drm_modeset_lock.c

  1/*
  2 * Copyright (C) 2014 Red Hat
  3 * Author: Rob Clark <robdclark@gmail.com>
  4 *
  5 * Permission is hereby granted, free of charge, to any person obtaining a
  6 * copy of this software and associated documentation files (the "Software"),
  7 * to deal in the Software without restriction, including without limitation
  8 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
  9 * and/or sell copies of the Software, and to permit persons to whom the
 10 * Software is furnished to do so, subject to the following conditions:
 11 *
 12 * The above copyright notice and this permission notice shall be included in
 13 * all copies or substantial portions of the Software.
 14 *
 15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 17 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 18 * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
 19 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 20 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 21 * OTHER DEALINGS IN THE SOFTWARE.
 22 */
 23
 24#include <linux/export.h>
 25
 26#include <drm/drm_atomic.h>
 27#include <drm/drm_crtc.h>
 28#include <drm/drm_device.h>
 29#include <drm/drm_modeset_lock.h>
 30#include <drm/drm_print.h>
 31
 32/**
 33 * DOC: kms locking
 34 *
 35 * As KMS moves toward more fine grained locking, and atomic ioctl where
 36 * userspace can indirectly control locking order, it becomes necessary
 37 * to use &ww_mutex and acquire-contexts to avoid deadlocks.  But because
 38 * the locking is more distributed around the driver code, we want a bit
 39 * of extra utility/tracking out of our acquire-ctx.  This is provided
 40 * by &struct drm_modeset_lock and &struct drm_modeset_acquire_ctx.
 41 *
 42 * For basic principles of &ww_mutex, see: Documentation/locking/ww-mutex-design.rst
 43 *
 44 * The basic usage pattern is to::
 45 *
 46 *     drm_modeset_acquire_init(ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE)
 47 *     retry:
 48 *     foreach (lock in random_ordered_set_of_locks) {
 49 *         ret = drm_modeset_lock(lock, ctx)
 50 *         if (ret == -EDEADLK) {
 51 *             ret = drm_modeset_backoff(ctx);
 52 *             if (!ret)
 53 *                 goto retry;
 54 *         }
 55 *         if (ret)
 56 *             goto out;
 57 *     }
 58 *     ... do stuff ...
 59 *     out:
 60 *     drm_modeset_drop_locks(ctx);
 61 *     drm_modeset_acquire_fini(ctx);
 62 *
 63 * For convenience this control flow is implemented in
 64 * DRM_MODESET_LOCK_ALL_BEGIN() and DRM_MODESET_LOCK_ALL_END() for the case
 65 * where all modeset locks need to be taken through drm_modeset_lock_all_ctx().
 66 *
 67 * If all that is needed is a single modeset lock, then the &struct
 68 * drm_modeset_acquire_ctx is not needed and the locking can be simplified
 69 * by passing a NULL instead of ctx in the drm_modeset_lock() call or
 70 * calling  drm_modeset_lock_single_interruptible(). To unlock afterwards
 71 * call drm_modeset_unlock().
 72 *
 73 * On top of these per-object locks using &ww_mutex there's also an overall
 74 * &drm_mode_config.mutex, for protecting everything else. Mostly this means
 75 * probe state of connectors, and preventing hotplug add/removal of connectors.
 76 *
 77 * Finally there's a bunch of dedicated locks to protect drm core internal
 78 * lists and lookup data structures.
 79 */
 80
 81static DEFINE_WW_CLASS(crtc_ww_class);
 82
 83#if IS_ENABLED(CONFIG_DRM_DEBUG_MODESET_LOCK)
 84static noinline depot_stack_handle_t __drm_stack_depot_save(void)
 85{
 86	unsigned long entries[8];
 87	unsigned int n;
 88
 89	n = stack_trace_save(entries, ARRAY_SIZE(entries), 1);
 90
 91	return stack_depot_save(entries, n, GFP_NOWAIT | __GFP_NOWARN);
 92}
 93
 94static void __drm_stack_depot_print(depot_stack_handle_t stack_depot)
 95{
 96	struct drm_printer p = drm_dbg_printer(NULL, DRM_UT_KMS, "drm_modeset_lock");
 97	unsigned long *entries;
 98	unsigned int nr_entries;
 99	char *buf;
100
101	buf = kmalloc(PAGE_SIZE, GFP_NOWAIT | __GFP_NOWARN);
102	if (!buf)
103		return;
104
105	nr_entries = stack_depot_fetch(stack_depot, &entries);
106	stack_trace_snprint(buf, PAGE_SIZE, entries, nr_entries, 2);
107
108	drm_printf(&p, "attempting to lock a contended lock without backoff:\n%s", buf);
109
110	kfree(buf);
111}
112
113static void __drm_stack_depot_init(void)
114{
115	stack_depot_init();
116}
117#else /* CONFIG_DRM_DEBUG_MODESET_LOCK */
118static depot_stack_handle_t __drm_stack_depot_save(void)
119{
120	return 0;
121}
122static void __drm_stack_depot_print(depot_stack_handle_t stack_depot)
123{
124}
125static void __drm_stack_depot_init(void)
126{
127}
128#endif /* CONFIG_DRM_DEBUG_MODESET_LOCK */
129
130/**
131 * drm_modeset_lock_all - take all modeset locks
132 * @dev: DRM device
133 *
134 * This function takes all modeset locks, suitable where a more fine-grained
135 * scheme isn't (yet) implemented. Locks must be dropped by calling the
136 * drm_modeset_unlock_all() function.
