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. Signed-off-by: Prateek Sood <[email protected]> --- kernel/locking/osq_lock.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index 05a3785..99b8d99 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -1,6 +1,7 @@ #include <linux/percpu.h> #include <linux/sched.h> #include <linux/osq_lock.h> +#include <linux/sched/rt.h> /* * An MCS like lock especially tailored for optimistic spinning for sleeping @@ -85,6 +86,7 @@ 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; @@ -118,8 +120,13 @@ bool osq_lock(struct optimistic_spin_queue *lock) while (!READ_ONCE(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(); -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
