Re: v2.6.21.5-rt19 (sched_getaffinity?)

[Ingo Molnar <mingo@elte.hu>]

* Fernando Lopez-Lezcano <nando@ccrma.Stanford.EDU> wrote:

> > does lockdep pinpoint anything?
> 
> Lots of stuff, and at the end the lock report for the problem. 
> Hopefully some of this will help... I have attached the whole bootup 
> sequence as logged in /var/log/messages.

yeah, it pinpointed the bug. It seems to be an interaction between 
RCU-preempt (Paul Cc:-ed) and sched_mc_power_savings_store(): 
detach_destroy_domains() uses synchronize_sched() which uses 
getaffinity, which takes sched_hotcpu_mutex, and 
arch_reinit_sched_domains does it too - see the lockdep report below. 
I've added a quick workaround below as well, which should keep your box 
from hanging.

	Ingo

=============================================
[ INFO: possible recursive locking detected ]
[ 2.6.22-0182.rt4.3.fc7.ccrmart #1
---------------------------------------------
sched-powersave/3251 is trying to acquire lock:
 (sched_hotcpu_mutex){--..}, at: [<c0424a37>] sched_getaffinity+0x14/0x94

but task is already holding lock:
 (sched_hotcpu_mutex){--..}, at: [<c04245a5>] arch_reinit_sched_domains+0xe/0x33

other info that might help us debug this:
1 lock held by sched-powersave/3251:
 #0:  (sched_hotcpu_mutex){--..}, at: [<c04245a5>] arch_reinit_sched_domains+0xe/0x33

stack backtrace:
 [<c040600c>] show_trace_log_lvl+0x1a/0x2f
 [<c0406ae8>] show_trace+0x12/0x14
 [<c0406b50>] dump_stack+0x16/0x18
 [<c0446f46>] __lock_acquire+0x172/0xb67
 [<c0447d03>] lock_acquire+0x56/0x6f
 [<c061d414>] _mutex_lock+0x2b/0x38
 [<c0424a37>] sched_getaffinity+0x14/0x94
 [<c0460841>] __synchronize_sched+0x11/0x5f
 [<c0423fa8>] detach_destroy_domains+0x2c/0x30
 [<c04245af>] arch_reinit_sched_domains+0x18/0x33
 [<c0424606>] sched_power_savings_store+0x3c/0x49
 [<c0424634>] sched_mc_power_savings_store+0xe/0x10
 [<c0561f11>] sysdev_class_store+0x20/0x25
 [<c04bbc6c>] sysfs_write_file+0xaf/0xd0
 [<c048183c>] vfs_write+0xaf/0x163
 [<c0481e8a>] sys_write+0x3d/0x61
 [<c040501a>] syscall_call+0x7/0xb
 =======================
thinkpad_acpi: ThinkPad ACPI Extras v0.14

--------------------->
Index: linux-rt.q/kernel/sched.c
===================================================================
--- linux-rt.q.orig/kernel/sched.c
+++ linux-rt.q/kernel/sched.c
@@ -6699,7 +6699,6 @@ static void detach_destroy_domains(const
 
 	for_each_cpu_mask(i, *cpu_map)
 		cpu_attach_domain(NULL, i);
-	synchronize_sched();
 	arch_destroy_sched_domains(cpu_map);
 }