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    SubjectRe: [Patch] shm bug introduced with pagecache in 2.3.11
    On Sat, 20 Nov 1999, Manfred Spraul wrote:

    > I give it up:
    > I've attached 2 more ideas, but one of them is a bit slow, and the other
    > one requires cmpxchg (ie only 486 and above).

    Interesting. Here's what I'm doing (it's subtle, but cute and efficient
    -- 1 instruction and an untaken branch for both readers and writers):

    count is initially set to a BIAS (I'm using 0x01000000). Readers
    decrement count by 1 and see a positive value if successful. Writers
    decrement by the BIAS and see a result of zero when acquired.

    This makes the recovery case a bit more challenging, but easily solved
    with a second value used to decide who is the next writer (and thus
    responsible for maintaining the bias while waiting).


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