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

> > i'm not sure what this means -- which of the transformations in
> > that patch is considered unsafe? here's a typical simplification:
>
> It is not unsafe, just generates slight worse code.
>
> current is inline assembler and the compiler doesn't know that it
> could cache it in a register because it is not marked pure for
> various reasons. That is why current is often cached explicitely in
> a local variable to tell the compiler that.

ah, i think i see, thanks. learn something every day.

> Before you run off and do that everywhere: it is also not a large
> win, just a small one unless current is used very often.

there's actually not that many explicit calls to either
set_task_state or __set_task_state in the entire tree, and a lot of
those don't count as they really are setting the state for a different
task or for some other reason. in fact, here's the entire list for
the whole tree:

$ grep -r set_task_state *
arch/powerpc/kernel/semaphore.c: __set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/powerpc/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/powerpc/kernel/semaphore.c: __set_task_state(tsk, TASK_RUNNING);
arch/powerpc/kernel/semaphore.c: __set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/powerpc/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/powerpc/kernel/semaphore.c: __set_task_state(tsk, TASK_RUNNING);
arch/alpha/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/alpha/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/parisc/kernel/semaphore.c: set_task_state(current, TASK_UNINTERRUPTIBLE);
arch/parisc/kernel/semaphore.c: set_task_state(current, TASK_INTERRUPTIBLE);
arch/mn10300/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/mn10300/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/mn10300/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/mn10300/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/s390/kernel/semaphore.c: __set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/s390/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/s390/kernel/semaphore.c: __set_task_state(tsk, TASK_RUNNING);
arch/s390/kernel/semaphore.c: __set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/s390/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/s390/kernel/semaphore.c: __set_task_state(tsk, TASK_RUNNING);
arch/s390/mm/fault.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/s390/mm/fault.c: set_task_state(tsk, TASK_RUNNING);
arch/frv/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/frv/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/frv/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/frv/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/mips/kernel/semaphore.c: __set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/mips/kernel/semaphore.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
arch/mips/kernel/semaphore.c: __set_task_state(tsk, TASK_RUNNING);
arch/mips/kernel/semaphore.c: __set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/mips/kernel/semaphore.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
arch/mips/kernel/semaphore.c: __set_task_state(tsk, TASK_RUNNING);
Documentation/scheduler/sched-coding.txt:set_task_state(tsk, state_value)
drivers/mmc/core/sdio_irq.c: set_task_state(current, TASK_INTERRUPTIBLE);
drivers/mmc/core/sdio_irq.c: set_task_state(current, TASK_RUNNING);
drivers/mfd/ucb1x00-ts.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
drivers/mfd/ucb1x00-ts.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
fs/aio.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
fs/aio.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
fs/aio.c: __set_task_state(tsk, TASK_RUNNING);
fs/aio.c: set_task_state(tsk, TASK_INTERRUPTIBLE);
fs/aio.c: set_task_state(tsk, TASK_RUNNING);
include/linux/sched.h:/* Convenience macros for the sake of set_task_state */
include/linux/sched.h:#define __set_task_state(tsk, state_value) \
include/linux/sched.h:#define set_task_state(tsk, state_value) \
kernel/ptrace.c: __set_task_state(child, TASK_STOPPED);
kernel/fork.c: __set_task_state(p, TASK_STOPPED);
kernel/mutex.c: __set_task_state(task, state);
lib/rwsem-spinlock.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
lib/rwsem-spinlock.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
lib/rwsem-spinlock.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
lib/rwsem-spinlock.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
lib/rwsem.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);
lib/rwsem.c: set_task_state(tsk, TASK_UNINTERRUPTIBLE);

once you ignore the semaphore stuff (and a bit of the rest, like the
stuff in sched.h), there's not really that much left that could be
rewritten with set_current_state() anyway.

rday
--

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

http://crashcourse.ca Waterloo, Ontario, CANADA
========================================================================


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