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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Message-ID: <1543959553.185366.213.camel@acm.org>
Date:   Tue, 04 Dec 2018 13:39:13 -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 16:08 -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.

Hi Waiman,

Peter asked me to add this check.

Bart.