rcu warnings cause stack overflow

[Heiko Carstens <heiko.carstens@de.ibm.com>]

Hi Frederic,

your patch 00f49e5729 "rcu: Warn when rcu_read_lock() is used in extended
quiescent state" adds a WARN_ON_ONCE to rcu_lock_acquire().
Actually this found a bug on s390 (thanks!) but it probably didn't work
as expected.
On architectures which implement WARN_ON_ONCE with an exception this
additional warning will lead to a stack overflow (if it triggers):

[   55.746956] Kernel stack overflow.
[   55.746966] Modules linked in: qeth_l3 binfmt_misc dm_multipath scsi_dh dm_mod qeth vmur ccwgroup [last unloaded: scsi_wait_
scan]
[   55.746999] CPU: 0 Not tainted 3.3.0-rc1-00167-gf8275f9 #90
[   55.747005] Process swapper/0 (pid: 0, task: 0000000000911100, ksp: 0000000000907d50)
[   55.747013] Krnl PSW : 0404000180000000 00000000005d5728 (illegal_op+0x1c/0x134)
[   55.747034]            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:0 CC:0 PM:0 EA:3
[   55.747043] Krnl GPRS: 0000000000000001 00000000005d570c 00000000009040e8 0000000000000002
[   55.747054]            00000000005d83dc ffffffffffffffff 0000000000000000 0400000000907cc8
[   55.747064]            0404100180000000 00000000005d8478 0000000000000008 00000000009040e8
[   55.747074]            0000000000904000 00000000005dc550 0000000000904048 0000000000904048
[   55.747096] Krnl Code: 00000000005d571c: b90400ef            lgr     %r14,%r15
[   55.747118]            00000000005d5720: b90400b2            lgr     %r11,%r2
[   55.747194]           #00000000005d5724: a7840001            brc     8,5d5726
[   55.747205]           >00000000005d5728: a7fbff18            aghi    %r15,-232
[   55.747216]            00000000005d572c: e3e0f0980024        stg     %r14,152(%r15)
[   55.747228]            00000000005d5732: e31020100004        lg      %r1,16(%r2)
[   55.747242]            00000000005d5738: 58c020a0            l       %r12,160(%r2)
[   55.747257]            00000000005d573c: 91012009            tm      9(%r2),1
[   55.747276] Call Trace:
[   55.747282] ([<00000000005d60b4>] pgm_check_handler+0x154/0x158)
[   55.747296]  [<00000000005d8478>] __atomic_notifier_call_chain+0xd8/0xfc
[   55.747309] ([<00000000005d83dc>] __atomic_notifier_call_chain+0x3c/0xfc)
[   55.747322]  [<00000000005d84c6>] atomic_notifier_call_chain+0x2a/0x3c
[   55.747335]  [<00000000005d852a>] notify_die+0x52/0x60
[   55.747349]  [<00000000005d57da>] illegal_op+0xce/0x134
[   55.747364]  [<00000000005d60b4>] pgm_check_handler+0x154/0x158

[...lots more of the same...] 

[   55.747379]  [<00000000005d8478>] __atomic_notifier_call_chain+0xd8/0xfc
[   55.747425] ([<00000000005d83dc>] __atomic_notifier_call_chain+0x3c/0xfc)
[   55.747432]  [<00000000005d84c6>] atomic_notifier_call_chain+0x2a/0x3c
[   55.747440]  [<00000000005d852a>] notify_die+0x52/0x60
[   55.747448]  [<00000000005d57da>] illegal_op+0xce/0x134
[   55.747457]  [<00000000005d60b4>] pgm_check_handler+0x154/0x158
[   55.747797]  [<00000000005d8478>] __atomic_notifier_call_chain+0xd8/0xfc
[   55.747806] ([<00000000005d83dc>] __atomic_notifier_call_chain+0x3c/0xfc)
[   55.747816]  [<00000000005d84c6>] atomic_notifier_call_chain+0x2a/0x3c
[   55.747826]  [<00000000005d852a>] notify_die+0x52/0x60
[   55.748456]  [<00000000005d57da>] illegal_op+0xce/0x134
[   55.748463]  [<00000000005d60b4>] pgm_check_handler+0x154/0x158
[   55.748472]  [<000000000017afa0>] select_task_rq_fair+0x1478/0x14b4
[   55.748483] ([<0000000000179bb8>] select_task_rq_fair+0x90/0x14b4)
[   55.748493]  [<0000000000170702>] try_to_wake_up+0x136/0x47c
[   55.748506]  [<000000000015b446>] autoremove_wake_function+0x26/0x58
[   55.748518]  [<000000000016693a>] __wake_up_common+0x76/0xb4
[   55.748530]  [<000000000016aed0>] __wake_up+0x4c/0x60
[   55.748541]  [<0000000000109ee0>] s390_handle_mcck+0x194/0x1f8
[   55.748557]  [<000000000010486a>] cpu_idle+0x192/0x1c0
[   55.748570]  [<0000000000977916>] start_kernel+0x402/0x410
[   55.748588]  [<0000000000100020>] _stext+0x20/0x80
[   55.748603] 2 locks held by swapper/0/0:
[   55.748612]  #0:  (crw_handler_wait_q.lock){......}, at: [<000000000016aeb6>] __wake_up+0x32/0x60
[   55.748648]  #1:  (&p->pi_lock){-.-.-.}, at: [<000000000017060c>] try_to_wake_up+0x40/0x47c
[   55.748663] Last Breaking-Event-Address:
[   55.748667]  [<0000000000000000>] 0x0

This simply happens because WARN_ON_ONCE causes an exception, the excpetion
handler wants to call a notifier call chain (notify_die), which again uses
rcu_read_lock(), which again causes an exception and so on...
Unfortunately WARN_ON_ONCE first causes an exception and only afterwards sets
the flag that the warning already happened. Seems to be quite some effort to
change this behaviour.

Removing the WARN_ON_ONCE will fix this and, if lockdep is turned on, still
will find illegal uses. But it won't work for lockdep off configs...
So we probably want something better than the patch below.

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 81c04f4..6da8ca4 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -239,13 +239,11 @@ static inline int rcu_is_cpu_idle(void)
 
 static inline void rcu_lock_acquire(struct lockdep_map *map)
 {
-	WARN_ON_ONCE(rcu_is_cpu_idle());
 	lock_acquire(map, 0, 0, 2, 1, NULL, _THIS_IP_);
 }
 
 static inline void rcu_lock_release(struct lockdep_map *map)
 {
-	WARN_ON_ONCE(rcu_is_cpu_idle());
 	lock_release(map, 1, _THIS_IP_);
 }