On Wed, Apr 06, 2016 at 08:59:15PM +0800, Xunlei Pang wrote: > A crash happened while I'm 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: > [<ffffffff810cf8aa>] ? enqueue_task_dl+0x2a/0x320 > [<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 > [<ffffffff810cfa15>] ? enqueue_task_dl+0x195/0x320 > [<ffffffff810d0bac>] ? prio_changed_dl+0x6c/0x90 > [<ffffffff810ed8b0>] do_futex+0x190/0x5b0 > [<ffffffff810edd50>] SyS_futex+0x80/0x180 > [<ffffffff8165a089>] system_call_fastpath+0x16/0x1b > RIP [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30 > > 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 a new pointer "pi_top_task" > in task_struct, and update it to be the top waiter task(this waiter > is updated under pi_lock) in rt_mutex_setprio() which is under > both pi_lock and rq lock, then ensure all its accessers be under > rq lock (or pi_lock), this can safely fix the crash. > > This patch is originated from "Peter Zijlstra", with several > tweaks and improvements by me.
I would suggest doing the rt_mutex_postunlock() thing as a separate patch, it has some merit outside of these changes and reduces the total amount of complexity in this patch. Also, I would very much like Thomas to ack this patch before I take it, but since its conference season this might take a little while. Esp. the change marked with XXX is something that I'm not sure about.
