[PATCH] mce: fix RCU lockdep from mce_log()

[Davidlohr Bueso <dave@gnu.org>]

Hi,

Please review this patch, I am not very familiar with MCE/RCU so I'm not sure that this is the correct fix (otherwise consider it a bug report :)).
This does "fix" the message though and I can use MCE normally.

Thanks,
Davidlohr


From: Davidlohr Bueso <dave@gnu.org>

Based on the following message:

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
arch/x86/kernel/cpu/mcheck/mce.c:1628 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1
no locks held by mcelog/2350.

stack backtrace:
Pid: 2350, comm: mcelog Tainted: G        W   2.6.37-rc1+ #7
Call Trace:
 [<ffffffff8108e6d4>] lockdep_rcu_dereference+0xa4/0xc0
 [<ffffffff810189e9>] mce_poll+0xa9/0xd0
 [<ffffffff81160585>] do_sys_poll+0x275/0x550
 [<ffffffff8115f0e0>] ? __pollwait+0x0/0xf0
 [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
 [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
 [<ffffffff8130431c>] ? rcu_read_lock_held+0x2c/0x30
 [<ffffffff8130549a>] ? radix_tree_lookup_element+0xda/0x100
 [<ffffffff81121f08>] ? __do_fault+0x128/0x470
 [<ffffffff81100bdb>] ? filemap_fault+0xdb/0x4e0
 [<ffffffff810ffe75>] ? unlock_page+0x25/0x30
 [<ffffffff81069ddf>] ? sigprocmask+0x3f/0x100
 [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
 [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
 [<ffffffff8108ff4d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
 [<ffffffff811608a7>] sys_ppoll+0x47/0x190
 [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
 [<ffffffff81835d39>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff810030eb>] system_call_fastpath+0x16/0x1b

At this point the arch/x86/kernel/cpu/mcheck/mce.c:1628 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1
no locks held by mcelog/2350.

stack backtrace:
Pid: 2350, comm: mcelog Tainted: G        W   2.6.37-rc1+ #7
Call Trace:
 [<ffffffff8108e6d4>] lockdep_rcu_dereference+0xa4/0xc0
 [<ffffffff810189e9>] mce_poll+0xa9/0xd0
 [<ffffffff81160585>] do_sys_poll+0x275/0x550
 [<ffffffff8115f0e0>] ? __pollwait+0x0/0xf0
 [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
 [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
 [<ffffffff8130431c>] ? rcu_read_lock_held+0x2c/0x30
 [<ffffffff8130549a>] ? radix_tree_lookup_element+0xda/0x100
 [<ffffffff81121f08>] ? __do_fault+0x128/0x470
 [<ffffffff81100bdb>] ? filemap_fault+0xdb/0x4e0
 [<ffffffff810ffe75>] ? unlock_page+0x25/0x30
 [<ffffffff81069ddf>] ? sigprocmask+0x3f/0x100
 [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
 [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
 [<ffffffff8108ff4d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
 [<ffffffff811608a7>] sys_ppoll+0x47/0x190
 [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
 [<ffffffff81835d39>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff810030eb>] system_call_fastpath+0x16/0x1b

At this point the lockdep_is_held(&mce_read_mutex) call is failing.
So check if the mce_read_mutex is held before derefencing instead of using rcu_dereference_check_mce()

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
---
 arch/x86/kernel/cpu/mcheck/mce.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 7a35b72..6f95b2c 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -1625,8 +1625,12 @@ out:
 static unsigned int mce_poll(struct file *file, poll_table *wait)
 {
 	poll_wait(file, &mce_wait, wait);
-	if (rcu_dereference_check_mce(mcelog.next))
-		return POLLIN | POLLRDNORM;
+
+	if (mutex_is_locked(&mce_read_mutex)) {
+		if (rcu_dereference_index_check(mcelog.next, 
+						rcu_read_lock_sched_held()))
+			return POLLIN | POLLRDNORM;
+	}
 	if (!mce_apei_read_done && apei_check_mce())
 		return POLLIN | POLLRDNORM;
 	return 0;
-- 
1.7.1