(Resending)
On Friday, April 28, 2017 07:32:36 PM Junaid Shahid wrote:
> In general, if kthread_unpark() and kthread_parkme() execute together,
> the kthread is supposed to be in an unparked state. This is because
> kthread_unpark() either wakes up the thread if it already got parked,
> or it cancels a prior kthread_park() call and hence renders the
> kthread_parkme() moot.
>
> However, if kthread_unpark() happens to execute between the time that
> kthread_parkme() checks the KTHREAD_SHOULD_STOP flag and sets the
> KTHREAD_IS_PARKED flag, then kthread_unpark() will not wake up the
> thread and it will remain in a parked state.
>
> This is fixed by making the checking of KTHREAD_SHOULD_STOP and the
> setting of KTHREAD_IS_PARKED atomic via a cmpxchg inside kthread_parkme.
>
> Signed-off-by: Junaid Shahid <juna...@google.com>
> ---
> kernel/kthread.c | 16 ++++++++++++----
> 1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 26db528c1d88..651f03baf62f 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -169,12 +169,20 @@ void *kthread_probe_data(struct task_struct *task)
>
> static void __kthread_parkme(struct kthread *self)
> {
> + ulong flags;
> +
> __set_current_state(TASK_PARKED);
> - while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
> - if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
> - complete(&self->parked);
> - schedule();
> + flags = self->flags;
> +
> + while (test_bit(KTHREAD_SHOULD_PARK, &flags)) {
> + if (cmpxchg(&self->flags, flags,
> + flags | (1 << KTHREAD_IS_PARKED)) == flags) {
> + if (!test_bit(KTHREAD_IS_PARKED, &flags))
> + complete(&self->parked);
> + schedule();
> + }
> __set_current_state(TASK_PARKED);
> + flags = self->flags;
> }
> clear_bit(KTHREAD_IS_PARKED, &self->flags);
> __set_current_state(TASK_RUNNING);
>
