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Date: Tue, 31 Mar 2009 10:17:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...e.hu>, Jan Beulich <jbeulich@...ell.com>,
tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
linux-kernel@...r.kernel.org, Gautham R Shenoy <ego@...ibm.com>,
Alexey Dobriyan <adobriyan@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] sysctl: lockdep support for sysctl reference
counting.
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?
> Signed-off-by: Eric Biederman <ebiederm@...stanetworks.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.
> +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?
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