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diff --git a/Documentation/locking/ww-mutex-design.rst b/Documentation/locking/ww-mutex-design.rst new file mode 100644 index 000000000..6a8f8beb9 --- /dev/null +++ b/Documentation/locking/ww-mutex-design.rst @@ -0,0 +1,393 @@ +====================================== +Wound/Wait Deadlock-Proof Mutex Design +====================================== + +Please read mutex-design.rst first, as it applies to wait/wound mutexes too. + +Motivation for WW-Mutexes +------------------------- + +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 +may 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 the TTM graphics subsystem 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/ticket, +from a global counter. In case of deadlock while locking all the buffers +associated with a execbuf, the one with the lowest reservation ticket (i.e. +the oldest task) wins, and the one with the higher reservation id (i.e. the +younger task) unlocks all of the buffers that it has already locked, and then +tries again. + +In the RDBMS literature, a reservation ticket is associated with a transaction. +and the deadlock handling approach is called Wait-Die. The name is based on +the actions of a locking thread when it encounters an already locked mutex. +If the transaction holding the lock is younger, the locking transaction waits. +If the transaction holding the lock is older, the locking transaction backs off +and dies. Hence Wait-Die. +There is also another algorithm called Wound-Wait: +If the transaction holding the lock is younger, the locking transaction +wounds the transaction holding the lock, requesting it to die. +If the transaction holding the lock is older, it waits for the other +transaction. Hence Wound-Wait. +The two algorithms are both fair in that a transaction will eventually succeed. +However, the Wound-Wait algorithm is typically stated to generate fewer backoffs +compared to Wait-Die, but is, on the other hand, associated with more work than +Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive +algorithm in that transactions are wounded by other transactions, and that +requires a reliable way to pick up the wounded condition and preempt the +running transaction. Note that this is not the same as process preemption. A +Wound-Wait transaction is considered preempted when it dies (returning +-EDEADLK) following a wound. + +Concepts +-------- + +Compared to normal mutexes two additional concepts/objects show up in the lock +interface for w/w mutexes: + +Acquire context: To ensure eventual forward progress it is important that a task +trying to acquire locks doesn't grab a new reservation id, but keeps the one it +acquired when starting the lock acquisition. This ticket is stored in the +acquire context. Furthermore the acquire context keeps track of debugging state +to catch w/w mutex interface abuse. An acquire context is representing a +transaction. + +W/w class: In contrast to normal mutexes the lock class needs to be explicit for +w/w mutexes, since it is required to initialize the acquire context. The lock +class also specifies what algorithm to use, Wound-Wait or Wait-Die. + +Furthermore there are three different class of w/w lock acquire functions: + +* Normal lock acquisition with a context, using ww_mutex_lock. + +* Slowpath lock acquisition on the contending lock, used by the task that just + killed its transaction after having dropped all already acquired locks. + These functions have the _slow postfix. + + From a simple semantics point-of-view the _slow functions are not strictly + required, since simply calling the normal ww_mutex_lock functions on the + contending lock (after having dropped all other already acquired locks) will + work correctly. After all if no other ww mutex has been acquired yet there's + no deadlock potential and hence the ww_mutex_lock call will block and not + prematurely return -EDEADLK. The advantage of the _slow functions is in + interface safety: + + - ww_mutex_lock has a __must_check int return type, whereas ww_mutex_lock_slow + has a void return type. Note that since ww mutex code needs loops/retries + anyway the __must_check doesn't result in spurious warnings, even though the + very first lock operation can never fail. + - When full debugging is enabled ww_mutex_lock_slow checks that all acquired + ww mutex have been released (preventing deadlocks) and makes sure that we + block on the contending lock (preventing spinning through the -EDEADLK + slowpath until the contended lock can be acquired). + +* Functions to only acquire a single w/w mutex, which results in the exact same + semantics as a normal mutex. This is done by calling ww_mutex_lock with a NULL + context. + + Again this is not strictly required. But often you only want to acquire a + single lock in which