Messages in this thread | | | Subject | Re: [PATCH 2/2] sysctl: lockdep support for sysctl reference counting. | From | Peter Zijlstra <> | Date | Tue, 31 Mar 2009 10:17:28 +0200 |
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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@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.
> +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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