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

From: Enke Chen
Date: Thu Feb 02 2012 - 20:54:44 EST

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:

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

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 ( 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.



To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at