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Subject[RFC][PATCH 6/7] lockdep: lock protection locks
On Fri, 2008-08-01 at 16:26 -0700, Linus Torvalds wrote:

> On Fri, 1 Aug 2008, David Miller wrote:
> >
> > Taking more than a few locks of the same class at once is bad
> > news and it's better to find an alternative method.
>
> It's not always wrong.
>
> If you can guarantee that anybody that takes more than one lock of a
> particular class will always take a single top-level lock _first_, then
> that's all good. You can obviously screw up and take the same lock _twice_
> (which will deadlock), but at least you cannot get into ABBA situations.
>
> So maybe the right thing to do is to just teach lockdep about "lock
> protection locks". That would have solved the multi-queue issues for
> networking too - all the actual network drivers would still have taken
> just their single queue lock, but the one case that needs to take all of
> them would have taken a separate top-level lock first.
>
> Never mind that the multi-queue locks were always taken in the same order:
> it's never wrong to just have some top-level serialization, and anybody
> who needs to take <n> locks might as well do <n+1>, because they sure as
> hell aren't going to be on _any_ fastpaths.
>
> So the simplest solution really sounds like just teaching lockdep about
> that one special case. It's not "nesting" exactly, although it's obviously
> related to it.

Do as Linus suggested. The lock protection lock is called nest_lock.

Note that we still have the MAX_LOCK_DEPTH (48) limit to consider, so anything
that spills that it still up shit creek.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/lockdep.h | 34 ++++++++++++++++++----------------
include/linux/rcuclassic.h | 2 +-
kernel/lockdep.c | 26 +++++++++++++++++++++-----
3 files changed, 40 insertions(+), 22 deletions(-)

Index: linux-2.6/kernel/lockdep.c
===================================================================
--- linux-2.6.orig/kernel/lockdep.c
+++ linux-2.6/kernel/lockdep.c
@@ -1372,18 +1372,32 @@ check_deadlock(struct task_struct *curr,
struct lockdep_map *next_instance, int read)
{
struct held_lock *prev;
+ struct held_lock *nest = NULL;
int i;

for (i = 0; i < curr->lockdep_depth; i++) {
prev = curr->held_locks + i;
+
+ if (prev->instance == next->nest_lock)
+ nest = prev;
+
if (hlock_class(prev) != hlock_class(next))
continue;
+
/*
* Allow read-after-read recursion of the same
* lock class (i.e. read_lock(lock)+read_lock(lock)):
*/
if ((read == 2) && prev->read)
return 2;
+
+ /*
+ * We're holding the nest_lock, which serializes this lock's
+ * nesting behaviour.
+ */
+ if (nest)
+ return 2;
+
return print_deadlock_bug(curr, prev, next);
}
return 1;
@@ -2507,7 +2521,7 @@ EXPORT_SYMBOL_GPL(lockdep_init_map);
*/
static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
int trylock, int read, int check, int hardirqs_off,
- unsigned long ip)
+ struct lockdep_map *nest_lock, unsigned long ip)
{
struct task_struct *curr = current;
struct lock_class *class = NULL;
@@ -2566,6 +2580,7 @@ static int __lock_acquire(struct lockdep
hlock->class_idx = class - lock_classes + 1;
hlock->acquire_ip = ip;
hlock->instance = lock;
+ hlock->nest_lock = nest_lock;
hlock->trylock = trylock;
hlock->read = read;
hlock->check = check;
@@ -2717,7 +2732,7 @@ found_it:
if (!__lock_acquire(hlock->instance,
hlock_class(hlock)->subclass, hlock->trylock,
hlock->read, hlock->check, hlock->hardirqs_off,
- hlock->acquire_ip))
+ hlock->nest_lock, hlock->acquire_ip))
return 0;
}

@@ -2778,7 +2793,7 @@ found_it:
if (!__lock_acquire(hlock->instance,
hlock_class(hlock)->subclass, hlock->trylock,
hlock->read, hlock->check, hlock->hardirqs_off,
- hlock->acquire_ip))
+ hlock->nest_lock, hlock->acquire_ip))
return 0;
}

@@ -2915,7 +2930,8 @@ EXPORT_SYMBOL_GPL(lock_set_subclass);
* and also avoid lockdep recursion:
*/
void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
- int trylock, int read, int check, unsigned long ip)
+ int trylock, int read, int check,
+ struct lockdep_map *nest_lock, unsigned long ip)
{
unsigned long flags;

