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    SubjectRe: [Patch] shm bug introduced with pagecache in 2.3.11

    On Sat, 20 Nov 1999, Arjan van de Ven wrote:
    > Do you think an implementation based on this (and the other examples from
    > the book) is acceptable? Since the implementation uses already available
    > primitives, no assembly or other architecture-specific stuff is required.

    Nope, not acceptable.

    The mm semaphore is one of the most timing-critical in the whole kernel.
    It usually has absolutely zero contention, but it needs to be FAST.

    Basically, a read-lock() must look something very very similar to the
    read-spinlock implementation, ie something like

    lock ; incl (%ecx)
    js fixup

    for the successful fall-through case. Two instructions, no more. That's
    what the spinlocks do, and that's also what the semaphores do (although in
    the case of a semaphore, it's a "decl" in that case.

    The "fixup" case is going to be more complex than for spinlocks: for
    spinlocks it's just a simple loop, while for semaphores you get all the
    complexity that you see in arch/i386/kernel/semaphore.c to handle the
    thing cleanly..

    The read-write semaphore should be doable with the same skeleton as the
    normal semaphores, although it needs two counters (regular semaphores have
    just "sleepers", rw-semaphores need to have "read_sleeper" and
    "write_sleeper" counts etc).


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