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    SubjectRe: [RFC][PATCH 3/7] sched,livepatch: Use task_try_func()
    On Wed 2021-09-22 13:05:09, Peter Zijlstra wrote:
    > Instead of frobbing around with scheduler internals, use the shiny new
    > task_try_func() interface.
    >
    > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    > ---
    > kernel/livepatch/transition.c | 84 ++++++++++++++++++------------------------
    > 1 file changed, 37 insertions(+), 47 deletions(-)
    >
    > --- a/kernel/livepatch/transition.c
    > +++ b/kernel/livepatch/transition.c
    > @@ -274,6 +266,22 @@ static int klp_check_stack(struct task_s
    > return 0;
    > }
    >
    > +static int klp_check_task(struct task_struct *task, void *arg)

    Please, call this klp_check_and_switch_task() to make it clear
    that it actually does the switch.

    > +{
    > + int ret;
    > +
    > + if (task_curr(task))
    > + return -EBUSY;
    > +
    > + ret = klp_check_stack(task, arg);
    > + if (ret)
    > + return ret;
    > +
    > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
    > + task->patch_state = klp_target_state;
    > + return 0;
    > +}
    > +
    > /*
    > * Try to safely switch a task to the target patch state. If it's currently
    > * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
    > @@ -305,36 +308,23 @@ static bool klp_try_switch_task(struct t
    > * functions. If all goes well, switch the task to the target patch
    > * state.
    > */
    > - rq = task_rq_lock(task, &flags);
    > -
    > - if (task_running(rq, task) && task != current) {
    > - snprintf(err_buf, STACK_ERR_BUF_SIZE,
    > - "%s: %s:%d is running\n", __func__, task->comm,
    > - task->pid);
    > - goto done;
    > + ret = task_try_func(task, klp_check_task, &old_name);
    > + switch (ret) {
    > + case -EBUSY:
    > + pr_debug("%s: %s:%d is running\n",
    > + __func__, task->comm, task->pid);
    > + break;
    > + case -EINVAL:
    > + pr_debug("%s: %s:%d has an unreliable stack\n",
    > + __func__, task->comm, task->pid);
    > + break;
    > + case -EADDRINUSE:
    > + pr_debug("%s: %s:%d is sleeping on function %s\n",
    > + __func__, task->comm, task->pid, old_name);
    > + break;

    I would prefer to be on the safe side and catch error codes that might
    eventually appear in the future.

    case 0:
    /* success */
    break;
    default:
    pr_debug("%s: Unknown error code (%d) when trying to switch %s:%d\n",
    __func__, ret, task->comm, task->pid);

    > }
    >
    > - ret = klp_check_stack(task, err_buf);
    > - if (ret)
    > - goto done;
    > -
    > - success = true;
    > -
    > - clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
    > - task->patch_state = klp_target_state;
    > -
    > -done:
    > - task_rq_unlock(rq, task, &flags);
    > -
    > - /*
    > - * Due to console deadlock issues, pr_debug() can't be used while
    > - * holding the task rq lock. Instead we have to use a temporary buffer
    > - * and print the debug message after releasing the lock.
    > - */
    > - if (err_buf[0] != '\0')
    > - pr_debug("%s", err_buf);
    > -
    > - return success;
    > + return !ret;
    > }

    Otherwise, it is great improvement!

    Best Regards,
    Petr

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