1======================================
2Wound/Wait Deadlock-Proof Mutex Design
3======================================
4
5Please read mutex-design.txt first, as it applies to wait/wound mutexes too.
6
7Motivation for WW-Mutexes
8-------------------------
9
10GPU's do operations that commonly involve many buffers.  Those buffers
11can be shared across contexts/processes, exist in different memory
12domains (for example VRAM vs system memory), and so on.  And with
13PRIME / dmabuf, they can even be shared across devices.  So there are
14a handful of situations where the driver needs to wait for buffers to
15become ready.  If you think about this in terms of waiting on a buffer
16mutex for it to become available, this presents a problem because
17there is no way to guarantee that buffers appear in a execbuf/batch in
18the same order in all contexts.  That is directly under control of
19userspace, and a result of the sequence of GL calls that an application
20makes.	Which results in the potential for deadlock.  The problem gets
21more complex when you consider that the kernel may need to migrate the
22buffer(s) into VRAM before the GPU operates on the buffer(s), which
23may in turn require evicting some other buffers (and you don't want to
24evict other buffers which are already queued up to the GPU), but for a
25simplified understanding of the problem you can ignore this.
26
27The algorithm that the TTM graphics subsystem came up with for dealing with
28this problem is quite simple.  For each group of buffers (execbuf) that need
29to be locked, the caller would be assigned a unique reservation id/ticket,
30from a global counter.  In case of deadlock while locking all the buffers
31associated with a execbuf, the one with the lowest reservation ticket (i.e.
32the oldest task) wins, and the one with the higher reservation id (i.e. the
33younger task) unlocks all of the buffers that it has already locked, and then
34tries again.
35
36In the RDBMS literature, a reservation ticket is associated with a transaction.
37and the deadlock handling approach is called Wait-Die. The name is based on
38the actions of a locking thread when it encounters an already locked mutex.
39If the transaction holding the lock is younger, the locking transaction waits.
40If the transaction holding the lock is older, the locking transaction backs off
41and dies. Hence Wait-Die.
42There is also another algorithm called Wound-Wait:
43If the transaction holding the lock is younger, the locking transaction
44wounds the transaction holding the lock, requesting it to die.
45If the transaction holding the lock is older, it waits for the other
46transaction. Hence Wound-Wait.
47The two algorithms are both fair in that a transaction will eventually succeed.
48However, the Wound-Wait algorithm is typically stated to generate fewer backoffs
49compared to Wait-Die, but is, on the other hand, associated with more work than
50Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive
51algorithm in that transactions are wounded by other transactions, and that
52requires a reliable way to pick up the wounded condition and preempt the
53running transaction. Note that this is not the same as process preemption. A
54Wound-Wait transaction is considered preempted when it dies (returning
55-EDEADLK) following a wound.
56
57Concepts
58--------
59
60Compared to normal mutexes two additional concepts/objects show up in the lock
61interface for w/w mutexes:
62
63Acquire context: To ensure eventual forward progress it is important the a task
64trying to acquire locks doesn't grab a new reservation id, but keeps the one it
65acquired when starting the lock acquisition. This ticket is stored in the
66acquire context. Furthermore the acquire context keeps track of debugging state
67to catch w/w mutex interface abuse. An acquire context is representing a
68transaction.
69
70W/w class: In contrast to normal mutexes the lock class needs to be explicit for
71w/w mutexes, since it is required to initialize the acquire context. The lock
72class also specifies what algorithm to use, Wound-Wait or Wait-Die.
73
74Furthermore there are three different class of w/w lock acquire functions:
75
76* Normal lock acquisition with a context, using ww_mutex_lock.
77
78* Slowpath lock acquisition on the contending lock, used by the task that just
79  killed its transaction after having dropped all already acquired locks.
80  These functions have the _slow postfix.
81
82  From a simple semantics point-of-view the _slow functions are not strictly
83  required, since simply calling the normal ww_mutex_lock functions on the
84  contending lock (after having dropped all other already acquired locks) will
85  work correctly. After all if no other ww mutex has been acquired yet there's
86  no deadlock potential and hence the ww_mutex_lock call will block and not
87  prematurely return -EDEADLK. The advantage of the _slow functions is in
88  interface safety:
89
90  - ww_mutex_lock has a __must_check int return type, whereas ww_mutex_lock_slow
91    has a void return type. Note that since ww mutex code needs loops/retries
92    anyway the __must_check doesn't result in spurious warnings, even though the
93    very first lock operation can never fail.
94  - When full debugging is enabled ww_mutex_lock_slow checks that all acquired
95    ww mutex have been released (preventing deadlocks) and makes sure that we
96    block on the contending lock (preventing spinning through the -EDEADLK
97    slowpath until the contended lock can be acquired).
98
99* Functions to only acquire a single w/w mutex, which results in the exact same
100  semantics as a normal mutex. This is done by calling ww_mutex_lock with a NULL
101  context.
102
103  Again this is not strictly required. But often you only want to acquire a
104  single lock in which case it's pointless to set up an acquire context (and so
105  better to avoid grabbing a deadlock avoidance ticket).
