locking/lockdep: Fix "USED" <- "IN-NMI" inversions

During the LPC RCU BoF Paul asked how come the "USED" <- "IN-NMI"
detector doesn't trip over rcu_read_lock()'s lockdep annotation.

Looking into this I found a very embarrasing typo in
verify_lock_unused():

	-	if (!(class->usage_mask & LOCK_USED))
	+	if (!(class->usage_mask & LOCKF_USED))

fixing that will indeed cause rcu_read_lock() to insta-splat :/

The above typo means that instead of testing for: 0x100 (1 <<
LOCK_USED), we test for 8 (LOCK_USED), which corresponds to (1 <<
LOCK_ENABLED_HARDIRQ).

So instead of testing for _any_ used lock, it will only match any lock
used with interrupts enabled.

The rcu_read_lock() annotation uses .check=0, which means it will not
set any of the interrupt bits and will thus never match.

In order to properly fix the situation and allow rcu_read_lock() to
correctly work, split LOCK_USED into LOCK_USED and LOCK_USED_READ and by
having .read users set USED_READ and test USED, pure read-recursive
locks are permitted.

Fixes: b798f9e31eaa ("lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20200902160323.GK1362448@hirez.programming.kicks-ass.net

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 54b74fabf40c7bd88633b717423403add6911d8c..2facbbd146ec234c27f3a87f08c515a59eec7159 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3969,13 +3969,18 @@ static int separate_irq_context(struct task_struct *curr,
 static int mark_lock(struct task_struct *curr, struct held_lock *this,
                             enum lock_usage_bit new_bit)
 {
-       unsigned int new_mask = 1 << new_bit, ret = 1;
+       unsigned int old_mask, new_mask, ret = 1;
 
        if (new_bit >= LOCK_USAGE_STATES) {
                DEBUG_LOCKS_WARN_ON(1);
                return 0;
        }
 
+       if (new_bit == LOCK_USED && this->read)
+               new_bit = LOCK_USED_READ;
+
+       new_mask = 1 << new_bit;
+
        /*
         * If already set then do not dirty the cacheline,
         * nor do any checks:
@@ -3988,13 +3993,22 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
        /*
         * Make sure we didn't race:
         */
-       if (unlikely(hlock_class(this)->usage_mask & new_mask)) {
-               graph_unlock();
-               return 1;
-       }
+       if (unlikely(hlock_class(this)->usage_mask & new_mask))
+               goto unlock;
 
+       old_mask = hlock_class(this)->usage_mask;
        hlock_class(this)->usage_mask |= new_mask;
 
+       /*
+        * Save one usage_traces[] entry and map both LOCK_USED and
+        * LOCK_USED_READ onto the same entry.
+        */
+       if (new_bit == LOCK_USED || new_bit == LOCK_USED_READ) {
+               if (old_mask & (LOCKF_USED | LOCKF_USED_READ))
+                       goto unlock;
+               new_bit = LOCK_USED;
+       }
+
        if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
                return 0;
 
@@ -4008,6 +4022,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
                        return 0;
        }
 
+unlock:
        graph_unlock();
 
        /*
@@ -4942,12 +4957,20 @@ static void verify_lock_unused(struct lockdep_map *lock, struct held_lock *hlock
 {
 #ifdef CONFIG_PROVE_LOCKING
        struct lock_class *class = look_up_lock_class(lock, subclass);
+       unsigned long mask = LOCKF_USED;
 
        /* if it doesn't have a class (yet), it certainly hasn't been used yet */
        if (!class)
                return;
 
-       if (!(class->usage_mask & LOCK_USED))
+       /*
+        * READ locks only conflict with USED, such that if we only ever use
+        * READ locks, there is no deadlock possible -- RCU.
+        */
+       if (!hlock->read)
+               mask |= LOCKF_USED_READ;
+
+       if (!(class->usage_mask & mask))
                return;
 
        hlock->class_idx = class - lock_classes;

diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index baca699b94e91d0026e86700b6f8678a6629d111..b0be1560ed17a44311ddde40ffb8b43c2ba6894b 100644 (file)
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -19,6 +19,7 @@ enum lock_usage_bit {
 #include "lockdep_states.h"
 #undef LOCKDEP_STATE
        LOCK_USED,
+       LOCK_USED_READ,
        LOCK_USAGE_STATES
 };
 
@@ -40,6 +41,7 @@ enum {
 #include "lockdep_states.h"
 #undef LOCKDEP_STATE
        __LOCKF(USED)
+       __LOCKF(USED_READ)
 };
 
 #define LOCKDEP_STATE(__STATE) LOCKF_ENABLED_##__STATE |