Re: [PATCH v2] locking/ww_mutex: Adjust to lockdep nest_lock requirements

[Boqun Feng]

On Tue, Oct 15, 2024 at 05:27:28PM +0200, Thomas Hellström wrote:
[..]
> > diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
> > index 6f6a5fc85b42..6750321e3e9a 100644
> > --- a/lib/locking-selftest.c
> > +++ b/lib/locking-selftest.c
> > @@ -1720,8 +1720,6 @@ static void ww_test_normal(void)
> >  {
> >         int ret;
> > 
> > -       WWAI(&t);
> > -
> >         /*
> >          * None of the ww_mutex codepaths should be taken in the
> > 'normal'
> >          * mutex calls. The easiest way to verify this is by using
> > the
> > @@ -1770,6 +1768,8 @@ static void ww_test_normal(void)
> >         ww_mutex_base_unlock(&o.base);
> >         WARN_ON(o.ctx != (void *)~0UL);
> > 
> > +       WWAI(&t);
> > +
> >         /* nest_lock */
> >         o.ctx = (void *)~0UL;
> >         ww_mutex_base_lock_nest_lock(&o.base, &t);
> > 
> > Please confirm whether this change is intended.
> 
> This fix looks correct and while this change was not intended, I think
> it makes sense and if this locking order is present in existing code
> apart from this selftest, it's probably easily fixable.
> 
> > 
> > The second is a case as follow:
> > 
> > 	ww_acquire_init(...);
> > 	spin_lock(...);
> > 	ww_mutex_lock(...); // this should trigger a context
> > 			    // invalidation. But the mutex was
> > 			    // initialized by ww_acquire_init() as a
> > 			    // LD_WAIT_INV lock.
> > 
> > The following could fix this:
> > 
> > diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> > index a401a2f31a77..45ff6f7a872b 100644
> > --- a/include/linux/ww_mutex.h
> > +++ b/include/linux/ww_mutex.h
> > @@ -156,8 +156,8 @@ static inline void ww_acquire_init(struct
> > ww_acquire_ctx *ctx,
> >         debug_check_no_locks_freed((void *)ctx, sizeof(*ctx));
> >         lockdep_init_map(&ctx->dep_map, ww_class->acquire_name,
> >                          &ww_class->acquire_key, 0);
> > -       lockdep_init_map(&ctx->first_lock_dep_map, ww_class-
> > >mutex_name,
> > -                        &ww_class->mutex_key, 0);
> > +       lockdep_init_map_wait(&ctx->first_lock_dep_map, ww_class-
> > >mutex_name,
> > +                             &ww_class->mutex_key, 0,
> > LD_WAIT_SLEEP);
> >         mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
> >         mutex_acquire_nest(&ctx->first_lock_dep_map, 0, 0, &ctx-
> > >dep_map, _RET_IP_);
> >  #endif
> > 
> > A v3 with all these fixed would look good to me, and I can add a
> > Tested-by tag to it. Thanks!
> 
> The fix here is a bit confusing. It looks like this test is crafted to
> fail because we take a sleeping ww_mutex inside a spinlock. But the
> ww_mutex lockdep map is already initialized as LD_WAIT_SLEEP. How come
> the first_lock_dep_map locking mode LD_WAIT_INV is used in the
> ww_mutex_lock()? Is that because of the lockdep hlock refcounting?
> 

No, it's not because of refcounting, actually in this case refcounting
won't happen because there is a spin_lock sitting in between:

held_locks stack:

	ww_lockdep_acquire
	  ww_lockdep_mutex
	    lock_A

because there is a lock_A here, the following "if" will be false for
ww_mutex_lock() in the test case:

	hlock = curr->held_locks + depth - 1;
	if (hlock->class_idx == class_idx && nest_lock) {

The reason why the wait types of 'first_lock_dep_map' matter is because
the lock class it shares with ww_mutex_lock() are registered at
*acquire* time. So because we do

	ww_acquire_init():
	  ...
	  lockdep_init_map(...);
	  ...
	  mutex_acquire_nest(...);
	...
	ww_mutex_lock():
	  __mutex_lock_common():
	    mutex_acquire_nest(...);

in the test case, these two mutex_acquire_nest()s use different
lockdep_maps but share the same key, therefore whoever call
mutex_acquire_nest() registers the lock class with its wait types.

So even though first_lock_dep_map is a fake lock, it has to have the
same wait types as a real mutex.

Does this make sense?