106
107Of course, all the usual variants for handling wake-ups due to signals are also
108provided.
109
110Usage
111-----
112
113The algorithm (Wait-Die vs Wound-Wait) is chosen by using either
114DEFINE_WW_CLASS() (Wound-Wait) or DEFINE_WD_CLASS() (Wait-Die)
115As a rough rule of thumb, use Wound-Wait iff you
116expect the number of simultaneous competing transactions to be typically small,
117and you want to reduce the number of rollbacks.
118
119Three different ways to acquire locks within the same w/w class. Common
120definitions for methods #1 and #2::
121
122  static DEFINE_WW_CLASS(ww_class);
123
124  struct obj {
125	struct ww_mutex lock;
126	/* obj data */
127  };
128
129  struct obj_entry {
130	struct list_head head;
131	struct obj *obj;
132  };
133
134Method 1, using a list in execbuf->buffers that's not allowed to be reordered.
135This is useful if a list of required objects is already tracked somewhere.
136Furthermore the lock helper can use propagate the -EALREADY return code back to
137the caller as a signal that an object is twice on the list. This is useful if
138the list is constructed from userspace input and the ABI requires userspace to
139not have duplicate entries (e.g. for a gpu commandbuffer submission ioctl)::
140
141  int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
142  {
143	struct obj *res_obj = NULL;
144	struct obj_entry *contended_entry = NULL;
145	struct obj_entry *entry;
146
147	ww_acquire_init(ctx, &ww_class);
148
149  retry:
150	list_for_each_entry (entry, list, head) {
151		if (entry->obj == res_obj) {
152			res_obj = NULL;
153			continue;
154		}
155		ret = ww_mutex_lock(&entry->obj->lock, ctx);
156		if (ret < 0) {
157			contended_entry = entry;
158			goto err;
159		}
160	}
161
162	ww_acquire_done(ctx);
163	return 0;
164
165  err:
166	list_for_each_entry_continue_reverse (entry, list, head)
167		ww_mutex_unlock(&entry->obj->lock);
168
169	if (res_obj)
170		ww_mutex_unlock(&res_obj->lock);
171
172	if (ret == -EDEADLK) {
173		/* we lost out in a seqno race, lock and retry.. */
174		ww_mutex_lock_slow(&contended_entry->obj->lock, ctx);
175		res_obj = contended_entry->obj;
176		goto retry;
177	}
178	ww_acquire_fini(ctx);
179
180	return ret;
181  }
182
183Method 2, using a list in execbuf->buffers that can be reordered. Same semantics
184of duplicate entry detection using -EALREADY as method 1 above. But the
185list-reordering allows for a bit more idiomatic code::
186
187  int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
188  {
189	struct obj_entry *entry, *entry2;
190
191	ww_acquire_init(ctx, &ww_class);
192
193	list_for_each_entry (entry, list, head) {
194		ret = ww_mutex_lock(&entry->obj->lock, ctx);
195		if (ret < 0) {
196			entry2 = entry;
197
198			list_for_each_entry_continue_reverse (entry2, list, head)
199				ww_mutex_unlock(&entry2->obj->lock);
200
201			if (ret != -EDEADLK) {
202				ww_acquire_fini(ctx);
203				return ret;
204			}
205
206			/* we lost out in a seqno race, lock and retry.. */
207			ww_mutex_lock_slow(&entry->obj->lock, ctx);
208
209			/*
210			 * Move buf to head of the list, this will point
211			 * buf->next to the first unlocked entry,
212			 * restarting the for loop.
213			 */
214			list_del(&entry->head);
215			list_add(&entry->head, list);
216		}
217	}
218
219	ww_acquire_done(ctx);
220	return 0;
221  }
222
223Unlocking works the same way for both methods #1 and #2::
224
225  void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
226  {
227	struct obj_entry *entry;
228
229	list_for_each_entry (entry, list, head)
230		ww_mutex_unlock(&entry->obj->lock);
231
232	ww_acquire_fini(ctx);
233  }
234
235Method 3 is useful if the list of objects is constructed ad-hoc and not upfront,
236e.g. when adjusting edges in a graph where each node has its own ww_mutex lock,
237and edges can only be changed when holding the locks of all involved nodes. w/w
238mutexes are a natural fit for such a case for two reasons:
239
240- They can handle lock-acquisition in any order which allows us to start walking
241  a graph from a starting point and then iteratively discovering new edges and
242  locking down the nodes those edges connect to.
243- Due to the -EALREADY return code signalling that a given objects is already
244  held there's no need for additional book-keeping to break cycles in the graph
245  or keep track off which looks are already held (when using more than one node
246  as a starting point).
247
248Note that this approach differs in two important ways from the above methods:
249
250- Since the list of objects is dynamically constructed (and might very well be
251  different when retrying due to hitting the -EDEADLK die condition) there's
252  no need to keep any object on a persistent list when it's not locked. We can
253  therefore move the list_head into the object itself.
254- On the other hand the dynamic object list construction also means that the -EALREADY return
255  code can't be propagated.
