Re: aio poll, io_pgetevents and a new in-kernel poll API V3

From: Jeff Moyer <jmoyer@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: viro@zeniv.linux.org.uk, Avi Kivity <avi@scylladb.com>,
	linux-aio@kvack.org, linux-fsdevel@vger.kernel.org,
	netdev@vger.kernel.org, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: aio poll, io_pgetevents and a new in-kernel poll API V3
Date: Thu, 18 Jan 2018 12:59:52 -0500	[thread overview]
Message-ID: <x49o9lrt5d3.fsf@segfault.boston.devel.redhat.com> (raw)
In-Reply-To: <20180118174228.GA27412@lst.de> (Christoph Hellwig's message of "Thu, 18 Jan 2018 18:42:28 +0100")

Christoph Hellwig <hch@lst.de> writes:

> On Thu, Jan 18, 2018 at 11:44:03AM -0500, Jeff Moyer wrote:
>> Jeff Moyer <jmoyer@redhat.com> writes:
>> 
>> > FYI, this kernel has issues.  It will boot up, but I don't have
>> > networking, and even rebooting doesn't succeed.  I'm looking into it.
>> 
>> A bisect lands on: eventfd: switch to ->poll_mask.  That's not super
>> helpful, though.  I did run the ltp eventfd2 tests, and they all pass.
>> 
>> The actual issue I get on boot is that several services don't start:
>>
>> [...]
>> 
>> Christoph, are you able to reproduce this?
>
> No, I can't reproduce any of that.  But I don't have a Fedora system
> either, so this might be a new systemd version doing funky things.
>
> The major change in this version was to call ->poll_mask before setting up
> the wait queue as well.  This does the right thing for poll and aio poll,
> but the more I dig into the epoll code the less sure I am it does the right
> thing for it, or in fact that epoll does the right thing in general..
>
> Do you still see it with the patch below applied?

No, that fixes it for me.

-Jeff

> diff --git a/fs/select.c b/fs/select.c
> index 707abe79536b..1784c1a29253 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -53,9 +53,9 @@ __poll_t vfs_poll(struct file *file, struct poll_table_struct *pt)
>  		head = vfs_get_poll_head(file, events);
>  		if (!head)
>  			return DEFAULT_POLLMASK;
> -		mask = file->f_op->poll_mask(file, events);
> -		if (mask)
> -			return mask;
> +//		mask = file->f_op->poll_mask(file, events);
> +//		if (mask)
> +//			return mask;
>  
>  		pt->_qproc(file, head, pt);
>  	}