[Skiboot] [PATCH] Fix possible deadlock with DEBUG build

[Vasant Hegde]

Sample output from Cédric:
-------------------------
[   88.294111649,7] cpu_idle_p9 called on cpu 0x063c with pm disabled
[   88.289365222,7] cpu_idle_p9 called on cpu 0x025f with pm disabled
[   88.289900684,7] cpu_idle_p9 called on cpu 0x045f with pm disabled
[   88.302621295,7] CHIPTOD: Base TFMR=0x2512000000000000
[   88.289899701,7] cpu_idle_p9 called on cpu 0x0456 with pm disabled
LOCK ERROR: Deadlock detected @0x30402740 (state: 0x0000000400000001)
[   88.332264757,3] ***********************************************
[   88.332300051,3] < assert failed at core/lock.c:32 >
[   88.332328282,3]     .
[   88.332347335,3]      .
[   88.332364894,3]       .
[   88.332377963,3]         OO__)
[   88.332395458,3]        <"__/
[   88.332412628,3]         ^ ^
[   88.332450246,3] Fatal TRAP at 00000000300286a0   .lock_error+0x64  MSR 9000000000021002
[   88.332501812,3] CFAR : 00000000300414f4 MSR  : 9000000000021002
[   88.332536539,3] SRR0 : 00000000300286a0 SRR1 : 9000000000021002
[   88.332574644,3] HSRR0: 0000000030020024 HSRR1: 9000000000001000
[   88.332610635,3] DSISR: 00000000         DAR  : 0000000000000000
[   88.332650628,3] LR   : 0000000030028690 CTR  : 00000000300f9fa0
[   88.332684451,3] CR   : 20002000         XER  : 00000000
[   88.332712767,3] GPR00: 0000000030028690 GPR16: 0000000032c98000
[   88.332748046,3] GPR01: 0000000032c9b0a0 GPR17: 0000000000000000
[   88.332784060,3] GPR02: 0000000030169d00 GPR18: 0000000000000000
[   88.332822091,3] GPR03: 0000000032c9b310 GPR19: 0000000000000000
[   88.332861357,3] GPR04: 0000000030041480 GPR20: 0000000000000000
[   88.332897229,3] GPR05: 0000000000000000 GPR21: 0000000000000000
[   88.332937051,3] GPR06: 0000000000000010 GPR22: 0000000000000000
[   88.332968463,3] GPR07: 0000000000000000 GPR23: 0000000000000000
[   88.333007333,3] GPR08: 000000000002cbb5 GPR24: 0000000000000000
[   88.333041971,3] GPR09: 0000000000000000 GPR25: 0000000000000000
[   88.333081073,3] GPR10: 0000000000000000 GPR26: 0000000000000003
[   88.333114301,3] GPR11: 3839616263646566 GPR27: 0000000000000211
[   88.333156040,3] GPR12: 0000000020002000 GPR28: 000000003042a134
[   88.333189222,3] GPR13: 0000000000000000 GPR29: 0000000030402740
[   88.333225638,3] GPR14: 0000000000000000 GPR30: 0000000000000001
[   88.333259730,3] GPR15: 0000000000000000 GPR31: 0000000000000000
CPU 0211 Backtrace:
 S: 0000000032c9b3b0 R: 0000000030028690   .lock_error+0x54
 S: 0000000032c9b440 R: 0000000030028828   .add_lock_request+0xd0
 S: 0000000032c9b4f0 R: 0000000030028a9c   .lock_caller+0x8c
 S: 0000000032c9b5a0 R: 0000000030021b30   .__mcount_stack_check+0x70
 S: 0000000032c9b650 R: 00000000300fabb0   .list_check_node+0x1c
 S: 0000000032c9b6f0 R: 00000000300fac98   .list_check+0x38
 S: 0000000032c9b790 R: 00000000300289bc   .try_lock_caller+0xac
 S: 0000000032c9b830 R: 0000000030028ad8   .lock_caller+0xc8
 S: 0000000032c9b8e0 R: 0000000030028d74   .lock_recursive_caller+0x54
 S: 0000000032c9b980 R: 0000000030020cb8   .console_write+0x48
 S: 0000000032c9ba30 R: 00000000300445a8   .vprlog+0xc8
 S: 0000000032c9bc20 R: 0000000030044630   ._prlog+0x50
 S: 0000000032c9bcb0 R: 0000000030029204   .cpu_idle_p9+0x74
 S: 0000000032c9bd40 R: 0000000030029628   .cpu_idle_pm+0x4c
 S: 0000000032c9bde0 R: 0000000030023fe0   .__secondary_cpu_entry+0xa0
 S: 0000000032c9be70 R: 0000000030024034   .secondary_cpu_entry+0x40
 S: 0000000032c9bf00 R: 0000000030003290   secondary_wait+0x8c

CPU 0x4:
  opal_run_pollers ->
    check_stacks -> takes stack_check_lock lock
      prlog ->
        console_write -> waits for con_lock

CPU 0x211
  cpu_idle_p9 ->
    prlog ->
      console_write -> Takes con_lock lock
       list_check_node -> tries to take stack_check_lock and hits deadlock.

I think we don't need to hold `stack_check_lock` while printing
backtraces. Instead it makes sense to hold backtrace lock (bt_lock)
and print output.

Reported-by: Cédric Le Goater <clg at kaod.org>
Signed-off-by: Vasant Hegde <hegdevasant at linux.vnet.ibm.com>
---
 core/stack.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/core/stack.c b/core/stack.c
index 05506f6b3..3edf98411 100644
--- a/core/stack.c
+++ b/core/stack.c
@@ -250,7 +250,7 @@ void check_stacks(void)
 		unlock(&stack_check_lock);
 	}
 	if (lowest) {
-		lock(&stack_check_lock);
+		lock(&bt_lock);
 		prlog(PR_NOTICE, "CPU %04x lowest stack mark %lld bytes left"
 		      " pc=%08llx token=%lld\n",
 		      lowest->pir, lowest->stack_bot_mark, lowest->stack_bot_pc,
@@ -258,7 +258,7 @@ void check_stacks(void)
 		backtrace_print(lowest->stack_bot_bt,
 				&lowest->stack_bot_bt_metadata,
 				NULL, NULL, true);
-		unlock(&stack_check_lock);
+		unlock(&bt_lock);
 	}
 
 	this_cpu()->in_mcount = false;
-- 
2.26.2