256
257Note also that methods #1 and #2 and method #3 can be combined, e.g. to first lock a
258list of starting nodes (passed in from userspace) using one of the above
259methods. And then lock any additional objects affected by the operations using
260method #3 below. The backoff/retry procedure will be a bit more involved, since
261when the dynamic locking step hits -EDEADLK we also need to unlock all the
262objects acquired with the fixed list. But the w/w mutex debug checks will catch
263any interface misuse for these cases.
264
265Also, method 3 can't fail the lock acquisition step since it doesn't return
266-EALREADY. Of course this would be different when using the _interruptible
267variants, but that's outside of the scope of these examples here::
268
269  struct obj {
270	struct ww_mutex ww_mutex;
271	struct list_head locked_list;
272  };
273
274  static DEFINE_WW_CLASS(ww_class);
275
276  void __unlock_objs(struct list_head *list)
277  {
278	struct obj *entry, *temp;
279
280	list_for_each_entry_safe (entry, temp, list, locked_list) {
281		/* need to do that before unlocking, since only the current lock holder is
282		allowed to use object */
283		list_del(&entry->locked_list);
284		ww_mutex_unlock(entry->ww_mutex)
285	}
286  }
287
288  void lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
289  {
290	struct obj *obj;
291
292	ww_acquire_init(ctx, &ww_class);
293
294  retry:
295	/* re-init loop start state */
296	loop {
297		/* magic code which walks over a graph and decides which objects
298		 * to lock */
299
300		ret = ww_mutex_lock(obj->ww_mutex, ctx);
301		if (ret == -EALREADY) {
302			/* we have that one already, get to the next object */
303			continue;
304		}
305		if (ret == -EDEADLK) {
306			__unlock_objs(list);
307
308			ww_mutex_lock_slow(obj, ctx);
309			list_add(&entry->locked_list, list);
310			goto retry;
311		}
312
313		/* locked a new object, add it to the list */
314		list_add_tail(&entry->locked_list, list);
315	}
316
317	ww_acquire_done(ctx);
318	return 0;
319  }
320
321  void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
322  {
323	__unlock_objs(list);
324	ww_acquire_fini(ctx);
325  }
326
327Method 4: Only lock one single objects. In that case deadlock detection and
328prevention is obviously overkill, since with grabbing just one lock you can't
329produce a deadlock within just one class. To simplify this case the w/w mutex
330api can be used with a NULL context.
331
332Implementation Details
333----------------------
334
335Design:
336^^^^^^^
337
338  ww_mutex currently encapsulates a struct mutex, this means no extra overhead for
339  normal mutex locks, which are far more common. As such there is only a small
340  increase in code size if wait/wound mutexes are not used.
341
342  We maintain the following invariants for the wait list:
343
344  (1) Waiters with an acquire context are sorted by stamp order; waiters
345      without an acquire context are interspersed in FIFO order.
346  (2) For Wait-Die, among waiters with contexts, only the first one can have
347      other locks acquired already (ctx->acquired > 0). Note that this waiter
348      may come after other waiters without contexts in the list.
349
350  The Wound-Wait preemption is implemented with a lazy-preemption scheme:
351  The wounded status of the transaction is checked only when there is
352  contention for a new lock and hence a true chance of deadlock. In that
353  situation, if the transaction is wounded, it backs off, clears the
354  wounded status and retries. A great benefit of implementing preemption in
355  this way is that the wounded transaction can identify a contending lock to
356  wait for before restarting the transaction. Just blindly restarting the
357  transaction would likely make the transaction end up in a situation where
358  it would have to back off again.
359
360  In general, not much contention is expected. The locks are typically used to
361  serialize access to resources for devices, and optimization focus should
362  therefore be directed towards the uncontended cases.
363
364Lockdep:
365^^^^^^^^
366
367  Special care has been taken to warn for as many cases of api abuse
368  as possible. Some common api abuses will be caught with
369  CONFIG_DEBUG_MUTEXES, but CONFIG_PROVE_LOCKING is recommended.
370
371  Some of the errors which will be warned about:
372   - Forgetting to call ww_acquire_fini or ww_acquire_init.
373   - Attempting to lock more mutexes after ww_acquire_done.
374   - Attempting to lock the wrong mutex after -EDEADLK and
375     unlocking all mutexes.
376   - Attempting to lock the right mutex after -EDEADLK,
377     before unlocking all mutexes.
378
379   - Calling ww_mutex_lock_slow before -EDEADLK was returned.
380
381   - Unlocking mutexes with the wrong unlock function.
382   - Calling one of the ww_acquire_* twice on the same context.
383   - Using a different ww_class for the mutex than for the ww_acquire_ctx.
384   - Normal lockdep errors that can result in deadlocks.
385
386  Some of the lockdep errors that can result in deadlocks:
387   - Calling ww_acquire_init to initialize a second ww_acquire_ctx before
388     having called ww_acquire_fini on the first.
389   - 'normal' deadlocks that can occur.
390
391FIXME:
392  Update this section once we have the TASK_DEADLOCK task state flag magic
393  implemented.
394