CC locking people.
On Tue, Nov 07, 2017 at 02:33:28AM +0000, Al Viro wrote:
On Tue, Nov 07, 2017 at 10:01:13AM +0800, Fengguang Wu wrote:
Hi,
Here is a warning in v4.14-rc8 -- it's not necessarily a new bug.
Why is it a bug at all?
[ 428.512005] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control:
RX
LKP: HOSTNAME vm-lkp-wsx03-openwrt-i386-8, MAC , kernel 4.14.0-rc8 158, serial
console /dev/ttyS0
[ 429.798345] Kernel tests: Boot OK!
[ 430.761760] [ 430.766166] =====================================
[ 430.775297] WARNING: bad unlock balance detected!
[ 430.784342] 4.14.0-rc8 #158 Not tainted
[ 430.792153] -------------------------------------
[ 430.801319] pidof/1024 is trying to release lock (rcu_preempt_state) at:
[ 430.813514] [<c10e4348>] rcu_read_unlock_special+0x5f8/0x620
[ 430.824041] but there are no more locks to release!
Er... yes? What of that? Since when is rcu_read_lock() not allowed to
be used under an rwsem?
[ 430.833342] [ 430.833342] other info that might help us debug this:
[ 430.845985] 2 locks held by pidof/1024:
[ 430.853826] #0: (&sb->s_type->i_mutex_key){....}, at: [<c1266efa>]
lookup_slow+0x8a/0x310
[ 430.869344] #1: (rcu_read_lock){....}, at: [<c128094e>]
d_alloc_parallel+0x7e/0xd10
No shit - we are doing RCU cache chain walk while holding ->i_rwsem. As in
down_read(&rwsem);
...
rcu_read_lock();
...
rcu_read_unlock();
Why is that a problem? If we are suddenly not allowed to have an RCU reader
section while holding any kind of a blocking lock, a *lot* of places in the
kernel are screwed.
Please, explain.
Good question! Actually it's not only you.
There are dozens of occurrences for this "unlock balance" warning
in RC8:
((console_sem).lock){-...}, at: up
(gcov_lock){+.+.}, at: gcov_enable_events
(&mm->mmap_sem){....}, at: __do_page_fault
(node_lock){+.+.}, at: gcov_event
(&p->pi_lock){-.-.}, at: try_to_wake_up
(rcu_read_lock){....}, at: aa_file_perm
(rcu_read_lock){....}, at: copy_namespaces
(rcu_read_lock){....}, at: d_alloc_parallel
(rcu_read_lock){....}, at: __d_lookup
(rcu_read_lock){....}, at: dput
(rcu_read_lock){....}, at: find_get_entry
(rcu_read_lock){....}, at: insert_retry
(rcu_read_lock){....}, at: mntput_no_expire
(rcu_read_lock){....}, at: netlink_insert
(rcu_read_lock){....}, at: rcu_read_lock
(rcu_read_lock){....}, at: rcu_torture_read_lock
(rcu_read_lock){....}, at: rhashtable_insert_slow
(rcu_read_lock){....}, at: rhashtable_walk_start
(rcu_read_lock){....}, at: sock_def_readable
(rcu_read_lock){....}, at: SyS_setpriority
(rcu_read_lock){....}, at: T.947
(rcu_read_lock){....}, at: T.949
(rcu_read_lock){....}, at: test_rhashtable
(rcu_read_lock){....}, at: test_rht_lookup
(rcu_read_lock){....}, at: threadfunc
(rcu_read_lock){....}, at: thread_lookup_test
(rcu_read_lock){....}, at: watchdog
(rcu_sched_state.exp_mutex){+.+.}, at: _synchronize_rcu_expedited
(&sb->s_type->i_mutex_key#3){+.+.}, at: start_creating
(&sb->s_type->i_mutex_key){....}, at: lookup_slow
(&sig->cred_guard_mutex){....}, at: prepare_bprm_creds
(sk_lock-AF_NETLINK){....}, at: netlink_insert
(tasklist_lock){....}, at: debug_show_all_locks
(tasklist_lock){.+.+}, at: debug_show_all_locks
(tty_mutex){....}, at: tty_open
(tty_mutex){+.+.}, at: tty_open
(tty_mutex){+.+.}, at: tty_release_struct
Maybe some recent core locking changes triggered this warning.
