Re: [PATCH] sched/vtime: Prevent unstable evaluation of WARN(vtime->state)

[Frederic Weisbecker]

Ping (again)?


On Thu, Jan 23, 2020 at 07:08:49PM +0100, Frederic Weisbecker wrote:
> From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> 
> As the vtime is sampled under loose seqcount protection by kcpustat, the
> vtime fields may change as the code flows. Where logic dictates a field
> has a static value, use a READ_ONCE.
> 
> Fixes: 74722bb223d0 ("sched/vtime: Bring up complete kcpustat accessor")
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  kernel/sched/cputime.c | 41 ++++++++++++++++++++++-------------------
>  1 file changed, 22 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index d43318a489f2..df3577149d2e 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -912,8 +912,10 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  	} while (read_seqcount_retry(&vtime->seqcount, seq));
>  }
>  
> -static int vtime_state_check(struct vtime *vtime, int cpu)
> +static int vtime_state_fetch(struct vtime *vtime, int cpu)
>  {
> +	int state = READ_ONCE(vtime->state);
> +
>  	/*
>  	 * We raced against a context switch, fetch the
>  	 * kcpustat task again.
> @@ -930,10 +932,10 @@ static int vtime_state_check(struct vtime *vtime, int cpu)
>  	 *
>  	 * Case 1) is ok but 2) is not. So wait for a safe VTIME state.
>  	 */
> -	if (vtime->state == VTIME_INACTIVE)
> +	if (state == VTIME_INACTIVE)
>  		return -EAGAIN;
>  
> -	return 0;
> +	return state;
>  }
>  
>  static u64 kcpustat_user_vtime(struct vtime *vtime)
> @@ -952,14 +954,15 @@ static int kcpustat_field_vtime(u64 *cpustat,
>  {
>  	struct vtime *vtime = &tsk->vtime;
>  	unsigned int seq;
> -	int err;
>  
>  	do {
> +		int state;
> +
>  		seq = read_seqcount_begin(&vtime->seqcount);
>  
> -		err = vtime_state_check(vtime, cpu);
> -		if (err < 0)
> -			return err;
> +		state = vtime_state_fetch(vtime, cpu);
> +		if (state < 0)
> +			return state;
>  
>  		*val = cpustat[usage];
>  
> @@ -972,7 +975,7 @@ static int kcpustat_field_vtime(u64 *cpustat,
>  		 */
>  		switch (usage) {
>  		case CPUTIME_SYSTEM:
> -			if (vtime->state == VTIME_SYS)
> +			if (state == VTIME_SYS)
>  				*val += vtime->stime + vtime_delta(vtime);
>  			break;
>  		case CPUTIME_USER:
> @@ -984,11 +987,11 @@ static int kcpustat_field_vtime(u64 *cpustat,
>  				*val += kcpustat_user_vtime(vtime);
>  			break;
>  		case CPUTIME_GUEST:
> -			if (vtime->state == VTIME_GUEST && task_nice(tsk) <= 0)
> +			if (state == VTIME_GUEST && task_nice(tsk) <= 0)
>  				*val += vtime->gtime + vtime_delta(vtime);
>  			break;
>  		case CPUTIME_GUEST_NICE:
> -			if (vtime->state == VTIME_GUEST && task_nice(tsk) > 0)
> +			if (state == VTIME_GUEST && task_nice(tsk) > 0)
>  				*val += vtime->gtime + vtime_delta(vtime);
>  			break;
>  		default:
> @@ -1039,23 +1042,23 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
>  {
>  	struct vtime *vtime = &tsk->vtime;
>  	unsigned int seq;
> -	int err;
>  
>  	do {
>  		u64 *cpustat;
>  		u64 delta;
> +		int state;
>  
>  		seq = read_seqcount_begin(&vtime->seqcount);
>  
> -		err = vtime_state_check(vtime, cpu);
> -		if (err < 0)
> -			return err;
> +		state = vtime_state_fetch(vtime, cpu);
> +		if (state < 0)
> +			return state;
>  
>  		*dst = *src;
>  		cpustat = dst->cpustat;
>  
>  		/* Task is sleeping, dead or idle, nothing to add */
> -		if (vtime->state < VTIME_SYS)
> +		if (state < VTIME_SYS)
>  			continue;
>  
>  		delta = vtime_delta(vtime);
> @@ -1064,15 +1067,15 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
>  		 * Task runs either in user (including guest) or kernel space,
>  		 * add pending nohz time to the right place.
>  		 */
> -		if (vtime->state == VTIME_SYS) {
> +		if (state == VTIME_SYS) {
>  			cpustat[CPUTIME_SYSTEM] += vtime->stime + delta;
> -		} else if (vtime->state == VTIME_USER) {
> +		} else if (state == VTIME_USER) {
>  			if (task_nice(tsk) > 0)
>  				cpustat[CPUTIME_NICE] += vtime->utime + delta;
>  			else
>  				cpustat[CPUTIME_USER] += vtime->utime + delta;
>  		} else {
> -			WARN_ON_ONCE(vtime->state != VTIME_GUEST);
> +			WARN_ON_ONCE(state != VTIME_GUEST);
>  			if (task_nice(tsk) > 0) {
>  				cpustat[CPUTIME_GUEST_NICE] += vtime->gtime + delta;
>  				cpustat[CPUTIME_NICE] += vtime->gtime + delta;
> @@ -1083,7 +1086,7 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
>  		}
>  	} while (read_seqcount_retry(&vtime->seqcount, seq));
>  
> -	return err;
> +	return 0;
>  }
>  
>  void kcpustat_cpu_fetch(struct kernel_cpustat *dst, int cpu)
> -- 
> 2.25.0
>