137 *
138 * This function is deprecated. It allocates a lock acquisition context and
139 * stores it in &drm_device.mode_config. This facilitate conversion of
140 * existing code because it removes the need to manually deal with the
141 * acquisition context, but it is also brittle because the context is global
142 * and care must be taken not to nest calls. New code should use the
143 * drm_modeset_lock_all_ctx() function and pass in the context explicitly.
144 */
145void drm_modeset_lock_all(struct drm_device *dev)
146{
147	struct drm_mode_config *config = &dev->mode_config;
148	struct drm_modeset_acquire_ctx *ctx;
149	int ret;
150
151	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL | __GFP_NOFAIL);
152	if (WARN_ON(!ctx))
153		return;
154
155	mutex_lock(&config->mutex);
156
157	drm_modeset_acquire_init(ctx, 0);
158
159retry:
160	ret = drm_modeset_lock_all_ctx(dev, ctx);
161	if (ret < 0) {
162		if (ret == -EDEADLK) {
163			drm_modeset_backoff(ctx);
164			goto retry;
165		}
166
167		drm_modeset_acquire_fini(ctx);
168		kfree(ctx);
169		return;
170	}
171	ww_acquire_done(&ctx->ww_ctx);
172
173	WARN_ON(config->acquire_ctx);
174
175	/*
176	 * We hold the locks now, so it is safe to stash the acquisition
177	 * context for drm_modeset_unlock_all().
178	 */
179	config->acquire_ctx = ctx;
180
181	drm_warn_on_modeset_not_all_locked(dev);
182}
183EXPORT_SYMBOL(drm_modeset_lock_all);
184
185/**
186 * drm_modeset_unlock_all - drop all modeset locks
187 * @dev: DRM device
188 *
189 * This function drops all modeset locks taken by a previous call to the
190 * drm_modeset_lock_all() function.
191 *
192 * This function is deprecated. It uses the lock acquisition context stored
193 * in &drm_device.mode_config. This facilitates conversion of existing
194 * code because it removes the need to manually deal with the acquisition
195 * context, but it is also brittle because the context is global and care must
196 * be taken not to nest calls. New code should pass the acquisition context
197 * directly to the drm_modeset_drop_locks() function.
198 */
199void drm_modeset_unlock_all(struct drm_device *dev)
200{
201	struct drm_mode_config *config = &dev->mode_config;
202	struct drm_modeset_acquire_ctx *ctx = config->acquire_ctx;
203
204	if (WARN_ON(!ctx))
205		return;
206
207	config->acquire_ctx = NULL;
208	drm_modeset_drop_locks(ctx);
209	drm_modeset_acquire_fini(ctx);
210
211	kfree(ctx);
212
213	mutex_unlock(&dev->mode_config.mutex);
214}
215EXPORT_SYMBOL(drm_modeset_unlock_all);
216
217/**
218 * drm_warn_on_modeset_not_all_locked - check that all modeset locks are locked
219 * @dev: device
220 *
221 * Useful as a debug assert.
222 */
223void drm_warn_on_modeset_not_all_locked(struct drm_device *dev)
224{
225	struct drm_crtc *crtc;
226
227	/* Locking is currently fubar in the panic handler. */
228	if (oops_in_progress)
229		return;
230
231	drm_for_each_crtc(crtc, dev)
232		WARN_ON(!drm_modeset_is_locked(&crtc->mutex));
233
234	WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
235	WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
236}
237EXPORT_SYMBOL(drm_warn_on_modeset_not_all_locked);
238
239/**
240 * drm_modeset_acquire_init - initialize acquire context
241 * @ctx: the acquire context
242 * @flags: 0 or %DRM_MODESET_ACQUIRE_INTERRUPTIBLE
243 *
244 * When passing %DRM_MODESET_ACQUIRE_INTERRUPTIBLE to @flags,
245 * all calls to drm_modeset_lock() will perform an interruptible
246 * wait.
