From: Al Viro <viro@ZenIV.linux.org.uk>
To: lkp@lists.01.org
Subject: Re: [d_alloc_parallel] WARNING: bad unlock balance detected!
Date: Tue, 07 Nov 2017 02:33:28 +0000 [thread overview]
Message-ID: <20171107023328.GU21978@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20171107020113.52ws4cqhonhk2zvw@wfg-t540p.sh.intel.com>
[-- Attachment #1: Type: text/plain, Size: 1595 bytes --]
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.
WARNING: multiple messages have this Message-ID (diff)
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Fengguang Wu <fengguang.wu@intel.com>
Cc: linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
David Howells <dhowells@redhat.com>,
Miklos Szeredi <mszeredi@suse.cz>,
lkp@01.org
Subject: Re: [d_alloc_parallel] WARNING: bad unlock balance detected!
Date: Tue, 7 Nov 2017 02:33:28 +0000 [thread overview]
Message-ID: <20171107023328.GU21978@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20171107020113.52ws4cqhonhk2zvw@wfg-t540p.sh.intel.com>
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.
next prev parent reply other threads:[~2017-11-07 2:33 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-07 2:01 [d_alloc_parallel] WARNING: bad unlock balance detected! Fengguang Wu
2017-11-07 2:01 ` Fengguang Wu
2017-11-07 2:33 ` Al Viro [this message]
2017-11-07 2:33 ` Al Viro
2017-11-07 4:03 ` Fengguang Wu
2017-11-07 4:03 ` Fengguang Wu
2017-11-07 8:51 ` Peter Zijlstra
2017-11-07 8:51 ` Peter Zijlstra
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