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Subject[PATCH] sched/vtime: Prevent unstable evaluation of WARN(vtime->state)
Date
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: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/cputime.c | 39 +++++++++++++++++++++------------------
1 file changed, 21 insertions(+), 18 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index d43318a489f2..96bbd52f43ae 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -914,6 +914,8 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)

static int vtime_state_check(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_check(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_check(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.rc0
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