CVE-2025-21897

In the Linux kernel, the following vulnerability has been resolved:

schedext: Fix picktask_scx() picking non-queued tasks when it's called without balance()

a6250aa251ea ("schedext: Handle cases where picktaskscx() is called without preceding balancescx()") added a workaround to handle the cases where picktaskscx() is called without prececing balancescx() which is due to a fair class bug where picktaksfair() may return NULL after a true return from balancefair().

The workaround detects when picktaskscx() is called without preceding balancescx() and emulates SCXRQBALKEEP and triggers kicking to avoid stalling. Unfortunately, the workaround code was testing whether @prev was on SCX to decide whether to keep the task running. This is incorrect as the task may be on SCX but no longer runnable.

This could lead to a non-runnable task to be returned from picktaskscx() which cause interesting confusions and failures. e.g. A common failure mode is the task ending up with (!onrq && oncpu) state which can cause potential wakers to busy loop, which can easily lead to deadlocks.

Fix it by testing whether @prev has SCXTASKQUEUED set. This makes @prevonscx only used in one place. Open code the usage and improve the comment while at it.