From: ebiederm@xmission•com (Eric W. Biederman)
To: Peter Zijlstra <peterz@infradead•org>
Cc: Andrew Morton <akpm@linux-foundation•org>,
Ingo Molnar <mingo@elte•hu>, Jan Beulich <jbeulich@novell•com>,
tglx@linutronix•de, mingo@redhat•com, hpa@zytor•com,
linux-kernel@vger•kernel.org, Gautham R Shenoy <ego@in•ibm.com>,
Alexey Dobriyan <adobriyan@gmail•com>,
netdev@vger•kernel.org
Subject: Re: [PATCH 2/2] sysctl: lockdep support for sysctl reference counting.
Date: Tue, 31 Mar 2009 06:40:54 -0700 [thread overview]
Message-ID: <m1vdppbyjd.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <1238487448.28248.1805.camel@twins> (Peter Zijlstra's message of "Tue\, 31 Mar 2009 10\:17\:28 +0200")
Peter Zijlstra <peterz@infradead•org> writes:
> On Sat, 2009-03-21 at 00:42 -0700, Eric W. Biederman wrote:
>> It is possible for get lock ordering deadlocks between locks
>> and waiting for the sysctl used count to drop to zero. We have
>> recently observed one of these in the networking code.
>>
>> So teach the sysctl code how to speak lockdep so the kernel
>> can warn about these kinds of rare issues proactively.
>
> It would be very good to extend this changelog with a more detailed
> explanation of the deadlock in question.
>
> Let me see if I got it right:
>
> We're holding a lock, while waiting for the refcount to drop to 0.
> Dropping that refcount is blocked on that lock.
>
> Something like that?
Exactly.
I must have written an explanation so many times that it got
lost when I wrote that commit message.
In particular the problem can be see with /proc/sys/net/ipv4/conf/*/forwarding.
The problem is that the handler for fowarding takes the rtnl_lock
with the reference count held.
Then we call unregister_sysctl_table under the rtnl_lock.
which waits for the reference count to go to zero.
>> Signed-off-by: Eric Biederman <ebiederm@aristanetworks•com>
>> ---
>> include/linux/sysctl.h | 4 ++
>> kernel/sysctl.c | 108 ++++++++++++++++++++++++++++++++++++++---------
>> 2 files changed, 91 insertions(+), 21 deletions(-)
>>
>> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
>> index 39d471d..ec9b1dd 100644
>> --- a/include/linux/sysctl.h
>> +++ b/include/linux/sysctl.h
>> @@ -28,6 +28,7 @@
>> #include <linux/kernel.h>
>> #include <linux/types.h>
>> #include <linux/compiler.h>
>> +#include <linux/lockdep.h>
>>
>> struct file;
>> struct completion;
>> @@ -1087,6 +1088,9 @@ struct ctl_table_header
>> struct ctl_table *attached_by;
>> struct ctl_table *attached_to;
>> struct ctl_table_header *parent;
>> +#ifdef CONFIG_PROVE_LOCKING
>> + struct lockdep_map dep_map;
>> +#endif
>> };
>>
>> /* struct ctl_path describes where in the hierarchy a table is added */
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index c5ef44f..ea8cc39 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1454,12 +1454,63 @@ static struct ctl_table dev_table[] = {
>>
>> static DEFINE_SPINLOCK(sysctl_lock);
>>
>> +#ifndef CONFIG_PROVE_LOCKING
>> +
>> +# define lock_sysctl() spin_lock(&sysctl_lock)
>> +# define unlock_sysctl() spin_unlock(&sysctl_lock)
>> +
>> +static inline void table_acquire_use(struct ctl_table_header *hdr) { }
>> +static inline void table_release_use(struct ctl_table_header *hdr) { }
>> +static inline void table_acquire(struct ctl_table_header *hdr) { }
>> +static inline void table_contended(struct ctl_table_header *hdr) { }
>> +static inline void table_acquired(struct ctl_table_header *hdr) { }
>> +static inline void table_release(struct ctl_table_header *hdr) { }
>> +
>> +#else /* CONFIG_PROVE_LOCKING */
>> +
>> +# define lock_sysctl() __raw_spin_lock(&sysctl_lock.raw_lock)
>> +# define unlock_sysctl() __raw_spin_unlock(&sysctl_lock.raw_lock)
>
> Uhmm, Please explain that -- without a proper explanation this is a NAK.
If the refcount is to be considered a lock. sysctl_lock must be considered
the internals of that lock. lockdep gets extremely confused otherwise.
Since the spinlock is static to this file I'm not especially worried
about it.