@@ -2930,7 +2946,7 @@ void lock_acquire(struct lockdep_map *lo

current->lockdep_recursion = 1;
__lock_acquire(lock, subclass, trylock, read, check,
- irqs_disabled_flags(flags), ip);
+ irqs_disabled_flags(flags), nest_lock, ip);
current->lockdep_recursion = 0;
raw_local_irq_restore(flags);
}
Index: linux-2.6/include/linux/lockdep.h
===================================================================
--- linux-2.6.orig/include/linux/lockdep.h
+++ linux-2.6/include/linux/lockdep.h
@@ -211,6 +211,7 @@ struct held_lock {
u64 prev_chain_key;
unsigned long acquire_ip;
struct lockdep_map *instance;
+ struct lockdep_map *nest_lock;
#ifdef CONFIG_LOCK_STAT
u64 waittime_stamp;
u64 holdtime_stamp;
@@ -297,7 +298,8 @@ extern void lockdep_init_map(struct lock
* 2: full validation
*/
extern void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
- int trylock, int read, int check, unsigned long ip);
+ int trylock, int read, int check,
+ struct lockdep_map *nest_lock, unsigned long ip);

extern void lock_release(struct lockdep_map *lock, int nested,
unsigned long ip);
@@ -319,7 +321,7 @@ static inline void lockdep_on(void)
{
}

-# define lock_acquire(l, s, t, r, c, i) do { } while (0)
+# define lock_acquire(l, s, t, r, c, n, i) do { } while (0)
# define lock_release(l, n, i) do { } while (0)
# define lock_set_subclass(l, s, i) do { } while (0)
# define lockdep_init() do { } while (0)
@@ -407,9 +409,9 @@ static inline void print_irqtrace_events

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# ifdef CONFIG_PROVE_LOCKING
-# define spin_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, i)
+# define spin_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, NULL, i)
# else
-# define spin_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, i)
+# define spin_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, NULL, i)
# endif
# define spin_release(l, n, i) lock_release(l, n, i)
#else
@@ -419,11 +421,11 @@ static inline void print_irqtrace_events

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# ifdef CONFIG_PROVE_LOCKING
-# define rwlock_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, i)
-# define rwlock_acquire_read(l, s, t, i) lock_acquire(l, s, t, 2, 2, i)
+# define rwlock_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, NULL, i)
+# define rwlock_acquire_read(l, s, t, i) lock_acquire(l, s, t, 2, 2, NULL, i)
# else
-# define rwlock_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, i)
-# define rwlock_acquire_read(l, s, t, i) lock_acquire(l, s, t, 2, 1, i)
+# define rwlock_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, NULL, i)
+# define rwlock_acquire_read(l, s, t, i) lock_acquire(l, s, t, 2, 1, NULL, i)
# endif
# define rwlock_release(l, n, i) lock_release(l, n, i)
#else
@@ -434,9 +436,9 @@ static inline void print_irqtrace_events

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# ifdef CONFIG_PROVE_LOCKING
-# define mutex_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, i)
+# define mutex_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, NULL, i)
# else
-# define mutex_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, i)
+# define mutex_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, NULL, i)
# endif
# define mutex_release(l, n, i) lock_release(l, n, i)
#else
@@ -446,11 +448,11 @@ static inline void print_irqtrace_events

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# ifdef CONFIG_PROVE_LOCKING
-# define rwsem_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, i)
-# define rwsem_acquire_read(l, s, t, i) lock_acquire(l, s, t, 1, 2, i)
+# define rwsem_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, NULL, i)
+# define rwsem_acquire_read(l, s, t, i) lock_acquire(l, s, t, 1, 2, NULL, i)
# else
-# define rwsem_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, i)
-# define rwsem_acquire_read(l, s, t, i) lock_acquire(l, s, t, 1, 1, i)
+# define rwsem_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, NULL, i)
+# define rwsem_acquire_read(l, s, t, i) lock_acquire(l, s, t, 1, 1, NULL, i)
# endif
# define rwsem_release(l, n, i) lock_release(l, n, i)
#else
@@ -461,9 +463,9 @@ static inline void print_irqtrace_events

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# ifdef CONFIG_PROVE_LOCKING
-# define map_acquire(l) lock_acquire(l, 0, 0, 0, 2, _THIS_IP_)
+# define map_acquire(l) lock_acquire(l, 0, 0, 0, 2, NULL, _THIS_IP_)
# else
-# define map_acquire(l) lock_acquire(l, 0, 0, 0, 1, _THIS_IP_)
+# define map_acquire(l) lock_acquire(l, 0, 0, 0, 1, NULL, _THIS_IP_)
# endif
# define map_release(l) lock_release(l, 1, _THIS_IP_)
#else
Index: linux-2.6/include/linux/rcuclassic.h
===================================================================
--- linux-2.6.orig/include/linux/rcuclassic.h
+++ linux-2.6/include/linux/rcuclassic.h
@@ -117,7 +117,7 @@ extern int rcu_needs_cpu(int cpu);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
extern struct lockdep_map rcu_lock_map;
# define rcu_read_acquire() \
- lock_acquire(&rcu_lock_map, 0, 0, 2, 1, _THIS_IP_)
+ lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
# define rcu_read_release() lock_release(&rcu_lock_map, 1, _THIS_IP_)
#else
# define rcu_read_acquire() do { } while (0)
--



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