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    SubjectRe: rcu warnings cause stack overflow
    On Wed, Feb 01, 2012 at 10:22:09AM -0800, Paul E. McKenney wrote:
    > On Wed, Feb 01, 2012 at 07:08:15PM +0100, Frederic Weisbecker wrote:
    > > On Wed, Feb 01, 2012 at 09:18:56AM -0800, Paul E. McKenney wrote:
    > > > On Wed, Feb 01, 2012 at 04:14:48PM +0100, Frederic Weisbecker wrote:
    > > > > On Wed, Feb 01, 2012 at 11:06:52AM +0100, Heiko Carstens wrote:
    > > > > > 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.
    > > > >
    > > > > Ah ok. Hmm, but why are you using an exception to implement WARN_ON()
    > > > > in s390? Is it to have a whole new stack for the warning path in order
    > > > > to avoid stack overflow from the place that called the WARN_ON() ?
    > > > >
    > > > > Anyway perhaps we need a recursion protection on WARN_ON_ONCE(), such
    > > > > as:
    > > >
    > > > This makes sense to me, but I am also taking Heiko's patch
    > > > removing the WARN_ON()s in favor of the lockdep checks now in
    > > > rcu_read_lock_held(). After all, the rcu_lock_acquire() WARN_ON_ONCE()
    > > > only appears if PROVE_LOCKING=y, so the added requirement is not a
    > > > big deal. Also, CONFIG_PROVE_RCU is in the testing requirements in
    > > > Documentation/SubmitChecklist.
    > > >
    > > > Thanx, Paul
    > >
    > > I think if we do this, we lose the debugging coverage on places that
    > > make use of rcu_read_lock{_sched,_bh}() without using rcu_dereference{_sched,_bh}().
    > >
    > > It seems walking the task list with list_for_each_entry_rcu() under rcu_read_lock() is
    > > one such example because rcu lists are using rcu_dereference_raw().
    >
    > Good point. I will convert these to rcu_lockdep_assert()s.

    But rcu_dereference_raw() can be used outside rcu_read_lock() as well.

    >
    > > Also I realize a problem with my below patch. The exception will call rcu_read_lock()
    > > from another callsite so we are still going to recurse. Only of one level but we will
    > > recurse. This could mess up the warning output, although it prevents from the
    > > stack overflow. If we still go that direction, we probably need a more global, or per
    > > cpu, recursion protection of these RCU warnings.
    >
    > Hmmm... Should this be global or specific to S390?

    I guess specific to S390. But it may be a good idea to protect against
    WARN() recursions in general. One would expect such function to be quite
    robust.


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