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    Subject[RFC][PATCH 1/7] sched,rcu: Rework try_invoke_on_locked_down_task()
    Give try_invoke_on_locked_down_task() a saner name and have it return
    an int so that the caller might distinguish between different reasons
    of failure.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    include/linux/wait.h | 3 ++-
    kernel/rcu/tasks.h | 12 ++++++------
    kernel/rcu/tree_stall.h | 8 ++++----
    kernel/sched/core.c | 11 +++++------
    4 files changed, 17 insertions(+), 17 deletions(-)

    --- a/include/linux/wait.h
    +++ b/include/linux/wait.h
    @@ -1160,6 +1160,7 @@ int autoremove_wake_function(struct wait
    (wait)->flags = 0; \
    } while (0)

    -bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg);
    +typedef int (*task_try_f)(struct task_struct *p, void *arg);
    +extern int task_try_func(struct task_struct *p, task_try_f func, void *arg);

    #endif /* _LINUX_WAIT_H */
    --- a/kernel/rcu/tasks.h
    +++ b/kernel/rcu/tasks.h
    @@ -928,7 +928,7 @@ static void trc_read_check_handler(void
    }

    /* Callback function for scheduler to check locked-down task. */
    -static bool trc_inspect_reader(struct task_struct *t, void *arg)
    +static int trc_inspect_reader(struct task_struct *t, void *arg)
    {
    int cpu = task_cpu(t);
    bool in_qs = false;
    @@ -939,7 +939,7 @@ static bool trc_inspect_reader(struct ta

    // If no chance of heavyweight readers, do it the hard way.
    if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
    - return false;
    + return -EINVAL;

    // If heavyweight readers are enabled on the remote task,
    // we can inspect its state despite its currently running.
    @@ -947,7 +947,7 @@ static bool trc_inspect_reader(struct ta
    n_heavy_reader_attempts++;
    if (!ofl && // Check for "running" idle tasks on offline CPUs.
    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
    - return false; // No quiescent state, do it the hard way.
    + return -EINVAL; // No quiescent state, do it the hard way.
    n_heavy_reader_updates++;
    if (ofl)
    n_heavy_reader_ofl_updates++;
    @@ -962,7 +962,7 @@ static bool trc_inspect_reader(struct ta
    t->trc_reader_checked = true;

    if (in_qs)
    - return true; // Already in quiescent state, done!!!
    + return 0; // Already in quiescent state, done!!!

    // The task is in a read-side critical section, so set up its
    // state so that it will awaken the grace-period kthread upon exit
    @@ -970,7 +970,7 @@ static bool trc_inspect_reader(struct ta
    atomic_inc(&trc_n_readers_need_end); // One more to wait on.
    WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
    WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
    - return true;
    + return 0;
    }

    /* Attempt to extract the state for the specified task. */
    @@ -992,7 +992,7 @@ static void trc_wait_for_one_reader(stru

    // Attempt to nail down the task for inspection.
    get_task_struct(t);
    - if (try_invoke_on_locked_down_task(t, trc_inspect_reader, NULL)) {
    + if (!task_try_func(t, trc_inspect_reader, NULL)) {
    put_task_struct(t);
    return;
    }
    --- a/kernel/rcu/tree_stall.h
    +++ b/kernel/rcu/tree_stall.h
    @@ -240,16 +240,16 @@ struct rcu_stall_chk_rdr {
    * Report out the state of a not-running task that is stalling the
    * current RCU grace period.
    */
    -static bool check_slow_task(struct task_struct *t, void *arg)
    +static int check_slow_task(struct task_struct *t, void *arg)
    {
    struct rcu_stall_chk_rdr *rscrp = arg;

    if (task_curr(t))
    - return false; // It is running, so decline to inspect it.
    + return -EBUSY; // It is running, so decline to inspect it.
    rscrp->nesting = t->rcu_read_lock_nesting;
    rscrp->rs = t->rcu_read_unlock_special;
    rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
    - return true;
    + return 0;
    }

    /*
    @@ -283,7 +283,7 @@ static int rcu_print_task_stall(struct r
    raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
    while (i) {
    t = ts[--i];
    - if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
    + if (task_try_func(t, check_slow_task, &rscr))
    pr_cont(" P%d", t->pid);
    else
    pr_cont(" P%d/%d:%c%c%c%c",
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4106,7 +4106,7 @@ try_to_wake_up(struct task_struct *p, un
    }

    /**
    - * try_invoke_on_locked_down_task - Invoke a function on task in fixed state
    + * task_try_func - Invoke a function on task in fixed state
    * @p: Process for which the function is to be invoked, can be @current.
    * @func: Function to invoke.
    * @arg: Argument to function.
    @@ -4119,14 +4119,13 @@ try_to_wake_up(struct task_struct *p, un
    * lightweight.
    *
    * Returns:
    - * @false if the task slipped out from under the locks.
    - * @true if the task was locked onto a runqueue or is sleeping.
    - * However, @func can override this by returning @false.
    + * -EAGAIN: we raced against task movement/state
    + * *: as returned by @func
    */
    -bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
    +int task_try_func(struct task_struct *p, task_try_f func, void *arg)
    {
    struct rq_flags rf;
    - bool ret = false;
    + int ret = -EAGAIN; /* raced, try again later */
    struct rq *rq;

    raw_spin_lock_irqsave(&p->pi_lock, rf.flags);

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