mm/kmemleak: avoid soft lockup when scanning task stacks

Overview

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

mm/kmemleak: avoid soft lockup when scanning task stacks

Patch series "mm/kmemleak: avoid soft lockup when scanning task", v3.

kmemleak_scan() scans every task stack under one rcu_read_lock() with no reschedule point, which can trip the soft lockup watchdog on hosts with very many threads.

That prints the following message, depending on the workload+host configuration:

  watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537]
   scan_block
   kmemleak_scan
   kmemleak_scan_thread
   kthread

Patch 1 walks the tasks with find_ge_pid() so the scan reschedules between tasks

Patches 2-3 let the scan loops stop early once a scan is interrupted.

This patch (of 3):

kmemleak_scan() walks every thread and scans its kernel stack under a single rcu_read_lock() with no reschedule point. On a host with very many threads -- amplified by KASAN/lockdep in debug builds -- this loop can hog a CPU long enough to trip the soft lockup watchdog:

watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] scan_block kmemleak_scan kmemleak_scan_thread kthread

A cond_resched() cannot be added directly: the loop runs inside an RCU read-side critical section.

Walk the tasks one PID at a time with find_ge_pid(), taking the RCU read lock only to look up and pin each task. The stack is then scanned with no lock held, so cond_resched() runs between tasks and the scan stops early on scan_should_stop(). This follows the next_tgid()/task_seq_get_next() iteration pattern and keeps each RCU critical section short.

Affected

• Linux >= c4b28963fd79457315783b3b0f21c01eb88cfdc1 < f1cd6c4c35243377882c47236281d03ee3be6eeb
• Linux >= c4b28963fd79457315783b3b0f21c01eb88cfdc1 < 9a1b12c06c192290b8479de48f66f1e75d89c4b7
• Linux >= c4b28963fd79457315783b3b0f21c01eb88cfdc1 < 3fc8044251de21555fb02c365fa681bab8b0db55
• Linux >= c4b28963fd79457315783b3b0f21c01eb88cfdc1 < 1838c704bcb4fd3556cf67513c6f97a39099e35c
• Linux >= c4b28963fd79457315783b3b0f21c01eb88cfdc1 < 5d10d4e19e6daa487f0cd0ea6cba472325de92f9
• Linux 5.10

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Low · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, … · updated 2026-09-15 · vex