On Mon, Jul 23, 2007 at 06:37:19AM -0700, Daniel Walker wrote: > On Sun, 2007-07-22 at 20:13 -0700, Paul E. McKenney wrote: > > On Sun, Jul 22, 2007 at 10:22:37AM -0700, Daniel Walker wrote: > > > > > > Strange rcu_read_unlock() which causes a imbalance, and boot hang.. I > > > didn't notice a reason for it, and removing it allows my system to make > > > progress. > > > > > > This should go into the preempt-realtime-sched.patch > > > > Strange. I have been getting boots, kernbench runs, and rcutorture > > tests despite its being present. Though perhaps it was the cause > > of the (non-fatal) "scheduling while atomic" I got during a kernbench > > run -- I was assuming it was my fault somehow. Non-reproducible. :-/ > > > > Thanx, Paul > > I had a strange config (for real time) .. All the new options turned > off, and classic RCU .. However, a regular PREEMPT_RT kernel booted > fine. The classic RCU made it easy to spot this since the preempt count > goes negative. Ah! That would happen in a CONFIG_PREEMPT kernel. The bug would be invisible in a non-CONFIG_PREEMPT kernel. > I was wondering what kind of side effects would happen with the > preemptible RCU .. Well, if you had only one rcu_read_lock() outstanding in the system, it would appear to be in a quiescent state, which would not be good. Given enough executions of that code, grace periods would cease and you would OOM. I didn't see any OOMs despite overnight rcutorture runs -- which would definitely have spotted halted grace periods -- so perhaps I just happened to never invoke sched_yield? > > BUG: sleeping function called from invalid context cc1(29651) at kernel/rtmutex.c:636 > > in_atomic():1 [00000001], irqs_disabled():0 > > [<c0119f50>] __might_sleep+0xf3/0xf9 > > [<c031600e>] __rt_spin_lock+0x21/0x3c > > [<c014102c>] get_zone_pcp+0x20/0x29 > > [<c0141a40>] free_hot_cold_page+0xdc/0x167 > > [<c013a3f4>] add_preempt_count+0x12/0xcc > > [<c0110d92>] pgd_dtor+0x0/0x1 > > [<c015d865>] quicklist_trim+0xb7/0xe3 > > [<c0111025>] check_pgt_cache+0x19/0x1c > > [<c0148df5>] free_pgtables+0x54/0x12c > > [<c013a3f4>] add_preempt_count+0x12/0xcc > > [<c014e5be>] unmap_region+0xeb/0x13b > > This looks a little like what Rui Nuno Capela is seeing , but his kernel > hangs. Hmmm... In this particular case, the test (kernbench) was fairly short, so maybe I just didn't let it run long enough. Thanx, Paul > > BUG: scheduling while atomic: cc1/0x00000002/29745, CPU#0 > > [<c03146c8>] __sched_text_start+0xb0/0x4fe > > [<c0116432>] try_to_wake_up+0x31b/0x326 > > [<c013a3f4>] add_preempt_count+0x12/0xcc > > [<c013a3f4>] add_preempt_count+0x12/0xcc > > [<c0314bfc>] schedule+0xe6/0x100 > > [<c03158db>] rt_spin_lock_slowlock+0xc5/0x145 > > [<c0316027>] __rt_spin_lock+0x3a/0x3c > > [<c014102c>] get_zone_pcp+0x20/0x29 > > [<c0141a40>] free_hot_cold_page+0xdc/0x167 > > [<c013a3f4>] add_preempt_count+0x12/0xcc > > [<c0110d92>] pgd_dtor+0x0/0x1 > > [<c015d865>] quicklist_trim+0xb7/0xe3 > > [<c0111025>] check_pgt_cache+0x19/0x1c > > [<c0148df5>] free_pgtables+0x54/0x12c > > [<c013a3f4>] add_preempt_count+0x12/0xcc > > [<c014e5be>] unmap_region+0xeb/0x13b > > [<c014e861>] do_munmap+0xea/0xff > > [<c014e8a7>] sys_munmap+0x31/0x40 > > [<c010276a>] syscall_call+0x7/0xb > > [<c0310000>] _shift_data_right_pages+0xb9/0xd1 > > ======================= > > --------------------------- > > | preempt count: 00000002 ] > > | 2-level deep critical section nesting: > > ---------------------------------------- > > .. [<c015d7c8>] .... quicklist_trim+0x1a/0xe3 > > .....[<00000000>] .. ( <= _stext+0x3fefff50/0x14) > > .. [<c031462b>] .... __sched_text_start+0x13/0x4fe > > .....[<00000000>] .. ( <= _stext+0x3fefff50/0x14) > > > > > Signed-Off-By: Daniel Walker <dwalker@xxxxxxxxxx> > > > > > > Index: linux-2.6.22.1/kernel/sched.c > > > =================================================================== > > > --- linux-2.6.22.1.orig/kernel/sched.c 2007-07-22 16:47:37.000000000 +0000 > > > +++ linux-2.6.22.1/kernel/sched.c 2007-07-22 16:16:48.000000000 +0000 > > > @@ -4900,7 +4900,6 @@ asmlinkage long sys_sched_yield(void) > > > * no need to preempt or enable interrupts: > > > */ > > > spin_unlock_no_resched(&rq->lock); > > > - rcu_read_unlock(); > > > > > > __schedule(); > > > > > > > > > > > > - > > > To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in > > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
