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    Subject[074/123] sched: Use rcu in sched_get/set_affinity()
    From: Thomas Gleixner <tglx@linutronix.de>

    commit 23f5d142519621b16cf2b378cf8adf4dcf01a616 upstream

    tasklist_lock is held read locked to protect the
    find_task_by_vpid() call and to prevent the task going away.
    sched_setaffinity acquires a task struct ref and drops tasklist
    lock right away. The access to the cpus_allowed mask is
    protected by rq->lock.

    rcu_read_lock() provides the same protection here.

    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    LKML-Reference: <20091209100706.789059966@linutronix.de>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    Signed-off-by: Mike Galbraith <efault@gmx.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    kernel/sched.c | 16 ++++++----------
    1 file changed, 6 insertions(+), 10 deletions(-)

    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -6563,22 +6563,18 @@ long sched_setaffinity(pid_t pid, const
    int retval;

    get_online_cpus();
    - read_lock(&tasklist_lock);
    + rcu_read_lock();

    p = find_process_by_pid(pid);
    if (!p) {
    - read_unlock(&tasklist_lock);
    + rcu_read_unlock();
    put_online_cpus();
    return -ESRCH;
    }

    - /*
    - * It is not safe to call set_cpus_allowed with the
    - * tasklist_lock held. We will bump the task_struct's
    - * usage count and then drop tasklist_lock.
    - */
    + /* Prevent p going away */
    get_task_struct(p);
    - read_unlock(&tasklist_lock);
    + rcu_read_unlock();

    if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
    retval = -ENOMEM;
    @@ -6664,7 +6660,7 @@ long sched_getaffinity(pid_t pid, struct
    int retval;

    get_online_cpus();
    - read_lock(&tasklist_lock);
    + rcu_read_lock();

    retval = -ESRCH;
    p = find_process_by_pid(pid);
    @@ -6680,7 +6676,7 @@ long sched_getaffinity(pid_t pid, struct
    task_rq_unlock(rq, &flags);

    out_unlock:
    - read_unlock(&tasklist_lock);
    + rcu_read_unlock();
    put_online_cpus();

    return retval;



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