From: Martin Wilck <mwilck@suse.com>
To: dm-devel@redhat.com, Andy Grover <agrover@redhat.com>
Cc: mpatocka@redhat.com, snitzer@redhat.com
Subject: Re: [PATCH 1/3] dm: a basic support for using the select or poll function
Date: Thu, 11 May 2017 21:49:14 +0200 [thread overview]
Message-ID: <1494532154.6093.30.camel@suse.com> (raw)
In-Reply-To: <01517603-ba44-bc02-8b70-96feef8050ed@redhat.com>
On Thu, 2017-05-11 at 11:10 -0700, Andy Grover wrote:
>
> I think getting one readiness notification for multiple events should
> be ok.
>
> But, the semantics of poll(2) are level-triggered, meaning if a fd
> is
> ready then it will stay ready through multiple invocations of poll()
> until something is done to satisfy it (e.g. for a TCP socket,
> read()ing
> all the available data). Therefore we can't make it "edge-triggered"
> by
> auto-clearing - it has to be a separate action.
Hm, perhaps.
Here is an idea: the best way to avoid the race mentioned by Mike might
be to set priv->global_event_nr to the highest event nr reported by the
DM_LIST_DEVICES ioctl when this ioctl returns. DM_LIST_DEVICES would
then represent your "separate action", which would fit quite well, and
the DM_DEV_ARM_POLL ioctl wouldn't be needed. It would be guaranteed
that if any event had occured after the previous DM_LIST_DEVICES,
whether or not that had been before or after the poll() call, userspace
would notice.
Cheers,
Martin
--
Dr. Martin Wilck <mwilck@suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)
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next prev parent reply other threads:[~2017-05-11 19:49 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-09 19:10 [PATCH 0/3] poll()able DM events Andy Grover
2017-05-09 19:10 ` [PATCH 1/3] dm: a basic support for using the select or poll function Andy Grover
2017-05-11 9:39 ` Martin Wilck
2017-05-11 9:43 ` Martin Wilck
2017-05-11 13:21 ` Mike Snitzer
2017-05-11 18:10 ` Andy Grover
2017-05-11 19:49 ` Martin Wilck [this message]
2017-05-11 20:21 ` Alasdair G Kergon
2017-05-11 20:45 ` Martin Wilck
2017-05-11 21:38 ` Alasdair G Kergon
2017-05-11 19:30 ` Martin Wilck
2017-05-11 21:46 ` Andy Grover
2017-05-12 6:48 ` Martin Wilck
2017-05-11 21:50 ` Alasdair G Kergon
2017-05-12 15:52 ` Mikulas Patocka
2017-05-09 19:10 ` [PATCH 2/3] dm-ioctl: add a new DM_DEV_ARM_POLL ioctl Andy Grover
2017-05-09 19:10 ` [PATCH 3/3] dm-ioctl: report event number in DM_LIST_DEVICES Andy Grover
2017-05-09 21:14 ` [PATCH 0/3] poll()able DM events Mike Snitzer
2017-09-17 2:41 ` [PATCH 3/3] dm-ioctl: report event number in DM_LIST_DEVICES Eugene Syromiatnikov
2017-09-19 16:47 ` Andy Grover
2017-09-20 11:29 ` [PATCH] dm-ioctl: fix alignment of event number in the device list Mikulas Patocka
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