Re: [PATCH 2/2] pipe: do not return POLLERR for fifo_poll

Re: [PATCH 2/2] pipe: do not return POLLERR for fifo_poll

[Andrew Morton]

Pekka Enberg <penberg@cs.helsinki.fi> wrote:
>
> This patch changes fifo_poll not to return POLLERR to take care of a FIXME
> in fs/pipe.c stating that "Most unices do not set POLLERR for fifos." The
> comment has been there since 2.3.99-pre3 so either apply this patch or
> alternatively, I can send a new one removing the unnecessary abstraction.
> 
> ...
> --- 2.6-mm.orig/fs/pipe.c
> +++ 2.6-mm/fs/pipe.c
> @@ -399,8 +399,8 @@ pipe_ioctl(struct inode *pino, struct fi
>  }
>  
>  /* No kernel lock held - fine */
> -static unsigned int
> -pipe_poll(struct file *filp, poll_table *wait)
> +static inline unsigned int
> +__pipe_poll(struct file *filp, poll_table *wait, int can_err)
>  {
>  	unsigned int mask;
>  	struct inode *inode = filp->f_dentry->d_inode;
> @@ -420,15 +420,24 @@ pipe_poll(struct file *filp, poll_table 
>  
>  	if (filp->f_mode & FMODE_WRITE) {
>  		mask |= (nrbufs < PIPE_BUFFERS) ? POLLOUT | POLLWRNORM : 0;
> -		if (!info->readers)
> +		if (can_err && !info->readers)
>  			mask |= POLLERR;
>  	}
>  
>  	return mask;
>  }
>  
> -/* FIXME: most Unices do not set POLLERR for fifos */
> -#define fifo_poll pipe_poll
> +static unsigned int
> +pipe_poll(struct file *filp, poll_table *wait)
> +{
> +	return __pipe_poll(filp, wait, 1);
> +}
> +
> +static unsigned int
> +fifo_poll(struct file *filp, poll_table *wait)
> +{
> +	return __pipe_poll(filp, wait, 0);
> +}
>  
>  static int
>  pipe_release(struct inode *inode, int decr, int decw)

A userspace-visible change, no?

So there's a risk in changing it.  What do we get in return?  Worried.

Re: [PATCH 2/2] pipe: do not return POLLERR for fifo_poll

[Pekka J Enberg]

Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> > This patch changes fifo_poll not to return POLLERR to take care of a FIXME
> > in fs/pipe.c stating that "Most unices do not set POLLERR for fifos." The
> > comment has been there since 2.3.99-pre3 so either apply this patch or
> > alternatively, I can send a new one removing the unnecessary abstraction.

On Thu, 25 Aug 2005, Andrew Morton wrote:
> A userspace-visible change, no?
> 
> So there's a risk in changing it.  What do we get in return?  Worried.

FWIW I have been running on this for few days now without any noticeable 
regressions.  We get a solved FIXME in return but like I said I am a happy 
to remove the redundant abstraction if this is too risky.

			Pekka

Re: [PATCH 2/2] pipe: do not return POLLERR for fifo_poll

[Manfred Spraul]

Pekka J Enberg wrote:

>FWIW I have been running on this for few days now without any noticeable 
>regressions.  We get a solved FIXME in return but like I said I am a happy 
>to remove the redundant abstraction if this is too risky.
>
>  
>
I would prefer just to remove the abstraction, together with a comment 
that Linux fifos behave exactly like pipes, unlike the behavior of most 
unices.

--
    Manfred

[PATCH] pipe: remove redundant fifo_poll abstraction

[Pekka Enberg]

On Fri, 2005-08-26 at 19:16 +0200, Manfred Spraul wrote:
> I would prefer just to remove the abstraction, together with a comment 
> that Linux fifos behave exactly like pipes, unlike the behavior of most 
> unices.

[PATCH] pipe: remove redundant fifo_poll abstraction

This patch removes a redundant fifo_poll() abstraction from fs/pipe.c and adds
a big fat comment stating we set POLLERR for FIFOs too on Linux unlike most
Unices.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
---

 pipe.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

Index: 2.6-mm/fs/pipe.c
===================================================================
--- 2.6-mm.orig/fs/pipe.c
+++ 2.6-mm/fs/pipe.c
@@ -419,6 +419,10 @@ pipe_poll(struct file *filp, poll_table 
 
 	if (filp->f_mode & FMODE_WRITE) {
 		mask |= (nrbufs < PIPE_BUFFERS) ? POLLOUT | POLLWRNORM : 0;
+		/*
+		 * Most Unices do not set POLLERR for FIFOs but on Linux they
+		 * behave exactly like pipes for poll().
+		 */
 		if (!PIPE_READERS(*inode))
 			mask |= POLLERR;
 	}
@@ -426,9 +430,6 @@ pipe_poll(struct file *filp, poll_table 
 	return mask;
 }
 
-/* FIXME: most Unices do not set POLLERR for fifos */
-#define fifo_poll pipe_poll
-
 static int
 pipe_release(struct inode *inode, int decr, int decw)
 {
@@ -572,7 +573,7 @@ struct file_operations read_fifo_fops = 
 	.read		= pipe_read,
 	.readv		= pipe_readv,
 	.write		= bad_pipe_w,
-	.poll		= fifo_poll,
+	.poll		= pipe_poll,
 	.ioctl		= pipe_ioctl,
 	.open		= pipe_read_open,
 	.release	= pipe_read_release,
@@ -584,7 +585,7 @@ struct file_operations write_fifo_fops =
 	.read		= bad_pipe_r,
 	.write		= pipe_write,
 	.writev		= pipe_writev,
-	.poll		= fifo_poll,
+	.poll		= pipe_poll,
 	.ioctl		= pipe_ioctl,
 	.open		= pipe_write_open,
 	.release	= pipe_write_release,
@@ -597,7 +598,7 @@ struct file_operations rdwr_fifo_fops = 
 	.readv		= pipe_readv,
 	.write		= pipe_write,
 	.writev		= pipe_writev,
-	.poll		= fifo_poll,
+	.poll		= pipe_poll,
 	.ioctl		= pipe_ioctl,
 	.open		= pipe_rdwr_open,
 	.release	= pipe_rdwr_release,