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Message-ID: <478B86DA.1010902@gmail.com>
Date: Mon, 14 Jan 2008 16:59:22 +0100
From: Jiri Slaby <jirislaby@gmail.com>
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To: Cyrill Gorcunov <gorcunov@gmail.com>
CC: Paul Gortmaker <p_gortmaker@yahoo.com>,
       LKML <linux-kernel@vger.kernel.org>, Andi Kleen <ak@suse.de>,
       Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: [PATCH] driver: ip27-rtc - convert ioctl to unlocked_ioctl
References: <20080113203223.GA6723@cvg> <478A8131.9050500@gmail.com> <478A82AD.3070904@gmail.com> <aa79d98a0801131332w45d6e0e9r13e96b69e7e1b70@mail.gmail.com> <aa79d98a0801132238o72266a91p78ae3a0c1476607c@mail.gmail.com> <478B7C4A.5090903@gmail.com> <20080114153806.GA6639@cvg>
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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).