247 */
248void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
249		uint32_t flags)
250{
251	memset(ctx, 0, sizeof(*ctx));
252	ww_acquire_init(&ctx->ww_ctx, &crtc_ww_class);
253	INIT_LIST_HEAD(&ctx->locked);
254
255	if (flags & DRM_MODESET_ACQUIRE_INTERRUPTIBLE)
256		ctx->interruptible = true;
257}
258EXPORT_SYMBOL(drm_modeset_acquire_init);
259
260/**
261 * drm_modeset_acquire_fini - cleanup acquire context
262 * @ctx: the acquire context
263 */
264void drm_modeset_acquire_fini(struct drm_modeset_acquire_ctx *ctx)
265{
266	ww_acquire_fini(&ctx->ww_ctx);
267}
268EXPORT_SYMBOL(drm_modeset_acquire_fini);
269
270/**
271 * drm_modeset_drop_locks - drop all locks
272 * @ctx: the acquire context
273 *
274 * Drop all locks currently held against this acquire context.
275 */
276void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx)
277{
278	if (WARN_ON(ctx->contended))
279		__drm_stack_depot_print(ctx->stack_depot);
280
281	while (!list_empty(&ctx->locked)) {
282		struct drm_modeset_lock *lock;
283
284		lock = list_first_entry(&ctx->locked,
285				struct drm_modeset_lock, head);
286
287		drm_modeset_unlock(lock);
288	}
289}
290EXPORT_SYMBOL(drm_modeset_drop_locks);
291
292static inline int modeset_lock(struct drm_modeset_lock *lock,
293		struct drm_modeset_acquire_ctx *ctx,
294		bool interruptible, bool slow)
295{
296	int ret;
297
298	if (WARN_ON(ctx->contended))
299		__drm_stack_depot_print(ctx->stack_depot);
300
301	if (ctx->trylock_only) {
302		lockdep_assert_held(&ctx->ww_ctx);
303
304		if (!ww_mutex_trylock(&lock->mutex, NULL))
305			return -EBUSY;
306		else
307			return 0;
308	} else if (interruptible && slow) {
309		ret = ww_mutex_lock_slow_interruptible(&lock->mutex, &ctx->ww_ctx);
310	} else if (interruptible) {
311		ret = ww_mutex_lock_interruptible(&lock->mutex, &ctx->ww_ctx);
312	} else if (slow) {
313		ww_mutex_lock_slow(&lock->mutex, &ctx->ww_ctx);
314		ret = 0;
315	} else {
316		ret = ww_mutex_lock(&lock->mutex, &ctx->ww_ctx);
317	}
318	if (!ret) {
319		WARN_ON(!list_empty(&lock->head));
320		list_add(&lock->head, &ctx->locked);
321	} else if (ret == -EALREADY) {
322		/* we already hold the lock.. this is fine.  For atomic
323		 * we will need to be able to drm_modeset_lock() things
324		 * without having to keep track of what is already locked
325		 * or not.
326		 */
327		ret = 0;
328	} else if (ret == -EDEADLK) {
329		ctx->contended = lock;
330		ctx->stack_depot = __drm_stack_depot_save();
331	}
332
333	return ret;
334}
335
336/**
337 * drm_modeset_backoff - deadlock avoidance backoff
338 * @ctx: the acquire context
339 *
340 * If deadlock is detected (ie. drm_modeset_lock() returns -EDEADLK),
341 * you must call this function to drop all currently held locks and
342 * block until the contended lock becomes available.
343 *
344 * This function returns 0 on success, or -ERESTARTSYS if this context
345 * is initialized with %DRM_MODESET_ACQUIRE_INTERRUPTIBLE and the
346 * wait has been interrupted.
