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    SubjectRe: CD writing in future Linux (stirring up a hornets' nest)
    On Sunday February 5, mrmacman_g4@mac.com wrote:
    >
    > If you specify O_EXCL (and not O_CREAT), it is implementation defined
    > what will happen (in the Linux case, this opens a block device for
    > exclusive access).

    With Linux, O_EXCL on a block devices isn't *exactly* exclusive
    access.
    It only provided you exclusive access against other people who ask for
    exclusive access, which includes in-kernel usage like mount, md, dm,
    and swap.

    So if you open a block device O_EXCL, it will fail if the block device
    is already open O_EXCL or is mounted, or in use by the kernel in some
    other way (including if a partition is open O_EXCL). An if you
    succeed in getting an O_EXCL open, then no-one else will be able to
    get an O_EXCL open, or mount the filesystem etc.
    Bit on open without O_EXCL will always succeed no matter whether
    someone has it O_EXCL or not.

    So it is a lot like an advisory exclusive lock on the whole block
    device.

    NeilBrown
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