Message-ID: <510900F0.9050800@canonical.com>
Date:	Wed, 30 Jan 2013 12:16:00 +0100
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Rob Clark <robdclark@...il.com>
CC:	Maarten Lankhorst <m.b.lankhorst@...il.com>,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 2/7] mutex: add support for reservation style locks

Op 30-01-13 02:07, Rob Clark schreef:
> On Tue, Jan 15, 2013 at 6:33 AM, Maarten Lankhorst
> <m.b.lankhorst@...il.com> wrote:
> Hi Maarten,
>
> This is a nice looking extension to avoid re-implementing a mutex in
> TTM/reservation code..  ofc, probably someone more familiar with mutex
> code should probably review, but probably a bit of explanation about
> what and why would be helpful.
>
>> mutex_reserve_lock, and mutex_reserve_lock_interruptible:
>>   Lock a buffer with a reservation_id set. reservation_id must not be set to 0,
>>   since this is a special value that means no reservation_id.
>>
>>   Normally if reservation_id is not set, or is older than the reservation_id that's
>>   currently set on the mutex, the behavior will be to wait normally.
>>
>>   However, if  the reservation_id is newer than the current reservation_id, -EAGAIN
>>   will be returned, and this function must unreserve all other mutexes and then redo
>>   a blocking lock with normal mutex calls to prevent a deadlock, then call
>>   mutex_locked_set_reservation on successful locking to set the reservation_id inside
>>   the lock.
> It might be a bit more clear to write up how this works from the
> perspective of the user of ticket_mutex, separately from the internal
> implementation first, and then how it works internally?  Ie, the
> mutex_set_reservation_fastpath() call is internal to the
> implementation of ticket_mutex, but -EAGAIN is something the caller of
> ticket_mutex shall deal with.  This might give a clearer picture of
> how TTM / reservation uses this to prevent deadlock, so those less
> familiar with TTM could better understand.
>
> Well, here is an attempt to start a write-up, which should perhaps
> eventually be folded into Documentation/ticket-mutex-design.txt.  But
> hopefully a better explanation of the problem and the solution will
> encourage some review of the ticket_mutex changes.
>
> ==========================
> Basic problem statement:
> ----- ------- ---------
> GPU's do operations that commonly involve many buffers.  Those buffers
> can be shared across contexts/processes, exist in different memory
> domains (for example VRAM vs system memory), and so on.  And with
> PRIME / dmabuf, they can even be shared across devices.  So there are
> a handful of situations where the driver needs to wait for buffers to
> become ready.  If you think about this in terms of waiting on a buffer
> mutex for it to become available, this presents a problem because
> there is no way to guarantee that buffers appear in a execbuf/batch in
> the same order in all contexts.  That is directly under control of
> userspace, and a result of the sequence of GL calls that an
> application makes.  Which results in the potential for deadlock.  The
> problem gets more complex when you consider that the kernel may need
> to migrate the buffer(s) into VRAM before the GPU operates on the
> buffer(s), which main in turn require evicting some other buffers (and
> you don't want to evict other buffers which are already queued up to
> the GPU), but for a simplified understanding of the problem you can
> ignore this.
>
> The algorithm that TTM came up with for dealing with this problem is
> quite simple.  For each group of buffers (execbuf) that need to be
> locked, the caller would be assigned a unique reservation_id, from a
> global counter.  In case of deadlock in the process of locking all the
> buffers associated with a execbuf, the one with the lowest
> reservation_id wins, and the one with the higher reservation_id
> unlocks all of the buffers that it has already locked, and then tries
> again.
>
> Originally TTM implemented this algorithm on top of an event-queue and
> atomic-ops, but Maarten Lankhorst realized that by merging this with
> the mutex code we could take advantage of the existing mutex fast-path
> code and result in a simpler solution, and so ticket_mutex was born.
> (Well, there where also some additional complexities with the original
> implementation when you start adding in cross-device buffer sharing
> for PRIME.. Maarten could probably better explain.)
>
> How it is used:
> --- -- -- -----
>
> A very simplified version:
>
>   int submit_execbuf(execbuf)
>   {
>       /* acquiring locks, before queuing up to GPU: */
>       seqno = assign_global_seqno();
You also need to make a 'lock' type for seqno, and lock it for lockdep purposes.
This will be a virtual lock that will only exist in lockdep, but it's needed for proper lockdep annotation.

See reservation_ticket_init/fini. It's also important that seqno must not be 0, ever.


>   retry:
>       for (buf in execbuf->buffers) {
>           ret = mutex_reserve_lock(&buf->lock, seqno);
The lockdep class for this lock must be the same for all reservations, and for maximum lockdep usability
you want all the buf->lock lockdep class for all objects across all devices to be the same too.

