>
> However, I honestly do not think this is necessary at all. But if it is the
> only way to get this in, then I'll do the work. The media/video subsystem really
> needs this functionality. Also note that previous versions of this patch have
> been in linux-next for months now.
>
> The first version of this patch was posted July 1st, 2011. I really hope that
> it won't take another six months to get a review from a fs developer. As this
> LWN article (http://lwn.net/Articles/450658/) said: 'There has been little
> discussion of the patch; it doesn't seem like there is any real reason for it
> not to go in for 3.1.'
>
> The earliest this can go in now is 3.4. The only reason it takes so long is
> that it has been almost impossible to get a Ack or comments or even just a
> simple reply from the fs developers. That is really frustrating, I'm sorry
> to say.
>
> Anyway, comments, reviews, etc. are very welcome.
>
> Regards,
>
> 	Hans

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