linux-kernel - Re: [PATCH] driver: ip27-rtc - convert ioctl to unlocked

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Date:	Mon, 14 Jan 2008 16:59:22 +0100
From:	Jiri Slaby <jirislaby@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	Paul Gortmaker <p_gortmaker@...oo.com>,
	LKML <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...e.de>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH] driver: ip27-rtc - convert ioctl to unlocked_ioctl

On 01/14/2008 04:38 PM, Cyrill Gorcunov wrote:
> Jiri, I mean rtc_open() is protected by spinlock+status from being
> opened simultaneously by a few processes. *But* lets imagine the
> following situation - this fd (file descriptor) is opened by one
> multithreaded application so all threads have an access to this
> fd. Then one thread reads rtc periodically thru unlocked_ioctl
> and another thread set new time from time to time. So the question
> I have - is it possible to get second thread stopped at attemption to
> get rtc spinlock while another thread is setting the new time? Or
> this situation never-ever could be? i'm not really familiar with
> process management in Linux and as result could be wrong.

Access to global variable 'rtc' (the rtc itself) is serialized through the 
spinlock, I see no problem there. If you call read-read-write-read from 4 tasks 
in userspace, it might be _still_ (no change) reordered to e.g. 
write-read-read-read by the scheduler.

In fact, the reading process is stopped while the another one is writing the 
time (due to spinlock).
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