Re: [PATCH] kthread: Fix race condition between kthread_parkme() and kthread_unpark()

[Junaid Shahid <junaids@google.com>]

(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 <junaids@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);
>