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Subjectschedule() && prev/current (Was: [PATCH 3/3] Make get_current() __attribute__((const)))
On 05/18, David Howells wrote:
>
> This doesn't break cached copies of current, whether they're cached by gcc in
> registers or on the stack. switch_to() will save all registers on the stack
> before actually switching, then when it switches current, it will also switch
> the stack and then pop back what was stored in the 'unclobbered' registers for
> the now active task and stack. Thus the copies of current that were cached
> just work.
>
> [...snip...]
>
> --- a/include/asm-generic/current.h
> +++ b/include/asm-generic/current.h
> @@ -3,7 +3,14 @@
>
> #include <linux/thread_info.h>
>
> -#define get_current() (current_thread_info()->task)
> +struct task_struct;
> +
> +static inline __attribute_const__
> +struct task_struct *get_current(void)
> +{
> + return current_thread_info()->task;
> +}

Can't ack this patch, but it looks correct to me.


And, looking at this patch I think that schedule() can be simplified
a little bit.

"sched: Reassign prev and switch_count when reacquire_kernel_lock() fail"
commit 6d558c3ac9b6508d26fd5cadccce51fc9d726b1c says:

Assume A->B schedule is processing,
...
Then on B's context,
...
prev and switch_count are related to A

How so? switch_count - yes, we should change it. But prev must be
equal to B, and it must be equal to current. When we return from
switch_to() registers/stack should be restored correctly, so we
can do

--- x/kernel/sched.c
+++ x/kernel/sched.c
@@ -3729,8 +3729,7 @@ need_resched_nonpreemptible:

post_schedule(rq);

- if (unlikely(reacquire_kernel_lock(current) < 0)) {
- prev = rq->curr;
+ if (unlikely(reacquire_kernel_lock(prev) < 0)) {
switch_count = &prev->nivcsw;
goto need_resched_nonpreemptible;
}

and in fact we can simplify this even more, no need to reassign
switch_count, we can just move the initial assignment down, under
"need_resched_nonpreemptible" label.

switch_to(prev, next, prev) changes "prev" inside context_switch()
but it should be stable inside of schedule().

Oleg.

--- x/kernel/sched.c
+++ x/kernel/sched.c
@@ -3708,7 +3708,6 @@ need_resched:
rq = cpu_rq(cpu);
rcu_sched_qs(cpu);
prev = rq->curr;
- switch_count = &prev->nivcsw;

release_kernel_lock(prev);
need_resched_nonpreemptible:
@@ -3722,6 +3721,7 @@ need_resched_nonpreemptible:
update_rq_clock(rq);
clear_tsk_need_resched(prev);

+ switch_count = &prev->nivcsw;
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
if (unlikely(signal_pending_state(prev->state, prev)))
prev->state = TASK_RUNNING;
@@ -3758,11 +3758,8 @@ need_resched_nonpreemptible:

post_schedule(rq);

- if (unlikely(reacquire_kernel_lock(current) < 0)) {
- prev = rq->curr;
- switch_count = &prev->nivcsw;
+ if (unlikely(reacquire_kernel_lock(prev))
goto need_resched_nonpreemptible;
- }

preempt_enable_no_resched();
if (need_resched())


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