Re: [PATCH] softlockup: ensure the task has been switched out once

[Mandeep Singh Baines <msb@google.com>]

Frederic Weisbecker (fweisbec@gmail.com) wrote:
> When we check if a task has been switched out since the last scan, we might
> have a race condition on the following scenario:
> 
> _ the task is freshly created and scheduled
> _ it puts its state to TASK_UNINTERRUPTIBLE and is not yet switched out
> _ check_hung_task() scans this task and will report a false positive because
>   t->nvcsw + t->nivcsw == t->last_switch_count == 0
> 
> Add a check for such cases.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
>  kernel/hung_task.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 0c924de..022a492 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -72,7 +72,13 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>  {
>  	unsigned long switch_count = t->nvcsw + t->nivcsw;
>  
> -	if (t->flags & PF_FROZEN)
> +	/*
> +	 * Ensure the task is not frozen.
> +	 * Also, when a freshly created task is scheduled once, changes
> +	 * its state to TASK_UNINTERRUPTIBLE without having ever been
> +	 * switched out once, it musn't be checked.
> +	 */
> +	if (unlikely(t->flags & PF_FROZEN || !switch_count))
>  		return;
>  
>  	if (switch_count != t->last_switch_count) {
> -- 
> 1.6.1
> 
> 


Looks good to me. Thanks for fixing this!

Acked-by: Mandeep Singh Baines <msb@google.com>