-
Xunlei Pang authored
A crash happened while I was playing with deadline PI rtmutex. BUG: unable to handle kernel NULL pointer dereference at 0000000000000018 IP: [<ffffffff810eeb8f>] 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: [<ffffffff810b658c>] enqueue_task+0x2c/0x80 [<ffffffff810ba763>] activate_task+0x23/0x30 [<ffffffff810d0ab5>] pull_dl_task+0x1d5/0x260 [<ffffffff810d0be6>] pre_schedule_dl+0x16/0x20 [<ffffffff8164e783>] __schedule+0xd3/0x900 [<ffffffff8164efd9>] schedule+0x29/0x70 [<ffffffff8165035b>] __rt_mutex_slowlock+0x4b/0xc0 [<ffffffff81650501>] rt_mutex_slowlock+0xd1/0x190 [<ffffffff810eeb33>] rt_mutex_timed_lock+0x53/0x60 [<ffffffff810ecbfc>] futex_lock_pi.isra.18+0x28c/0x390 [<ffffffff810ed8b0>] do_futex+0x190/0x5b0 [<ffffffff810edd50>] 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 <peterz@infradead.org> Signed-off-by:
Xunlei Pang <xlpang@redhat.com> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by:
Steven Rostedt <rostedt@goodmis.org> Reviewed-by:
Thomas Gleixner <tglx@linutronix.de> 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.orgSigned-off-by:
Thomas Gleixner <tglx@linutronix.de>
e96a7705