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    SubjectRe: [PATCH] RWSEM: Rewrite rwsem.c and rwsem-spinlock.c more simply.
    On Tue, 25 Mar 2008, Andi Kleen wrote:

    > "Robert P. J. Day" <> writes:
    > > Rewrite these source files more simply by deleting the superfluous
    > > "tsk" task_struct pointer and rephrasing in terms of the "current"
    > > task pointer.
    > This is likely a code pessimization because "current" is inline
    > assembler and many gcc versions cannot CSE it.

    i'm not sure what this means -- which of the transformations in that
    patch is considered unsafe? here's a typical simplification:

    - tsk = current;
    - set_task_state(tsk, TASK_UNINTERRUPTIBLE);
    + set_current_state(TASK_UNINTERRUPTIBLE);

    there's all sorts of usage of set_current_state() throughout the
    tree. how is simplifying the code in these two files in exactly the
    same way any different? or am i missing something because this
    involves semaphores?


    p.s. given this bit from sched.h:

    #define __set_task_state(tsk, state_value) \
    do { (tsk)->state = (state_value); } while (0)
    #define set_task_state(tsk, state_value) \
    set_mb((tsk)->state, (state_value))


    #define __set_current_state(state_value) \
    do { current->state = (state_value); } while (0)
    #define set_current_state(state_value) \
    set_mb(current->state, (state_value))

    it's not clear why set_current_state() and __current_state() are
    defined the way they are when it would seem to be simpler (and less
    error-prone) to just write:

    #define __set_current_state(sv) __set_task_state(current, sv)
    #define set_current_state(sv) set_task_state(current, sv)

    the law of parsimony and all that. or, once again, is there
    something subtle i'm not seeing?


    Robert P. J. Day
    Linux Consulting, Training and Annoying Kernel Pedantry:
    Have classroom, will lecture. Waterloo, Ontario, CANADA

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