The __ticket_mutex_init in reservation_object_init does just that for you. :-)

>           switch (ret) {
>           case 0:
>               /* we got the lock */
>               break;
>           case -EAGAIN:
>               /* someone with a lower seqno, so unreserve and try again: */
>               for (buf2 in reverse order starting before buf in
> execbuf->buffers)
>                   mutex_unreserve_unlock(&buf2->lock);
>               goto retry;
Almost correct, you need to re-regrab buf->lock after unreserving all other buffers with mutex_reserve_lock_slow, then goto retry, and skip over this bo when doing the normal locking.

The difference between mutex_reserve_lock and mutex_reserve_lock_slow is that mutex_reserve_lock_slow will block indefinitely where mutex_reserve_lock would return -EAGAIN.
mutex_reserve_lock_slow does not return an error code. mutex_reserve_lock_intr_slow can return -EINTR if interrupted.
>           default:
>               goto err;
>           }
>       }
>
>       /* now everything is good to go, submit job to GPU: */
>       ...
>   }
>
>   int finish_execbuf(execbuf)
>   {
>       /* when GPU is finished: */
>       for (buf in execbuf->buffers)
>           mutex_unreserve_unlock(&buf->lock);
>   }
> ==========================
Thanks for taking the effort into writing this.
> anyways, for the rest of the patch, I'm still going through the
> mutex/ticket_mutex code in conjunction with the reservation/fence
> patches, so for now just a couple very superficial comments.
>
>>   These functions will return -EDEADLK instead of -EAGAIN if reservation_id is the same
>>   as the reservation_id that's attempted to lock the mutex with, since in that case you
>>   presumably attempted to lock the same lock twice.
>>
>> mutex_reserve_lock_slow and mutex_reserve_lock_intr_slow:
>>   Similar to mutex_reserve_lock, except it won't backoff with -EAGAIN. This is useful
>>   after mutex_reserve_lock failed with -EAGAIN, and you unreserved all buffers so no
>>   deadlock can occur.
>>
>> mutex_unreserve_unlock:
>>    Unlock a buffer reserved with the previous calls.
>>
>> Missing at the moment, maybe TODO?
>>   * lockdep warnings when wrongly calling mutex_unreserve_unlock or mutex_unlock,
>>     depending on whether reservation_id was set previously or not.
>>     - Does lockdep have something for this or do I need to extend struct mutex?
>>
>>   * Check if lockdep warns if you unlock a lock that other locks were nested to.
>>     - spin_lock(m); spin_lock_nest_lock(a, m); spin_unlock(m); spin_unlock(a);
>>       would be nice if it gave a splat. Have to recheck if it does, though..
>>
>> Design:
>>   I chose for ticket_mutex to encapsulate struct mutex, so the extra memory usage and
>>   atomic set on init will only happen when you deliberately create a ticket lock.
>>
>>   Since the mutexes are mostly meant to protect buffer object serialization in ttm, not
>>   much contention is expected. I could be slightly smarter with wakeups, but this would
>>   be at the expense at adding a field to struct mutex_waiter. Because this would add
>>   overhead to all cases where ticket_mutexes are not used, and ticket_mutexes are less
>>   performance sensitive anyway since they only protect buffer objects, I didn't want to
>>   do this. It's still better than ttm always calling wake_up_all, which does a
>>   unconditional spin_lock_irqsave/irqrestore.
>>
>>   I needed this in kernel/mutex.c because of the extensions to __lock_common, which are
>>   hopefully optimized away for all normal paths.
>>
>> Changes since RFC patch v1:
>>  - Updated to use atomic_long instead of atomic, since the reservation_id was a long.
>>  - added mutex_reserve_lock_slow and mutex_reserve_lock_intr_slow
>>  - removed mutex_locked_set_reservation_id (or w/e it was called)
>>