>> +static inline void table_acquire_use(struct ctl_table_header *hdr)
>> +{
>> + lock_acquire(&hdr->dep_map, 0, 0, 1, 2, NULL, _RET_IP_);
>> + lock_acquired(&hdr->dep_map, _RET_IP_);
>> +}
>> +
>> +static inline void table_release_use(struct ctl_table_header *hdr)
>> +{
>> + lock_release(&hdr->dep_map, 0, _RET_IP_);
>> +}
>> +
>> +static inline void table_acquire(struct ctl_table_header *hdr)
>> +{
>> + lock_acquire(&hdr->dep_map, 0, 0, 0, 2, NULL, _RET_IP_);
>> +}
>> +
>> +static inline void table_contended(struct ctl_table_header *hdr)
>> +{
>> + lock_contended(&hdr->dep_map, _RET_IP_);
>> +}
>> +
>> +static inline void table_acquired(struct ctl_table_header *hdr)
>> +{
>> + lock_acquired(&hdr->dep_map, _RET_IP_);
>> +}
>> +
>> +static inline void table_release(struct ctl_table_header *hdr)
>> +{
>> + lock_release(&hdr->dep_map, 0, _RET_IP_);
>> +}
>> +
>> +#endif /* CONFIG_PROVE_LOCKING */
>> +
>> /* called under sysctl_lock */
>> static int use_table(struct ctl_table_header *p)
>> {
>> if (unlikely(p->unregistering))
>> return 0;
>> p->used++;
>> + table_acquire_use(p);
>> return 1;
>> }
>>
>> @@ -1469,6 +1520,8 @@ static void unuse_table(struct ctl_table_header *p)
>> if (!--p->used)
>> if (unlikely(p->unregistering))
>> complete(p->unregistering);
>> +
>> + table_release_use(p);
>> }
>>
>> /* called under sysctl_lock, will reacquire if has to wait */
>> @@ -1478,47 +1531,54 @@ static void start_unregistering(struct ctl_table_header *p)
>> * if p->used is 0, nobody will ever touch that entry again;
>> * we'll eliminate all paths to it before dropping sysctl_lock
>> */
>> + table_acquire(p);
>> if (unlikely(p->used)) {
>> struct completion wait;
>> + table_contended(p);
>> +
>> init_completion(&wait);
>> p->unregistering = &wait;
>> - spin_unlock(&sysctl_lock);
>> + unlock_sysctl();
>> wait_for_completion(&wait);
>> - spin_lock(&sysctl_lock);
>> + lock_sysctl();
>> } else {
>> /* anything non-NULL; we'll never dereference it */
>> p->unregistering = ERR_PTR(-EINVAL);
>> }
>> + table_acquired(p);
>> +
>> /*
>> * do not remove from the list until nobody holds it; walking the
>> * list in do_sysctl() relies on that.
>> */
>> list_del_init(&p->ctl_entry);
>> +
>> + table_release(p);
>> }
>>
>
>> @@ -1951,7 +2011,13 @@ struct ctl_table_header *__register_sysctl_paths(
>> return NULL;
>> }
>> #endif
>> - spin_lock(&sysctl_lock);
>> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
>> + {
>> + static struct lock_class_key __key;
>> + lockdep_init_map(&header->dep_map, "sysctl_used", &__key, 0);
>> + }
>> +#endif
>
> This means every sysctl thingy gets the same class, is that
> intended/desired?
There is only one place we initialize it, and as far as I know really
only one place we take it. Which is the definition of a lockdep
class as far as I know.
Eric
next prev parent reply other threads:[~2009-03-31 13:41 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <49B91A7E.76E4.0078.0@novell.com>
[not found] ` <tip-6a09dfa870ba0ed21b1124539968a36b42660661@git.kernel.org>
[not found] ` <1236934491.5188.209.camel@laptop>
[not found] ` <49BA33BE.76E4.0078.0@novell.com>
[not found] ` <1236937423.22914.3698.camel@twins>
[not found] ` <20090313103828.GB31094@elte.hu>
[not found] ` <m1d4cd9n4k.fsf@fess.ebiederm.org>
[not found] ` <20090320085205.GB16021@elte.hu>
[not found] ` <m14oxo8qis.fsf@fess.ebiederm.org>
[not found] ` <20090320182404.GA31629@elte.hu>
[not found] ` <1237575134.4667.5.camel@laptop>
[not found] ` <1237577688.4667.68.camel@laptop>
2009-03-21 7:39 ` [PATCH 0/2] sysctl: lockdep support Eric W. Biederman
2009-03-21 7:40 ` [PATCH 1/2] sysctl: Don't take the use count of multiple heads at a time Eric W. Biederman
2009-03-21 7:42 ` [PATCH 2/2] sysctl: lockdep support for sysctl reference counting Eric W. Biederman
2009-03-30 22:26 ` Andrew Morton
2009-03-30 22:53 ` Eric W. Biederman
2009-03-30 23:18 ` Andrew Morton
2009-03-30 23:50 ` Eric W. Biederman
2009-03-31 8:10 ` Peter Zijlstra
2009-03-31 8:47 ` Eric W. Biederman
2009-03-31 8:17 ` Peter Zijlstra
2009-03-31 13:40 ` Eric W. Biederman [this message]
2009-03-31 15:35 ` Peter Zijlstra
2009-03-31 22:44 ` Eric W. Biederman
2009-04-10 9:18 ` Andrew Morton
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=m1vdppbyjd.fsf@fess.ebiederm.org \
--to=ebiederm@xmission$(echo .)com \
--cc=adobriyan@gmail$(echo .)com \
--cc=akpm@linux-foundation$(echo .)org \
--cc=ego@in$(echo .)ibm.com \
--cc=hpa@zytor$(echo .)com \
--cc=jbeulich@novell$(echo .)com \
--cc=linux-kernel@vger$(echo .)kernel.org \
--cc=mingo@elte$(echo .)hu \
--cc=mingo@redhat$(echo .)com \
--cc=netdev@vger$(echo .)kernel.org \
--cc=peterz@infradead$(echo .)org \
--cc=tglx@linutronix$(echo .)de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox