linux-kernel - Re: [PATCH 25/27] locking/lockdep: Add support for dynamic keys

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Date:   Mon, 03 Dec 2018 09:07:00 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tj@...nel.org, johannes.berg@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/27] locking/lockdep: Add support for dynamic keys

On Thu, 2018-11-29 at 11:10 +0100, Peter Zijlstra wrote:
> On Wed, Nov 28, 2018 at 03:43:23PM -0800, Bart Van Assche wrote:
> > +/* hash_entry is used to keep track of dynamically allocated keys. */
> >  struct lock_class_key {
> > +	struct hlist_node		hash_entry;
> >  	struct lockdep_subclass_key	subkeys[MAX_LOCKDEP_SUBCLASSES];
> >  };
> 
> One consideration; and maybe we should have a BUILD_BUG for that, is
> that this object should be no larger than the smallest lock primitive.
> 
> That typically is raw_spinlock_t, which normally is 4 bytes, but with
> lockdep on that at least also includes struct lockdep_map.
> 
> So what we want is:
> 
> 	sizeof(lock_class_key) <= sizeof(raw_spinlock_t)
> 
> Otherwise, two consecutive spinlocks could end up with key overlap in
> their subclass range.
> 
> Now, I think that is still valid after this patch, but it is something
> that gave me pause.

How about adding this as an additional patch before patch 25/27?

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 9a7cca6dc3d4..ce05b9b419f4 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -725,6 +725,15 @@ static bool assign_lock_key(struct lockdep_map *lock)
 {
 	unsigned long can_addr, addr = (unsigned long)lock;
 
+	/*
+	 * lockdep_free_key_range() assumes that struct lock_class_key
+	 * objects do not overlap. Since we use the address of lock
+	 * objects as class key for static objects, check whether the
+	 * size of lock_class_key objects does not exceed the size of
+	 * the smallest lock object.
+	 */
+	BUILD_BUG_ON(sizeof(struct lock_class_key) > sizeof(raw_spinlock_t));
+
 	if (__is_kernel_percpu_address(addr, &can_addr))
 		lock->key = (void *)can_addr;
 	else if (__is_module_percpu_address(addr, &can_addr))

Thanks,

Bart.