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    SubjectRe: [PATCH] i386 rw_semaphores fix
    Andrew Morton wrote:
    > I think that's a very good approach. Sure, it's suboptimal when there
    > are three or more waiters (and they're the right type and order). But
    > that never happens. Nice design idea.


    > These numbers are infinity :)

    I know, but I think Linus may be happy with the resolution for the moment. It
    can be extended later by siphoning off excess quantities of waiters into a
    separate counter (as is done now) and by making the access count use a larger
    part of the variable.

    Unfortunately, managing the count and siphoned-off count together is tricky.

    > You need sterner testing stuff :) I hit the BUG at the end of rwsem_wake()
    > in about a second running rwsem-4. Removed the BUG and everything stops
    > in D state.
    > Grab rwsem-4 from
    > ...

    Will do.

    > It's very simple. But running fully in-kernel shortens the
    > code paths enormously and allows you to find those little
    > timing windows.

    I thought I'd got them all by using an activity counter incremented by both
    read and write lockers.

    > - rwsemdebug(FMT, ...) doesn't compile with egcs-1.1.2. Need
    > to remove the comma.

    This is tricky... you get all sorts of horrible warnings with gcc-2.96 if you
    remove the comma. What I've done is now ANSI-C99 compliant, but egcs is not.

    > - The comments in down_write and down_read() are inaccurate.
    > RWSEM_ACTIVE_WRITE_BIAS is 0xffff0001, not 0x00010001


    > - It won't compile when WAITQUEUE_DEBUG is turned on. I
    > guess you knew that.

    Currently putting in separate debugging stuff for rwsems.

    > - The comments above the functions in semaphore.h need
    > updating.

    Done. (BTW in the latest patch, they're actually split out into separate
    header files as per Linus's suggestion).

    > - What on earth does __xg() do? (And why do people write
    > code like that without explaining why? Don't answer this
    > one).

    Stolen from the xchg() macro/function, but I'm not sure what it does. Plus I
    don't use it now.

    > - Somewhat offtopic: the `asm' statements in semaphore.c
    > are really dangerous.

    Now all got .text in.

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