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    SubjectRe: Locks used in the FAT file system are non-atomic and in fact, don't work on SMP systems
    Date
    Alan,

    Already tried this -- it seems to work (this is what the buffer cache is
    using). The other versions of "lock" however are probably broken. Up() and
    Down() also work great. I think the FAT file system probably needs to do
    what you suggest so it wont be broken.

    Jeff

    ----- Original Message -----
    From: Alan Cox <alan@lxorguk.ukuu.org.uk>
    To: Jeff Merkey <jmerkey@timpanogas.com>
    Cc: <linux-kernel@vger.rutgers.edu>
    Sent: Thursday, August 26, 1999 12:01 PM
    Subject: Re: Locks used in the FAT file system are non-atomic and in fact,
    don't work on SMP systems


    > > Lock()
    > > {
    > > while (lock) sleep_on(&wait);
    > > lock =3D 1;
    > > }
    > >
    > > Unlock()
    > > {
    > > lock =3D 0;
    > > wake_up(&wait);
    > > }
    > >
    > > Two processes can enter Lock() while lock is equal to 0, and both set =
    > > it. We have seen this occur, and it seems broken. =20
    >
    > Really
    >
    > CPU 0 CPU 1
    > while(lock) - its 0 while(lock) - its 0
    > lock=1 lock=1
    >
    > am I missing something. I think you want to be using atomic test and set
    > operations. (test_and_set_bit)
    >
    > Alan
    >
    >


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