Regards,
Boqun

> Thanks,
> Thomas
> 
> 
> 
> > 
> > Regards,
> > Boqun
> > 
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Will Deacon <will@xxxxxxxxxx>
> > > Cc: Waiman Long <longman@xxxxxxxxxx>
> > > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > Cc: Maarten Lankhorst <maarten@xxxxxxxxxxxx>
> > > Cc: Christian König <christian.koenig@xxxxxxx>
> > > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > > ---
> > >  include/linux/ww_mutex.h       | 14 ++++++++++++++
> > >  kernel/locking/test-ww_mutex.c |  8 +++++---
> > >  2 files changed, 19 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> > > index bb763085479a..a401a2f31a77 100644
> > > --- a/include/linux/ww_mutex.h
> > > +++ b/include/linux/ww_mutex.h
> > > @@ -65,6 +65,16 @@ struct ww_acquire_ctx {
> > >  #endif
> > >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > >  	struct lockdep_map dep_map;
> > > +	/**
> > > +	 * @first_lock_dep_map: fake lockdep_map for first locked
> > > ww_mutex.
> > > +	 *
> > > +	 * lockdep requires the lockdep_map for the first locked
> > > ww_mutex
> > > +	 * in a ww transaction to remain in memory until all
> > > ww_mutexes of
> > > +	 * the transaction have been unlocked. Ensure this by
> > > keeping a
> > > +	 * fake locked ww_mutex lockdep map between
> > > ww_acquire_init() and
> > > +	 * ww_acquire_fini().
> > > +	 */
> > > +	struct lockdep_map first_lock_dep_map;
> > >  #endif
> > >  #ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
> > >  	unsigned int deadlock_inject_interval;
> > > @@ -146,7 +156,10 @@ static inline void ww_acquire_init(struct
> > > ww_acquire_ctx *ctx,
> > >  	debug_check_no_locks_freed((void *)ctx, sizeof(*ctx));
> > >  	lockdep_init_map(&ctx->dep_map, ww_class->acquire_name,
> > >  			 &ww_class->acquire_key, 0);
> > > +	lockdep_init_map(&ctx->first_lock_dep_map, ww_class-
> > > >mutex_name,
> > > +			 &ww_class->mutex_key, 0);
> > >  	mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
> > > +	mutex_acquire_nest(&ctx->first_lock_dep_map, 0, 0, &ctx-
> > > >dep_map, _RET_IP_);
> > >  #endif
> > >  #ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
> > >  	ctx->deadlock_inject_interval = 1;
> > > @@ -185,6 +198,7 @@ static inline void ww_acquire_done(struct
> > > ww_acquire_ctx *ctx)
> > >  static inline void ww_acquire_fini(struct ww_acquire_ctx *ctx)
> > >  {
> > >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > > +	mutex_release(&ctx->first_lock_dep_map, _THIS_IP_);
> > >  	mutex_release(&ctx->dep_map, _THIS_IP_);
> > >  #endif
> > >  #ifdef DEBUG_WW_MUTEXES
> > > diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-
> > > ww_mutex.c
> > > index 10a5736a21c2..5d58b2c0ef98 100644
> > > --- a/kernel/locking/test-ww_mutex.c
> > > +++ b/kernel/locking/test-ww_mutex.c
> > > @@ -62,7 +62,8 @@ static int __test_mutex(unsigned int flags)
> > >  	int ret;
> > >  
> > >  	ww_mutex_init(&mtx.mutex, &ww_class);
> > > -	ww_acquire_init(&ctx, &ww_class);
> > > +	if (flags & TEST_MTX_CTX)
> > > +		ww_acquire_init(&ctx, &ww_class);
> > >  
> > >  	INIT_WORK_ONSTACK(&mtx.work, test_mutex_work);
> > >  	init_completion(&mtx.ready);
> > > @@ -90,7 +91,8 @@ static int __test_mutex(unsigned int flags)
> > >  		ret = wait_for_completion_timeout(&mtx.done,
> > > TIMEOUT);
> > >  	}
> > >  	ww_mutex_unlock(&mtx.mutex);
> > > -	ww_acquire_fini(&ctx);
> > > +	if (flags & TEST_MTX_CTX)
> > > +		ww_acquire_fini(&ctx);
> > >  
> > >  	if (ret) {
> > >  		pr_err("%s(flags=%x): mutual exclusion failure\n",
> > > @@ -679,7 +681,7 @@ static int __init test_ww_mutex_init(void)
> > >  	if (ret)
> > >  		return ret;
> > >  
> > > -	ret = stress(2047, hweight32(STRESS_ALL)*ncpus,
> > > STRESS_ALL);
> > > +	ret = stress(2046, hweight32(STRESS_ALL)*ncpus,
> > > STRESS_ALL);
> > >  	if (ret)
> > >  		return ret;
> > >  
> > > -- 
> > > 2.46.0
> > > 
>