The runqueue locks are special in that the owner changes over a context switch. To ensure that this is accounted for in CONFIG_DEBUG_SPINLOCK builds, finish_lock_switch updates rq->lock.owner while the lock is held.
However, this happens *after* prev->on_cpu is cleared, which allows prev to be scheduled on another CPU. If prev then attempts to acquire the same rq lock, before the updated rq->lock.owner is made visible, it will see itself as the owner. This can happen in virtual environments, where a vCPU can be preempted for an arbitrarily long period between updating prev->on_cpu and rq->lock.owner, as has been observed on arm64 and x86 under KVM, e.g. BUG: spinlock recursion on CPU#1, syz-executor1/1521 lock: 0xffff80001a764080, .magic: dead4ead, .owner: syz-executor1/1521, .owner_cpu: 0 CPU: 1 PID: 1521 Comm: syz-executor1 Not tainted 4.15.0 #3 Hardware name: linux,dummy-virt (DT) Call trace: dump_backtrace+0x0/0x330 arch/arm64/kernel/time.c:52 show_stack+0x20/0x30 arch/arm64/kernel/traps.c:151 __dump_stack lib/dump_stack.c:17 [inline] dump_stack+0xd0/0x120 lib/dump_stack.c:53 spin_dump+0x150/0x1f0 kernel/locking/spinlock_debug.c:67 spin_bug kernel/locking/spinlock_debug.c:75 [inline] debug_spin_lock_before kernel/locking/spinlock_debug.c:84 [inline] do_raw_spin_lock+0x1e4/0x250 kernel/locking/spinlock_debug.c:112 __raw_spin_lock include/linux/spinlock_api_smp.h:143 [inline] _raw_spin_lock+0x44/0x50 kernel/locking/spinlock.c:144 __task_rq_lock+0xc0/0x288 kernel/sched/core.c:103 wake_up_new_task+0x384/0x798 kernel/sched/core.c:2465 _do_fork+0x1b4/0xa78 kernel/fork.c:2069 SYSC_clone kernel/fork.c:2154 [inline] SyS_clone+0x48/0x60 kernel/fork.c:2132 el0_svc_naked+0x20/0x24 Let's avoid this updating rq->lock.owner before clearing prev->on_cpu. As the latter is a release, it ensures that the new owner is visible to prev if it is scheduled on another CPU. Signed-off-by: Mark Rutland <mark.rutl...@arm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Ingo Molnar <mi...@kernel.org> --- kernel/sched/sched.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index b19552a212de..4f0d2e3701c3 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1342,6 +1342,10 @@ static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next) static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev) { +#ifdef CONFIG_DEBUG_SPINLOCK + /* this is a valid case when another task releases the spinlock */ + rq->lock.owner = current; +#endif #ifdef CONFIG_SMP /* * After ->on_cpu is cleared, the task can be moved to a different CPU. @@ -1355,10 +1359,6 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev) */ smp_store_release(&prev->on_cpu, 0); #endif -#ifdef CONFIG_DEBUG_SPINLOCK - /* this is a valid case when another task releases the spinlock */ - rq->lock.owner = current; -#endif /* * If we are tracking spinlock dependencies then we have to * fix up the runqueue lock - which gets 'carried over' from -- 2.11.0