In particular, some warnings show up since this commit.
commit cde50a67397c0da7d11795d4b4418384022ab8e6
Author: Levin, Alexander (Sasha Levin) <alexander.le...@verizon.com>
AuthorDate: Sun Jun 18 14:06:01 2017 +0000
Commit: Ingo Molnar <mi...@kernel.org>
CommitDate: Tue Jun 20 11:53:09 2017 +0200
locking/rtmutex: Don't initialize lockdep when not required
pi_mutex isn't supposed to be tracked by lockdep, but just
passing NULLs for name and key will cause lockdep to spew a
warning and die, which is not what we want it to do.
Skip lockdep initialization if the caller passed NULLs for
name and key, suggesting such initialization isn't desired.
Signed-off-by: Sasha Levin <alexander.le...@verizon.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Fixes: f5694788ad8d ("rt_mutex: Add lockdep annotations")
Link:
http://lkml.kernel.org/r/20170618140548.4763-1-alexander.le...@verizon.com
Signed-off-by: Ingo Molnar <mi...@kernel.org>
---
kernel/locking/rtmutex.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 43123533e9b10..78069895032a9 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1661,6 +1661,7 @@ void __rt_mutex_init(struct rt_mutex *lock, const char
*name,
lock->waiters = RB_ROOT;
lock->waiters_leftmost = NULL;
+ if (name && key)
debug_rt_mutex_init(lock, name, key);
}
EXPORT_SYMBOL_GPL(__rt_mutex_init);
018956d641 locking/selftest: Add RT-mutex support
cde50a6739 locking/rtmutex: Don't initialize lockdep when not required
+-------------------------------------------------------+------------+------------+
| | 018956d641 |
cde50a6739 |
+-------------------------------------------------------+------------+------------+
| boot_successes | 60 | 22
|
| boot_failures | 42 | 15
|
| WARNING:at_kernel/locking/lockdep.c:#lockdep_init_map | 42 |
|
| is_trying_to_release_lock(rcu_preempt_state)at | 0 | 15
|
+-------------------------------------------------------+------------+------------+
[ 8.863582]
[ 8.863770] =====================================
[ 8.864214] WARNING: bad unlock balance detected!
[ 8.864704] 4.12.0-rc4-00028-gcde50a6 #1 Not tainted
[ 8.865223] -------------------------------------
[ 8.865718] swapper/1 is trying to release lock (rcu_preempt_state) at:
[ 8.866385] [<ffffffff810b9a93>] rcu_read_unlock_special+0x253/0x2c0
[ 8.867097] but there are no more locks to release!
[ 8.867659]
[ 8.867659] other info that might help us debug this:
[ 8.868546] 1 lock held by swapper/1:
[ 8.869141] #0: (rcu_read_lock){......}, at: [<ffffffff8124c1a3>]
insert_retry+0x23/0x520
[ 8.870608]
[ 8.870608] stack backtrace:
[ 8.871299] CPU: 0 PID: 1 Comm: swapper Not tainted
4.12.0-rc4-00028-gcde50a6 #1
[ 8.872510] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.9.3-20161025_171302-gandalf
04/01/2014
[ 8.874123] Call Trace:
[ 8.874559] dump_stack+0x19/0x1b
[ 8.875151] print_unlock_imbalance_bug+0xd7/0xe0
[ 8.875947] lock_release+0x352/0x390
[ 8.876579] rt_mutex_unlock+0x27/0x60
[ 8.877208] ? _raw_spin_unlock_irqrestore+0x56/0x70
[ 8.877998] rcu_read_unlock_special+0x253/0x2c0
[ 8.878766] __rcu_read_unlock+0x3f/0x60
[ 8.879388] insert_retry+0x207/0x520
[ 8.880173] ? do_early_param+0x8f/0x8f
[ 8.880838] test_rht_init+0x144/0x9f6
[ 8.881493] ? kstrtol_from_user+0xa0/0xa0
[ 8.882184] ? __test_string_get_size+0x87/0xa8
[ 8.882975] ? test_string_helpers_init+0x197/0x197
[ 8.883807] ? do_early_param+0x8f/0x8f
[ 8.884456] do_one_initcall+0x95/0x180
[ 8.885130] ? do_early_param+0x8f/0x8f
[ 8.885778] kernel_init_freeable+0x1bd/0x247
[ 8.886511] ? rest_init+0x130/0x130
[ 8.887133] kernel_init+0x9/0xf0
[ 8.887703] ret_from_fork+0x2a/0x40
Regards,
Fengguang