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@...onical.com>
>> ---
>>  include/linux/mutex.h |  86 +++++++++++++-
>>  kernel/mutex.c        | 317 +++++++++++++++++++++++++++++++++++++++++++++++---
>>  2 files changed, 387 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>> index 9121595..602c247 100644
>> --- a/include/linux/mutex.h
>> +++ b/include/linux/mutex.h
>> @@ -62,6 +62,11 @@ struct mutex {
>>  #endif
>>  };
>>
>> +struct ticket_mutex {
>> +       struct mutex base;
>> +       atomic_long_t reservation_id;
>> +};
>> +
>>  /*
>>   * This is the control structure for tasks blocked on mutex,
>>   * which resides on the blocked task's kernel stack:
>> @@ -109,12 +114,24 @@ static inline void mutex_destroy(struct mutex *lock) {}
>>                 __DEBUG_MUTEX_INITIALIZER(lockname) \
>>                 __DEP_MAP_MUTEX_INITIALIZER(lockname) }
>>
>> +#define __TICKET_MUTEX_INITIALIZER(lockname) \
>> +               { .base = __MUTEX_INITIALIZER(lockname) \
>> +               , .reservation_id = ATOMIC_LONG_INIT(0) }
>> +
>>  #define DEFINE_MUTEX(mutexname) \
>>         struct mutex mutexname = __MUTEX_INITIALIZER(mutexname)
>>
>>  extern void __mutex_init(struct mutex *lock, const char *name,
>>                          struct lock_class_key *key);
>>
>> +static inline void __ticket_mutex_init(struct ticket_mutex *lock,
>> +                                      const char *name,
>> +                                      struct lock_class_key *key)
>> +{
>> +       __mutex_init(&lock->base, name, key);
>> +       atomic_long_set(&lock->reservation_id, 0);
>> +}
>> +
>>  /**
>>   * mutex_is_locked - is the mutex locked
>>   * @lock: the mutex to be queried
>> @@ -133,26 +150,91 @@ static inline int mutex_is_locked(struct mutex *lock)
>>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>  extern void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
>>  extern void _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest_lock);
>> +
>>  extern int __must_check mutex_lock_interruptible_nested(struct mutex *lock,
>>                                         unsigned int subclass);
>>  extern int __must_check mutex_lock_killable_nested(struct mutex *lock,
>>                                         unsigned int subclass);
>>
>> +extern int __must_check _mutex_reserve_lock(struct ticket_mutex *lock,
>> +                                       struct lockdep_map *nest_lock,
>> +                                       unsigned long reservation_id);
>> +
>> +extern int __must_check _mutex_reserve_lock_interruptible(struct ticket_mutex *,
>> +                                       struct lockdep_map *nest_lock,
>> +                                       unsigned long reservation_id);
>> +
>> +extern void _mutex_reserve_lock_slow(struct ticket_mutex *lock,
>> +                                    struct lockdep_map *nest_lock,
>> +                                    unsigned long reservation_id);
>> +
>> +extern int __must_check _mutex_reserve_lock_intr_slow(struct ticket_mutex *,
>> +                                       struct lockdep_map *nest_lock,
>> +                                       unsigned long reservation_id);
>> +
>>  #define mutex_lock(lock) mutex_lock_nested(lock, 0)
>>  #define mutex_lock_interruptible(lock) mutex_lock_interruptible_nested(lock, 0)
>>  #define mutex_lock_killable(lock) mutex_lock_killable_nested(lock, 0)
>>
>>  #define mutex_lock_nest_lock(lock, nest_lock)                          \
>>  do {                                                                   \
>> -       typecheck(struct lockdep_map *, &(nest_lock)->dep_map);         \
>> +       typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
> looks like that was unintended whitespace change..`
I think it was intentional, as it would be just above 80 lines otherwise.

