There is a potential bug in "yield_to"

Hi, All

I found there may be a potential bug in "yield_to":

        local_irq_save(flags);
        rq = this_rq();

again:

//task's rq may already changed in "sched_move_task"

        p_rq = task_rq(p);
        double_rq_lock(rq, p_rq);
        while (task_rq(p) != p_rq) {
                double_rq_unlock(rq, p_rq);
                goto again;
        }

I think it may happen in this scene:

cpu 0 cpu 1(task a)

yield_to { disable_irq
sched_move_task { rq = this_rq();
task_rq_lock(task a) double_rq_lock

hold lock of rq 1
set_task_rq //task rq changed
release lock of rq 1

hold lock of rq 1 but task b no longer there

set rq 1's current to skip which is not task a

which means we hold a rq's lock but it's current is not the one should do yield.

Only "sched_move_task" will cause this issue as it will move the task which is still running.

The bug will make the task who want to do yield failed to set skip buddy to himself, but to a innocent task instead, not very harmful and almost impossible to occur in normal, but should we fix it with another check "rq == this_rq()"?

Quoted 41 lines · Michael Wang

Hi, All

I found there may be a potential bug in "yield_to":

        local_irq_save(flags);
        rq = this_rq();

again:

//task's rq may already changed in "sched_move_task"

        p_rq = task_rq(p);
        double_rq_lock(rq, p_rq);
        while (task_rq(p) != p_rq) {
                double_rq_unlock(rq, p_rq);
                goto again;
        }

I think it may happen in this scene:

cpu 0 cpu 1(task a)

yield_to { disable_irq
sched_move_task { rq = this_rq();
task_rq_lock(task a) double_rq_lock

hold lock of rq 1
set_task_rq //task rq changed
release lock of rq 1

hold lock of rq 1 but task b no longer there

set rq 1's current to skip which is not task a

which means we hold a rq's lock but it's current is not the one should do yield.

Quoted 7 lines

Only "sched_move_task" will cause this issue as it will move the task which is still running.

The bug will make the task who want to do yield failed to set skip buddy to himself, but to a innocent task instead, not very harmful and almost impossible to occur in normal, but should we fix it with another check "rq == this_rq()"?

Uhm, what?!

We've got interrupts disabled, this_rq() cannot ever possibly change, so rq is always correct.

Only p_rq can change, and we have an again loop on that, so what's the problem again?