linux-kernel - Re: [PATCH v2 21/24] locking/lockdep: Verify whether lock objects are small enough to be used as class keys

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Date:   Tue, 04 Dec 2018 16:06:13 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Waiman Long <longman@...hat.com>, mingo@...hat.com
Cc:     peterz@...radead.org, tj@...nel.org, johannes.berg@...el.com,
        linux-kernel@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH v2 21/24] locking/lockdep: Verify whether lock objects
 are small enough to be used as class keys

On Tue, 2018-12-04 at 16:50 -0500, Waiman Long wrote:
> On 12/04/2018 04:39 PM, Bart Van Assche wrote:
> > On Tue, 2018-12-04 at 16:08 -0500, Waiman Long wrote:
> > > On 12/03/2018 07:28 PM, Bart Van Assche wrote:
> > > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > > Cc: Waiman Long <longman@...hat.com>
> > > > Cc: Johannes Berg <johannes@...solutions.net>
> > > > Signed-off-by: Bart Van Assche <bvanassche@....org>
> > > > ---
> > > >  kernel/locking/lockdep.c | 9 +++++++++
> > > >  1 file changed, 9 insertions(+)
> > > > 
> > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > > index c936fce5b9d7..b4772e5fc176 100644
> > > > --- a/kernel/locking/lockdep.c
> > > > +++ b/kernel/locking/lockdep.c
> > > > @@ -727,6 +727,15 @@ 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))
> > > 
> > > I don't understand what this check is for. lock_class_key and spinlock
> > > are different objects. Their relative size shouldn't matter.
> > 
> > Peter asked me to add this check.
> 
> I haven't finished reviewing all your patches yet. Maybe one of the
> subsequent patches requires this. If that is the case, you should move
> this patch after that.

Hi Waiman,

The existing (v4.20-rc5) code and the comment I added should be sufficient to
understand this patch.

Bart.