case it's pointless to set up an acquire context (and so + better to avoid grabbing a deadlock avoidance ticket). + +Of course, all the usual variants for handling wake-ups due to signals are also +provided. + +Usage +----- + +The algorithm (Wait-Die vs Wound-Wait) is chosen by using either +DEFINE_WW_CLASS() (Wound-Wait) or DEFINE_WD_CLASS() (Wait-Die) +As a rough rule of thumb, use Wound-Wait iff you +expect the number of simultaneous competing transactions to be typically small, +and you want to reduce the number of rollbacks. + +Three different ways to acquire locks within the same w/w class. Common +definitions for methods #1 and #2:: + + static DEFINE_WW_CLASS(ww_class); + + struct obj { + struct ww_mutex lock; + /* obj data */ + }; + + struct obj_entry { + struct list_head head; + struct obj *obj; + }; + +Method 1, using a list in execbuf->buffers that's not allowed to be reordered. +This is useful if a list of required objects is already tracked somewhere. +Furthermore the lock helper can use propagate the -EALREADY return code back to +the caller as a signal that an object is twice on the list. This is useful if +the list is constructed from userspace input and the ABI requires userspace to +not have duplicate entries (e.g. for a gpu commandbuffer submission ioctl):: + + int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) + { + struct obj *res_obj = NULL; + struct obj_entry *contended_entry = NULL; + struct obj_entry *entry; + + ww_acquire_init(ctx, &ww_class); + + retry: + list_for_each_entry (entry, list, head) { + if (entry->obj == res_obj) { + res_obj = NULL; + continue; + } + ret = ww_mutex_lock(&entry->obj->lock, ctx); + if (ret < 0) { + contended_entry = entry; + goto err; + } + } + + ww_acquire_done(ctx); + return 0; + + err: + list_for_each_entry_continue_reverse (entry, list, head) + ww_mutex_unlock(&entry->obj->lock); + + if (res_obj) + ww_mutex_unlock(&res_obj->lock); + + if (ret == -EDEADLK) { + /* we lost out in a seqno race, lock and retry.. */ + ww_mutex_lock_slow(&contended_entry->obj->lock, ctx); + res_obj = contended_entry->obj; + goto retry; + } + ww_acquire_fini(ctx); + + return ret; + } + +Method 2, using a list in execbuf->buffers that can be reordered. Same semantics +of duplicate entry detection using -EALREADY as method 1 above. But the +list-reordering allows for a bit more idiomatic code:: + + int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) + { + struct obj_entry *entry, *entry2; + + ww_acquire_init(ctx, &ww_class); + + list_for_each_entry (entry, list, head) { + ret = ww_mutex_lock(&entry->obj->lock, ctx); + if (ret < 0) { + entry2 = entry; + + list_for_each_entry_continue_reverse (entry2, list, head) + ww_mutex_unlock(&entry2->obj->lock); + + if (ret != -EDEADLK) { + ww_acquire_fini(ctx); + return ret; + } + + /* we lost out in a seqno race, lock and retry.. */ + ww_mutex_lock_slow(&entry->obj->lock, ctx); + + /* + * Move buf to head of the list, this will point + * buf->next to the first unlocked entry, + * restarting the for loop. + */ + list_del(&entry->head); + list_add(&entry->head, list); + } + } + + ww_acquire_done(ctx); + return 0; + } + +Unlocking works the same way for both methods #1 and #2:: + + void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) + { + struct obj_entry *entry; + + list_for_each_entry (entry, list, head) + ww_mutex_unlock(&entry->obj->lock); + + ww_acquire_fini(ctx); + } + +Method 3 is useful if the list of objects is constructed ad-hoc and not upfront, +e.g. when adjusting edges in a graph where each node has its own ww_mutex lock, +and edges can only be changed when holding the locks of all involved nodes. w/w +mutexes are a natural fit for such a case for two reasons: + +- They can handle lock-acquisition in any order which allows us to start walking + a graph from a starting point and then iteratively discovering new edges and + locking down the nodes those edges connect to. +- Due to the -EALREADY return code signalling that a given objects is already + held there's no need for additional book-keeping to break cycles in the graph + or keep track off which looks are already held (when using more than one node + as a starting point). + +Note that this approach differs in two important ways from the above methods: + +- Since the list of objects is dynamically constructed (and might very well be + different when retrying due to hitting the -EDEADLK die condition) there's + no need to keep any object on a persistent list when it's not locked. We can + therefore move the list_head into the object itself. +- On the other hand the dynamic object list construction also means that the -EALREADY return + code can't be propagated. + +Note also that methods #1 and #2 and method #3 can be combined, e.g. to first lock a +list of starting nodes (passed in from userspace) using