>>         _mutex_lock_nest_lock(lock, &(nest_lock)->dep_map);             \
>>  } while (0)
>>
>> +#define mutex_reserve_lock(lock, nest_lock, reservation_id)            \
>> +({                                                                     \
>> +       typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
>> +       _mutex_reserve_lock(lock, &(nest_lock)->dep_map, reservation_id);       \
>> +})
>> +
>> +#define mutex_reserve_lock_interruptible(lock, nest_lock, reservation_id)      \
>> +({                                                                     \
>> +       typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
>> +       _mutex_reserve_lock_interruptible(lock, &(nest_lock)->dep_map,  \
>> +                                          reservation_id);             \
>> +})
>> +
>> +#define mutex_reserve_lock_slow(lock, nest_lock, reservation_id)       \
>> +do {                                                                   \
>> +       typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
>> +       _mutex_reserve_lock_slow(lock, &(nest_lock)->dep_map, reservation_id);  \
>> +} while (0)
>> +
>> +#define mutex_reserve_lock_intr_slow(lock, nest_lock, reservation_id)  \
>> +({                                                                     \
>> +       typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
>> +       _mutex_reserve_lock_intr_slow(lock, &(nest_lock)->dep_map,      \
>> +                                     reservation_id);                  \
>> +})
>> +
>>  #else
>>  extern void mutex_lock(struct mutex *lock);
>>  extern int __must_check mutex_lock_interruptible(struct mutex *lock);
>>  extern int __must_check mutex_lock_killable(struct mutex *lock);
>>
>> +extern int __must_check _mutex_reserve_lock(struct ticket_mutex *lock,
>> +                                           unsigned long reservation_id);
>> +extern int __must_check _mutex_reserve_lock_interruptible(struct ticket_mutex *,
>> +                                               unsigned long reservation_id);
>> +
>> +extern void _mutex_reserve_lock_slow(struct ticket_mutex *lock,
>> +                                    unsigned long reservation_id);
>> +extern int __must_check _mutex_reserve_lock_intr_slow(struct ticket_mutex *,
>> +                                               unsigned long reservation_id);
>> +
>> +#define mutex_reserve_lock(lock, nest_lock, reservation_id)            \
>> +       _mutex_reserve_lock(lock, reservation_id)
>> +
>> +#define mutex_reserve_lock_interruptible(lock, nest_lock, reservation_id)      \
>> +       _mutex_reserve_lock_interruptible(lock, reservation_id)
>> +
>> +#define mutex_reserve_lock_slow(lock, nest_lock, reservation_id)       \
>> +       _mutex_reserve_lock_slow(lock, reservation_id)
>> +
>> +#define mutex_reserve_lock_intr_slow(lock, nest_lock, reservation_id)  \
>> +       _mutex_reserve_lock_intr_slow(lock, reservation_id)
>> +
>>  # define mutex_lock_nested(lock, subclass) mutex_lock(lock)
>>  # define mutex_lock_interruptible_nested(lock, subclass) mutex_lock_interruptible(lock)
>>  # define mutex_lock_killable_nested(lock, subclass) mutex_lock_killable(lock)
>> @@ -167,6 +249,8 @@ extern int __must_check mutex_lock_killable(struct mutex *lock);
>>   */
>>  extern int mutex_trylock(struct mutex *lock);
>>  extern void mutex_unlock(struct mutex *lock);
>> +extern void mutex_unreserve_unlock(struct ticket_mutex *lock);
>> +
>>  extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
>>
>>  #ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX
>> diff --git a/kernel/mutex.c b/kernel/mutex.c
>> index a307cc9..8282729 100644
>> --- a/kernel/mutex.c
>> +++ b/kernel/mutex.c
>> @@ -126,16 +126,119 @@ void __sched mutex_unlock(struct mutex *lock)
>>
>>  EXPORT_SYMBOL(mutex_unlock);
>>
>> +/**
>> + * mutex_unreserve_unlock - release the mutex
>> + * @lock: the mutex to be released
>> + *
>> + * Unlock a mutex that has been locked by this task previously
>> + * with _mutex_reserve_lock*.
>> + *
>> + * This function must not be used in interrupt context. Unlocking
>> + * of a not locked mutex is not allowed.
>> + */
>> +void __sched mutex_unreserve_unlock(struct ticket_mutex *lock)
>> +{
>> +       /*
>> +        * mark mutex as no longer part of a reservation, next
>> +        * locker can set this again
>> +        */
>> +       atomic_long_set(&lock->reservation_id, 0);
>> +
>> +       /*
>> +        * The unlocking fastpath is the 0->1 transition from 'locked'
>> +        * into 'unlocked' state:
>> +        */
>> +#ifndef CONFIG_DEBUG_MUTEXES
>> +       /*
>> +        * When debugging is enabled we must not clear the owner before time,
>> +        * the slow path will always be taken, and that clears the owner field
>> +        * after verifying that it was indeed current.
>> +        */
>> +       mutex_clear_owner(&lock->base);
>> +#endif
>> +       __mutex_fastpath_unlock(&lock->base.count, __mutex_unlock_slowpath);
>> +}
>> +EXPORT_SYMBOL(mutex_unreserve_unlock);
>> +
>> +static inline int __sched
>> +__mutex_lock_check_reserve(struct mutex *lock, unsigned long reservation_id)
>> +{
>> +       struct ticket_mutex *m = container_of(lock, struct ticket_mutex, base);
>> +       unsigned long cur_id;
>> +
>> +       cur_id = atomic_long_read(&m->reservation_id);
>> +       if (!cur_id)
>> +               return 0;
>> +
>> +       if (unlikely(reservation_id == cur_id))
>> +               return -EDEADLK;
>> +
>> +       if (unlikely(reservation_id - cur_id <= LONG_MAX))
>> +               return -EAGAIN;
>> +
>> +       return 0;
>> +}
>> +
>> +/*
>> + * after acquiring lock with fastpath or when we lost out in contested
>> + * slowpath, set reservation_id and wake up any waiters so they can recheck.
>> + */
> I think that is a bit misleading, if I'm understanding correctly this
> is called once you get the lock (but in either fast or slow path)
Yes, but strictly speaking it does not need to be called on slow path, it will do an atomic_xchg to set reservation_id,
see that it had already set reservation_id, and skip the rest. :-)

Maybe I should just return !__builtin_constant_p(reservation_id) in __mutex_lock_common to distinguish between fastpath and slowpath.
That would cause __mutex_lock_common to return 0 for normal mutexes, 1 when reservation_id is set and slowpath is used. This would
allow me to check whether mutex_set_reservation_fastpath needs to be called or not, and tighten up lockdep detection of mismatched
mutex_reserve_lock / mutex_lock with mutex_unlock / mutex_reserve_unlock.

~Maarte
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