From e96a7705e7d3fef96aec9b590c63b2f6f7d2ba22 Mon Sep 17 00:00:00 2001 From: Xunlei Pang Date: Thu, 23 Mar 2017 15:56:08 +0100 Subject: sched/rtmutex/deadline: Fix a PI crash for deadline tasks A crash happened while I was playing with deadline PI rtmutex. BUG: unable to handle kernel NULL pointer dereference at 0000000000000018 IP: [] rt_mutex_get_top_task+0x1f/0x30 PGD 232a75067 PUD 230947067 PMD 0 Oops: 0000 [#1] SMP CPU: 1 PID: 10994 Comm: a.out Not tainted Call Trace: [] enqueue_task+0x2c/0x80 [] activate_task+0x23/0x30 [] pull_dl_task+0x1d5/0x260 [] pre_schedule_dl+0x16/0x20 [] __schedule+0xd3/0x900 [] schedule+0x29/0x70 [] __rt_mutex_slowlock+0x4b/0xc0 [] rt_mutex_slowlock+0xd1/0x190 [] rt_mutex_timed_lock+0x53/0x60 [] futex_lock_pi.isra.18+0x28c/0x390 [] do_futex+0x190/0x5b0 [] SyS_futex+0x80/0x180 This is because rt_mutex_enqueue_pi() and rt_mutex_dequeue_pi() are only protected by pi_lock when operating pi waiters, while rt_mutex_get_top_task(), will access them with rq lock held but not holding pi_lock. In order to tackle it, we introduce new "pi_top_task" pointer cached in task_struct, and add new rt_mutex_update_top_task() to update its value, it can be called by rt_mutex_setprio() which held both owner's pi_lock and rq lock. Thus "pi_top_task" can be safely accessed by enqueue_task_dl() under rq lock. Originally-From: Peter Zijlstra Signed-off-by: Xunlei Pang Signed-off-by: Peter Zijlstra (Intel) Acked-by: Steven Rostedt Reviewed-by: Thomas Gleixner Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170323150216.157682758@infradead.org Signed-off-by: Thomas Gleixner --- include/linux/sched.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/linux/sched.h') diff --git a/include/linux/sched.h b/include/linux/sched.h index d67eee84fd43..1ea2eee7bc4f 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -775,6 +775,8 @@ struct task_struct { /* PI waiters blocked on a rt_mutex held by this task: */ struct rb_root pi_waiters; struct rb_node *pi_waiters_leftmost; + /* Updated under owner's pi_lock and rq lock */ + struct task_struct *pi_top_task; /* Deadlock detection and priority inheritance handling: */ struct rt_mutex_waiter *pi_blocked_on; #endif -- cgit v1.2.3