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To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Sat, 06 Nov 2004 19:15:13 EST."
             <200411070015.iA70FDxm015465@khavrinen.lcs.mit.edu> 
Date: Sun, 07 Nov 2004 08:45:59 +0100
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Sender: phk@critter.freebsd.dk
cc: arch@FreeBSD.org
Subject: Re: Multi-threading access to device drivers. 
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In message <200411070015.iA70FDxm015465@khavrinen.lcs.mit.edu>, Garrett Wollman writes:
>In article <10847.1099784360@critter.freebsd.dk> you write:
>>Assume a process with two threads on two CPUs, both
>>doing read(fd, buf, len) at the same time.
>>
>>Should we let both reads into the driver at the same time ?
>>
>>If so, which uio_offset do we hand them ?
>
>POSIX is quite clear on the matter:
>
>> All of the functions chmod (), close (), fchmod (), fcntl (), fstat
>> (), ftruncate (), lseek (), open (), read (), readlink (), stat (),
>> symlink ( ), and write ( ) shall be atomic with respect to each other
>> in the effects specified in IEEE Std 1003.1-2001 when they operate on
>> regular files. If two threads each call one of these functions, each
>> call shall either see all of the specified effects of the other call,
>> or none of them.
>
>(One of the "specified effects" is advancing the file offset.)

Yes, but notice the "... when they operate on regular files." bit.

It doesn't say anything about devices.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.