locking/lockdep: Improve the deadlock scenario print for sync and read lock

Lock scenario print is always a weak spot of lockdep splats. Improvement
can be made if we rework the dependency search and the error printing.

However without touching the graph search, we can improve a little for
the circular deadlock case, since we have the to-be-added lock
dependency, and know whether these two locks are read/write/sync.

In order to know whether a held_lock is sync or not, a bit was
"stolen" from ->references, which reduce our limit for the same lock
class nesting from 2^12 to 2^11, and it should still be good enough.

Besides, since we now have bit in held_lock for sync, we don't need the
"hardirqoffs being 1" trick, and also we can avoid the __lock_release()
if we jump out of __lock_acquire() before the held_lock stored.

With these changes, a deadlock case evolved with read lock and sync gets
a better print-out from:

	[...]  Possible unsafe locking scenario:
	[...]
	[...]        CPU0                    CPU1
	[...]        ----                    ----
	[...]   lock(srcuA);
	[...]                                lock(srcuB);
	[...]                                lock(srcuA);
	[...]   lock(srcuB);

to

	[...]  Possible unsafe locking scenario:
	[...]
	[...]        CPU0                    CPU1
	[...]        ----                    ----
	[...]   rlock(srcuA);
	[...]                                lock(srcuB);
	[...]                                lock(srcuA);
	[...]   sync(srcuB);

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 14d9dbedc6c1fa2b64bcc02cc55b41c36d2616c7..b32256e9e94434e9189688ef4b5dde29e6369b84 100644 (file)
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -134,7 +134,8 @@ struct held_lock {
        unsigned int read:2;        /* see lock_acquire() comment */
        unsigned int check:1;       /* see lock_acquire() comment */
        unsigned int hardirqs_off:1;
-       unsigned int references:12;                                     /* 32 bits */
+       unsigned int sync:1;
+       unsigned int references:11;                                     /* 32 bits */
        unsigned int pin_count;
 };
 

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 3ee3b278789d8ac7ab4032360ec5675fe38b9329..dcd1d5bfc1e077a57da1253d0eaab90e61100bd2 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1881,6 +1881,8 @@ print_circular_lock_scenario(struct held_lock *src,
        struct lock_class *source = hlock_class(src);
        struct lock_class *target = hlock_class(tgt);
        struct lock_class *parent = prt->class;
+       int src_read = src->read;
+       int tgt_read = tgt->read;
 
        /*
         * A direct locking problem where unsafe_class lock is taken
@@ -1908,7 +1910,10 @@ print_circular_lock_scenario(struct held_lock *src,
        printk(" Possible unsafe locking scenario:\n\n");
        printk("       CPU0                    CPU1\n");
        printk("       ----                    ----\n");
-       printk("  lock(");
+       if (tgt_read != 0)
+               printk("  rlock(");
+       else
+               printk("  lock(");
        __print_lock_name(target);
        printk(KERN_CONT ");\n");
        printk("                               lock(");
@@ -1917,7 +1922,12 @@ print_circular_lock_scenario(struct held_lock *src,
        printk("                               lock(");
        __print_lock_name(target);
        printk(KERN_CONT ");\n");
-       printk("  lock(");
+       if (src_read != 0)
+               printk("  rlock(");
+       else if (src->sync)
+               printk("  sync(");
+       else
+               printk("  lock(");
        __print_lock_name(source);
        printk(KERN_CONT ");\n");
        printk("\n *** DEADLOCK ***\n\n");
@@ -4531,7 +4541,13 @@ mark_usage(struct task_struct *curr, struct held_lock *hlock, int check)
                                        return 0;
                }
        }
-       if (!hlock->hardirqs_off) {
+
+       /*
+        * For lock_sync(), don't mark the ENABLED usage, since lock_sync()
+        * creates no critical section and no extra dependency can be introduced
+        * by interrupts
+        */
+       if (!hlock->hardirqs_off && !hlock->sync) {
                if (hlock->read) {
                        if (!mark_lock(curr, hlock,
                                        LOCK_ENABLED_HARDIRQ_READ))
@@ -4910,7 +4926,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
 static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
                          int trylock, int read, int check, int hardirqs_off,
                          struct lockdep_map *nest_lock, unsigned long ip,
-                         int references, int pin_count)
+                         int references, int pin_count, int sync)
 {
        struct task_struct *curr = current;
        struct lock_class *class = NULL;
@@ -4961,7 +4977,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
        class_idx = class - lock_classes;
 
-       if (depth) { /* we're holding locks */
+       if (depth && !sync) {
+               /* we're holding locks and the new held lock is not a sync */
                hlock = curr->held_locks + depth - 1;
                if (hlock->class_idx == class_idx && nest_lock) {
                        if (!references)
@@ -4995,6 +5012,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
        hlock->trylock = trylock;
        hlock->read = read;
        hlock->check = check;
+       hlock->sync = !!sync;
        hlock->hardirqs_off = !!hardirqs_off;
        hlock->references = references;
 #ifdef CONFIG_LOCK_STAT
@@ -5056,6 +5074,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
        if (!validate_chain(curr, hlock, chain_head, chain_key))
                return 0;
 
+       /* For lock_sync(), we are done here since no actual critical section */
+       if (hlock->sync)
+               return 1;
+
        curr->curr_chain_key = chain_key;
        curr->lockdep_depth++;
        check_chain_key(curr);
@@ -5197,7 +5219,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
                                    hlock->read, hlock->check,
                                    hlock->hardirqs_off,
                                    hlock->nest_lock, hlock->acquire_ip,
-                                   hlock->references, hlock->pin_count)) {
+                                   hlock->references, hlock->pin_count, 0)) {
                case 0:
                        return 1;
                case 1:
@@ -5667,7 +5689,7 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
        lockdep_recursion_inc();
        __lock_acquire(lock, subclass, trylock, read, check,
-                      irqs_disabled_flags(flags), nest_lock, ip, 0, 0);
+                      irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 0);
        lockdep_recursion_finish();
        raw_local_irq_restore(flags);
 }
@@ -5700,11 +5722,6 @@ EXPORT_SYMBOL_GPL(lock_release);
  * APIs are used to wait for one or multiple critical sections (on other CPUs
  * or threads), and it means that calling these APIs inside these critical
  * sections is potential deadlock.
- *
- * This annotation acts as an acquire+release annotation pair with hardirqoff
- * being 1. Since there's no critical section, no interrupt can create extra
- * dependencies "inside" the annotation, hardirqoff == 1 allows us to avoid
- * false positives.
  */
 void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
               int check, struct lockdep_map *nest_lock, unsigned long ip)
@@ -5718,10 +5735,9 @@ void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
        check_flags(flags);
 
        lockdep_recursion_inc();
-       __lock_acquire(lock, subclass, 0, read, check, 1, nest_lock, ip, 0, 0);
-
-       if (__lock_release(lock, ip))
-               check_chain_key(current);
+       __lock_acquire(lock, subclass, 0, read, check,
+                      irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 1);
+       check_chain_key(current);
        lockdep_recursion_finish();
        raw_local_irq_restore(flags);
 }