Re: Multi-threading access to device drivers.

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 11/07/04

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    To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
Date: Sun, 07 Nov 2004 08:45:59 +0100

    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.
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