347 */
348int drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
349{
350	struct drm_modeset_lock *contended = ctx->contended;
351
352	ctx->contended = NULL;
353	ctx->stack_depot = 0;
354
355	if (WARN_ON(!contended))
356		return 0;
357
358	drm_modeset_drop_locks(ctx);
359
360	return modeset_lock(contended, ctx, ctx->interruptible, true);
361}
362EXPORT_SYMBOL(drm_modeset_backoff);
363
364/**
365 * drm_modeset_lock_init - initialize lock
366 * @lock: lock to init
367 */
368void drm_modeset_lock_init(struct drm_modeset_lock *lock)
369{
370	ww_mutex_init(&lock->mutex, &crtc_ww_class);
371	INIT_LIST_HEAD(&lock->head);
372	__drm_stack_depot_init();
373}
374EXPORT_SYMBOL(drm_modeset_lock_init);
375
376/**
377 * drm_modeset_lock - take modeset lock
378 * @lock: lock to take
379 * @ctx: acquire ctx
380 *
381 * If @ctx is not NULL, then its ww acquire context is used and the
382 * lock will be tracked by the context and can be released by calling
383 * drm_modeset_drop_locks().  If -EDEADLK is returned, this means a
384 * deadlock scenario has been detected and it is an error to attempt
385 * to take any more locks without first calling drm_modeset_backoff().
386 *
387 * If the @ctx is not NULL and initialized with
388 * %DRM_MODESET_ACQUIRE_INTERRUPTIBLE, this function will fail with
389 * -ERESTARTSYS when interrupted.
390 *
391 * If @ctx is NULL then the function call behaves like a normal,
392 * uninterruptible non-nesting mutex_lock() call.
393 */
394int drm_modeset_lock(struct drm_modeset_lock *lock,
395		struct drm_modeset_acquire_ctx *ctx)
396{
397	if (ctx)
398		return modeset_lock(lock, ctx, ctx->interruptible, false);
399
400	ww_mutex_lock(&lock->mutex, NULL);
401	return 0;
402}
403EXPORT_SYMBOL(drm_modeset_lock);
404
405/**
406 * drm_modeset_lock_single_interruptible - take a single modeset lock
407 * @lock: lock to take
408 *
409 * This function behaves as drm_modeset_lock() with a NULL context,
410 * but performs interruptible waits.
411 *
412 * This function returns 0 on success, or -ERESTARTSYS when interrupted.
413 */
414int drm_modeset_lock_single_interruptible(struct drm_modeset_lock *lock)
415{
416	return ww_mutex_lock_interruptible(&lock->mutex, NULL);
417}
418EXPORT_SYMBOL(drm_modeset_lock_single_interruptible);
419
420/**
421 * drm_modeset_unlock - drop modeset lock
422 * @lock: lock to release
423 */
424void drm_modeset_unlock(struct drm_modeset_lock *lock)
425{
426	list_del_init(&lock->head);
427	ww_mutex_unlock(&lock->mutex);
428}
429EXPORT_SYMBOL(drm_modeset_unlock);
430
431/**
432 * drm_modeset_lock_all_ctx - take all modeset locks
433 * @dev: DRM device
434 * @ctx: lock acquisition context
435 *
436 * This function takes all modeset locks, suitable where a more fine-grained
437 * scheme isn't (yet) implemented.
438 *
439 * Unlike drm_modeset_lock_all(), it doesn't take the &drm_mode_config.mutex
440 * since that lock isn't required for modeset state changes. Callers which
441 * need to grab that lock too need to do so outside of the acquire context
442 * @ctx.
443 *
444 * Locks acquired with this function should be released by calling the
445 * drm_modeset_drop_locks() function on @ctx.
446 *
447 * See also: DRM_MODESET_LOCK_ALL_BEGIN() and DRM_MODESET_LOCK_ALL_END()
448 *
449 * Returns: 0 on success or a negative error-code on failure.
450 */
451int drm_modeset_lock_all_ctx(struct drm_device *dev,
452			     struct drm_modeset_acquire_ctx *ctx)
453{
454	struct drm_private_obj *privobj;
455	struct drm_crtc *crtc;
456	struct drm_plane *plane;
457	int ret;
458
459	ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
460	if (ret)
461		return ret;
462
463	drm_for_each_crtc(crtc, dev) {
464		ret = drm_modeset_lock(&crtc->mutex, ctx);
465		if (ret)
466			return ret;
467	}
468
469	drm_for_each_plane(plane, dev) {
470		ret = drm_modeset_lock(&plane->mutex, ctx);
471		if (ret)
472			return ret;
473	}
474
475	drm_for_each_privobj(privobj, dev) {
476		ret = drm_modeset_lock(&privobj->lock, ctx);
477		if (ret)
478			return ret;
479	}
480
481	return 0;
482}
483EXPORT_SYMBOL(drm_modeset_lock_all_ctx);