Date:   Tue, 25 Jun 2019 16:41:21 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     christian.koenig@....com
Cc:     peterz@...radead.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dma-buf: add reservation_context for deadlock
 handling

On Tue, Jun 25, 2019 at 04:36:28PM +0200, Christian König wrote:
> Am 25.06.19 um 16:16 schrieb Daniel Vetter:
> > On Tue, Jun 25, 2019 at 03:55:06PM +0200, Christian König wrote:
> > > The ww_mutex framework allows for detecting deadlocks when multiple
> > > threads try to acquire the same set of locks in different order.
> > > 
> > > The problem is that handling those deadlocks was the burden of the user of
> > > the ww_mutex implementation and at least some users didn't got that right
> > > on the first try.
> > > 
> > > So introduce a new reservation_context object which can be used to
> > > simplify the deadlock handling. This is done by tracking all locked
> > > reservation objects in the context as well as the last contended
> > > reservation object.
> > > 
> > > When a deadlock occurse we now unlock all previously locked object and
> > > acquire the contended lock in the slow path. After this is done -EDEADLK
> > > is still returned to signal that all other locks now need to be
> > > re-acquired again.
> > > 
> > > Signed-off-by: Christian König <christian.koenig@....com>
> > > ---
> > >   drivers/dma-buf/reservation.c | 82 +++++++++++++++++++++++++++++++++++
> > >   include/linux/reservation.h   | 38 ++++++++++++++++
> > >   2 files changed, 120 insertions(+)
> > > 
> > > diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> > > index 4d32e2c67862..9e53e42b053a 100644
> > > --- a/drivers/dma-buf/reservation.c
> > > +++ b/drivers/dma-buf/reservation.c
> > > @@ -55,6 +55,88 @@ EXPORT_SYMBOL(reservation_seqcount_class);
> > >   const char reservation_seqcount_string[] = "reservation_seqcount";
> > >   EXPORT_SYMBOL(reservation_seqcount_string);
> > > +/**
> > > + * reservation_context_init - initialize a reservation context
> > > + * @ctx: the context to initialize
> > > + *
> > > + * Start using this reservation context to lock reservation objects for update.
> > Bunch of hyperlinks here for more consistent story would be really nice in
> > the kerneldoc.
> > 
> > > + */
> > > +void reservation_context_init(struct reservation_context *ctx)
> > > +{
> > > +	ww_acquire_init(&ctx->ctx, &reservation_ww_class);
> > > +	init_llist_head(&ctx->locked);
> > > +	ctx->contended = NULL;
> > > +}
> > > +EXPORT_SYMBOL(reservation_context_init);
> > > +
> > > +/**
> > > + * reservation_context_unlock_all - unlock all reservation objects
> > > + * @ctx: the context which holds the reservation objects
> > > + *
> > > + * Unlocks all reservation objects locked with this context.
> > > + */
> > > +void reservation_context_unlock_all(struct reservation_context *ctx)
> > I'd just call this reservation_unlock_all or so. Feel free to ignore the
> > bikeshed.
> > 
> > > +{
> > > +	struct reservation_object *obj, *next;
> > > +
> > > +	if (ctx->contended)
> > > +		ww_mutex_unlock(&ctx->contended->lock);
> > > +	ctx->contended = NULL;
> > > +
> > > +	llist_for_each_entry_safe(obj, next, ctx->locked.first, locked)
> > > +		ww_mutex_unlock(&obj->lock);
> > > +	init_llist_head(&ctx->locked);
> > > +}
> > > +EXPORT_SYMBOL(reservation_context_unlock_all);
> > > +
> > > +/**
> > > + * reservation_context_lock - lock a reservation object with deadlock handling
> > > + * @ctx: the context which should be used to lock the object
> > > + * @obj: the object which needs to be locked
> > > + * @interruptible: if we should wait interruptible or not
> > > + *
> > > + * Use @ctx to lock the reservation object. If a deadlock is detected we backoff
> > > + * by releasing all locked objects and use the slow path to lock the reservation
> > > + * object. After successfully locking in the slow path -EDEADLK is returned to
> > > + * signal that all other locks must be re-taken as well.
> > > + */
> > > +int reservation_context_lock(struct reservation_context *ctx,
> > > +			     struct reservation_object *obj,
> > > +			     bool interruptible)
> > reservation_lock_ctx is what we generally used in drm_modeset_lock, I like
> > that bikeshed a bit better.
> 
> Actually doesn't sound that good if you ask me.
> 
> Is reservation_lock_ctx the name of the function or the name of the
> structure?

Ah we put the ctx argument last for everything, i.e. operates on the
reservation_object as the main thing, but with the context-aware variant.

> 
> > Also to stay in style I think the explicit set of functions is much
> > better, i.e. reservation_lock_ctx, reservation_lock_interruptible_ctx and
> > reservation_trylock_ctx (later useful for lru applications where you still
> > want to drop the entire pile with resrvation_unlock_ctx).
> 
> The problem is that I then will duplicate a lot of logic between
> reservation_lock_ctx and reservation_lock_interruptible_ctx.

Yeah I know, but that seems to be the style for locking functions. It's
mildly tedious in the shared code, but I do think cleaner and easier to
read in the actual users.