one of the above +methods. And then lock any additional objects affected by the operations using +method #3 below. The backoff/retry procedure will be a bit more involved, since +when the dynamic locking step hits -EDEADLK we also need to unlock all the +objects acquired with the fixed list. But the w/w mutex debug checks will catch +any interface misuse for these cases. + +Also, method 3 can't fail the lock acquisition step since it doesn't return +-EALREADY. Of course this would be different when using the _interruptible +variants, but that's outside of the scope of these examples here:: + + struct obj { + struct ww_mutex ww_mutex; + struct list_head locked_list; + }; + + static DEFINE_WW_CLASS(ww_class); + + void __unlock_objs(struct list_head *list) + { + struct obj *entry, *temp; + + list_for_each_entry_safe (entry, temp, list, locked_list) { + /* need to do that before unlocking, since only the current lock holder is + allowed to use object */ + list_del(&entry->locked_list); + ww_mutex_unlock(entry->ww_mutex) + } + } + + void lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) + { + struct obj *obj; + + ww_acquire_init(ctx, &ww_class); + + retry: + /* re-init loop start state */ + loop { + /* magic code which walks over a graph and decides which objects + * to lock */ + + ret = ww_mutex_lock(obj->ww_mutex, ctx); + if (ret == -EALREADY) { + /* we have that one already, get to the next object */ + continue; + } + if (ret == -EDEADLK) { + __unlock_objs(list); + + ww_mutex_lock_slow(obj, ctx); + list_add(&entry->locked_list, list); + goto retry; + } + + /* locked a new object, add it to the list */ + list_add_tail(&entry->locked_list, list); + } + + ww_acquire_done(ctx); + return 0; + } + + void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) + { + __unlock_objs(list); + ww_acquire_fini(ctx); + } + +Method 4: Only lock one single objects. In that case deadlock detection and +prevention is obviously overkill, since with grabbing just one lock you can't +produce a deadlock within just one class. To simplify this case the w/w mutex +api can be used with a NULL context. + +Implementation Details +---------------------- + +Design: +^^^^^^^ + + ww_mutex currently encapsulates a struct mutex, this means no extra overhead for + normal mutex locks, which are far more common. As such there is only a small + increase in code size if wait/wound mutexes are not used. + + We maintain the following invariants for the wait list: + + (1) Waiters with an acquire context are sorted by stamp order; waiters + without an acquire context are interspersed in FIFO order. + (2) For Wait-Die, among waiters with contexts, only the first one can have + other locks acquired already (ctx->acquired > 0). Note that this waiter + may come after other waiters without contexts in the list. + + The Wound-Wait preemption is implemented with a lazy-preemption scheme: + The wounded status of the transaction is checked only when there is + contention for a new lock and hence a true chance of deadlock. In that + situation, if the transaction is wounded, it backs off, clears the + wounded status and retries. A great benefit of implementing preemption in + this way is that the wounded transaction can identify a contending lock to + wait for before restarting the transaction. Just blindly restarting the + transaction would likely make the transaction end up in a situation where + it would have to back off again. + + In general, not much contention is expected. The locks are typically used to + serialize access to resources for devices, and optimization focus should + therefore be directed towards the uncontended cases. + +Lockdep: +^^^^^^^^ + + Special care has been taken to warn for as many cases of api abuse + as possible. Some common api abuses will be caught with + CONFIG_DEBUG_MUTEXES, but CONFIG_PROVE_LOCKING is recommended. + + Some of the errors which will be warned about: + - Forgetting to call ww_acquire_fini or ww_acquire_init. + - Attempting to lock more mutexes after ww_acquire_done. + - Attempting to lock the wrong mutex after -EDEADLK and + unlocking all mutexes. + - Attempting to lock the right mutex after -EDEADLK, + before unlocking all mutexes. + + - Calling ww_mutex_lock_slow before -EDEADLK was returned. + + - Unlocking mutexes with the wrong unlock function. + - Calling one of the ww_acquire_* twice on the same context. + - Using a different ww_class for the mutex than for the ww_acquire_ctx. + - Normal lockdep errors that can result in deadlocks. + + Some of the lockdep errors that can result in deadlocks: + - Calling ww_acquire_init to initialize a second ww_acquire_ctx before + having called ww_acquire_fini on the first. + - 'normal' deadlocks that can occur. + +FIXME: + Update this section once we have the TASK_DEADLOCK task state flag magic + implemented. |