osq_lock: avoid live-lock issue for RT task (8057ad31) · Commits · e / devices / android_kernel_xiaomi_markw

Live Lock due to task spinning while unqueue of CPU osq_node from optimistic_spin_queue. Task T1 had decremented mutex count to acquire the lock on CPU0. Before setting owner it got preempted. On CPU1 task T2 acquired osq_lock and started spinning on owner of mutex with preemption disabled. CPU1 runq has one task, so need_resched will not be set. On CPU0 task T3 tried to acquire osq_lock to spin on the same mutex. At this time following scenario causes soft lockup: After preemption of task T1, RT task T3 tried to acquire the same mutex. It will start spinning on the osq_lock until the lock is available or need_resched is set. For RT task, need_resched will not be set. Task T3 will not be able to bail out of the infinite loop. Change-Id: Icd599bdcebda23b22dcd5e6f9be8842bb8f2f5b6 Signed-off-by:

Prateek Sood <prsood@codeaurora.org>

kernel/locking/mcs_spinlock.c

+8 −1

Original line number	Diff line number	Diff line
		#include <linux/percpu.h>
		#include <linux/sched.h>
		#include "mcs_spinlock.h"
		#include <linux/sched/rt.h>

		#ifdef CONFIG_SMP

		@@ -87,6 +88,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
		{
		struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
		struct optimistic_spin_node prev, next;
		struct task_struct *task = current;
		int curr = encode_cpu(smp_processor_id());
		int old;

		@@ -114,8 +116,13 @@ bool osq_lock(struct optimistic_spin_queue *lock)
		while (!smp_load_acquire(&node->locked)) {
		/*
		* If we need to reschedule bail... so we can block.
		* If a task spins on owner on a CPU after acquiring
		* osq_lock while a RT task spins on another CPU to
		* acquire osq_lock, it will starve the owner from
		* completing if owner is to be scheduled on the same CPU.
		* It will be a live lock.
		*/
		if (need_resched())
		if (need_resched() \|\| rt_task(task))
		goto unqueue;

		cpu_relax_lowlatency();