[PATCH V2] sched,psi: fix the 'int' underflow for psi

[Charan Teja Reddy]

psi_group_cpu->tasks, represented by the unsigned int, stores the number
of tasks that could be stalled on a psi resource(io/mem/cpu).
Decrementing these counters at zero leads to wrapping which further
leads to the psi_group_cpu->state_mask is being set with the respective
pressure state. This could result into the unnecessary time sampling for
the pressure state thus cause the spurious psi events. This can further
lead to wrong actions being taken at the user land based on these psi
events.
Though psi_bug is set under these conditions but that just for debug
purpose. Fix it by decrementing the ->tasks count only when it is
non-zero.

Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
---
 
Changes in V1: https://lore.kernel.org/patchwork/patch/1413894/

 kernel/sched/psi.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 967732c..651218d 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -711,14 +711,15 @@ static void psi_group_change(struct psi_group *group, int cpu,
 	for (t = 0, m = clear; m; m &= ~(1 << t), t++) {
 		if (!(m & (1 << t)))
 			continue;
-		if (groupc->tasks[t] == 0 && !psi_bug) {
+		if (groupc->tasks[t]) {
+			groupc->tasks[t]--;
+		} else if (!psi_bug) {
 			printk_deferred(KERN_ERR "psi: task underflow! cpu=%d t=%d tasks=[%u %u %u %u] clear=%x set=%x\n",
 					cpu, t, groupc->tasks[0],
 					groupc->tasks[1], groupc->tasks[2],
 					groupc->tasks[3], clear, set);
 			psi_bug = 1;
 		}
-		groupc->tasks[t]--;
 	}
 
 	for (t = 0; set; set &= ~(1 << t), t++)
-- 
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