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SubjectRe: [PATCH] sched/vtime: Prevent unstable evaluation of WARN(vtime->state)
Ping (again)?


On Thu, Jan 23, 2020 at 07:08:49PM +0100, Frederic Weisbecker wrote:
> From: Chris Wilson <chris@chris-wilson.co.uk>
>
> 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@chris-wilson.co.uk>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> 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
>

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