> > That's what all the other locking things do. ttm_bo_reserve has a long
> > list of parameters, and I can never remember which is which. I don't think
> > that's a great style.
> 
> Yeah, I don't really like that either. It is one of the reasons why I want
> to get rid of it.
> 
> But duplicating implementations is not a good idea either. We could go down
> the wait_event_* wait of doing thins and implement everything in macros, but
> I don't really like that either.
> 
> > Another option for interruptible vs. not is to store that in the
> > reservation_context and dtrt. Since generally interruptible or not is a
> > propery of the top-level handler - you need be able to pass EDEADLCK all
> > the way up anyway.

^^ this is what I recommend if you like neither.

> > 
> > > +{
> > > +	int ret = 0;
> > > +
> > > +	if (unlikely(ctx->contended == obj))
> > > +		ctx->contended = NULL;
> > Imo cleaner to handle that with EALREADY filtering from the ww_mutex_lock.
> 
> How do you want to do this? EALREADY handling is different for different
> users of this API.

Oh right the amdgpu CS semantics of throwing an errno if a bo is listed
twice. In drm_modeset_lock we could rely on this fully. Maybe add a
comment that except for the contended case we have to pass EALREADY back
to callers for reasons? Your kerneldoc is lacking a "Returns:" paragraph
anyway, good to list the options there.

Cheers, Daniel


> 
> Christian.
> 
> > 
> > > +	else if (interruptible)
> > > +		ret = ww_mutex_lock_interruptible(&obj->lock, &ctx->ctx);
> > > +	else
> > > +		ret = ww_mutex_lock(&obj->lock, &ctx->ctx);
> > > +
> > > +	if (likely(!ret)) {
> > > +		/* don't use llist_add here, we have separate locking */
> > > +		obj->locked.next = ctx->locked.first;
> > > +		ctx->locked.first = &obj->locked;
> > > +		return 0;
> > > +	}
> > > +	if (unlikely(ret != -EDEADLK))
> > > +		return ret;
> > > +
> > > +	reservation_context_unlock_all(ctx);
> > > +
> > > +	if (interruptible) {
> > > +		ret = ww_mutex_lock_slow_interruptible(&obj->lock, &ctx->ctx);
> > > +		if (unlikely(ret))
> > > +			return ret;
> > > +	} else {
> > > +		ww_mutex_lock_slow(&obj->lock, &ctx->ctx);
> > > +	}
> > > +
> > > +	ctx->contended = obj;
> > > +	return -EDEADLK;
> > > +}
> > > +EXPORT_SYMBOL(reservation_context_lock);
> > > +
> > >   /**
> > >    * reservation_object_reserve_shared - Reserve space to add shared fences to
> > >    * a reservation_object.
> > > diff --git a/include/linux/reservation.h b/include/linux/reservation.h
> > > index ee750765cc94..a8a52e5d3e80 100644
> > > --- a/include/linux/reservation.h
> > > +++ b/include/linux/reservation.h
> > > @@ -44,11 +44,48 @@
> > >   #include <linux/slab.h>
> > >   #include <linux/seqlock.h>
> > >   #include <linux/rcupdate.h>
> > > +#include <linux/llist.h>
> > >   extern struct ww_class reservation_ww_class;
> > >   extern struct lock_class_key reservation_seqcount_class;
> > >   extern const char reservation_seqcount_string[];
> > > +/**
> > > + * struct reservation_context - context to lock reservation objects
> > > + * @ctx: ww_acquire_ctx used for deadlock detection
> > > + * @locked: list of reservation objects locked in this context
> > > + * @contended: contended reservation object
> > > + */
> > > +struct reservation_context {
> > > +	struct ww_acquire_ctx ctx;
> > > +	struct llist_head locked;
> > > +	struct reservation_object *contended;
> > > +};
> > > +
> > > +/**
> > > + * reservation_context_done - wrapper for ww_acquire_done
> > > + * @ctx: the reservation context which is done with locking
> > > + */
> > > +static inline void reservation_context_done(struct reservation_context *ctx)
> > > +{
> > > +	ww_acquire_done(&ctx->ctx);
> > > +}
> > > +
> > > +/**
> > > + * reservation_context_fini - wrapper for ww_acquire_fini
> > > + * @ctx: the reservation context which is finished
> > > + */
> > > +static inline void reservation_context_fini(struct reservation_context *ctx)
> > > +{
> > > +	ww_acquire_fini(&ctx->ctx);
> > > +}
> > > +
> > > +void reservation_context_init(struct reservation_context *ctx);
> > > +void reservation_context_unlock_all(struct reservation_context *ctx);
> > > +int reservation_context_lock(struct reservation_context *ctx,
> > > +			     struct reservation_object *obj,
> > > +			     bool interruptible);
> > Needs a __must_check.
> > 
> > > +
> > >   /**
> > >    * struct reservation_object_list - a list of shared fences
> > >    * @rcu: for internal use
> > > @@ -71,6 +108,7 @@ struct reservation_object_list {
> > >    */
> > >   struct reservation_object {
> > >   	struct ww_mutex lock;
> > > +	struct llist_node locked;
> > >   	seqcount_t seq;
> > >   	struct dma_fence __rcu *fence_excl;
> > Aside from the nits&bikesheds, I like.
> > -Daniel
> > 
> > > -- 
> > > 2.17.1
> > > 
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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