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-Crossrelease
-============
-
-Started by Byungchul Park <byungchul.park@lge.com>
-
-Contents:
-
- (*) Background
-
- - What causes deadlock
- - How lockdep works
-
- (*) Limitation
-
- - Limit lockdep
- - Pros from the limitation
- - Cons from the limitation
- - Relax the limitation
-
- (*) Crossrelease
-
- - Introduce crossrelease
- - Introduce commit
-
- (*) Implementation
-
- - Data structures
- - How crossrelease works
-
- (*) Optimizations
-
- - Avoid duplication
- - Lockless for hot paths
-
- (*) APPENDIX A: What lockdep does to work aggresively
-
- (*) APPENDIX B: How to avoid adding false dependencies
-
-
-==========
-Background
-==========
-
-What causes deadlock
---------------------
-
-A deadlock occurs when a context is waiting for an event to happen,
-which is impossible because another (or the) context who can trigger the
-event is also waiting for another (or the) event to happen, which is
-also impossible due to the same reason.
-
-For example:
-
- A context going to trigger event C is waiting for event A to happen.
- A context going to trigger event A is waiting for event B to happen.
- A context going to trigger event B is waiting for event C to happen.
-
-A deadlock occurs when these three wait operations run at the same time,
-because event C cannot be triggered if event A does not happen, which in
-turn cannot be triggered if event B does not happen, which in turn
-cannot be triggered if event C does not happen. After all, no event can
-be triggered since any of them never meets its condition to wake up.
-
-A dependency might exist between two waiters and a deadlock might happen
-due to an incorrect releationship between dependencies. Thus, we must
-define what a dependency is first. A dependency exists between them if:
-
- 1. There are two waiters waiting for each event at a given time.
- 2. The only way to wake up each waiter is to trigger its event.
- 3. Whether one can be woken up depends on whether the other can.
-
-Each wait in the example creates its dependency like:
-
- Event C depends on event A.
- Event A depends on event B.
- Event B depends on event C.
-
- NOTE: Precisely speaking, a dependency is one between whether a
- waiter for an event can be woken up and whether another waiter for
- another event can be woken up. However from now on, we will describe
- a dependency as if it's one between an event and another event for
- simplicity.
-
-And they form circular dependencies like:
-
- -> C -> A -> B -
- / \
- \ /
- ----------------
-
- where 'A -> B' means that event A depends on event B.
-
-Such circular dependencies lead to a deadlock since no waiter can meet
-its condition to wake up as described.
-
-CONCLUSION
-
-Circular dependencies cause a deadlock.
-
-
-How lockdep works
------------------
-
-Lockdep tries to detect a deadlock by checking dependencies created by
-lock operations, acquire and release. Waiting for a lock corresponds to
-waiting for an event, and releasing a lock corresponds to triggering an
-event in the previous section.
-
-In short, lockdep does:
-
- 1. Detect a new dependency.
- 2. Add the dependency into a global graph.
- 3. Check if that makes dependencies circular.
- 4. Report a deadlock or its possibility if so.
-
-For example, consider a graph built by lockdep that looks like:
-
- A -> B -
- \
- -> E
- /
- C -> D -
-
- where A, B,..., E are different lock classes.
-
-Lockdep will add a dependency into the graph on detection of a new
-dependency. For example, it will add a dependency 'E -> C' when a new
-dependency between lock E and lock C is detected. Then the graph will be:
-
- A -> B -
- \
- -> E -
- / \
- -> C -> D - \
- / /
- \ /
- ------------------
-
- where A, B,..., E are different lock classes.
-
-This graph contains a subgraph which demonstrates circular dependencies:
-
- -> E -
- / \
- -> C -> D - \
- / /
- \ /
- ------------------
-
- where C, D and E are different lock classes.
-
-This is the condition under which a deadlock might occur. Lockdep
-reports it on detection after adding a new dependency. This is the way
-how lockdep works.
-
-CONCLUSION
-
-Lockdep detects a deadlock or its possibility by checking if circular
-dependencies were created after adding each new dependency.
-
-
-==========
-Limitation
-==========
-
-Limit lockdep
--------------
-
-Limiting lockdep to work on only typical locks e.g. spin locks and
-mutexes, which are released within the acquire context, the
-implementation becomes simple but its capacity for detection becomes
-limited. Let's check pros and cons in next section.
-
-
-Pros from the limitation
-------------------------
-
-Given the limitation, when acquiring a lock, locks in a held_locks
-cannot be released if the context cannot acquire it so has to wait to
-acquire it, which means all waiters for the locks in the held_locks are
-stuck. It's an exact case to create dependencies between each lock in
-the held_locks and the lock to acquire.
-
-For example:
-
- CONTEXT X
- ---------
- acquire A
- acquire B /* Add a dependency 'A -> B' */
- release B
- release A
-
- where A and B are different lock classes.
-
-When acquiring lock A, the held_locks of CONTEXT X is empty thus no
-dependency is added. But when acquiring lock B, lockdep detects and adds
-a new dependency 'A -> B' between lock A in the held_locks and lock B.
-They can be simply added whenever acquiring each lock.
-
-And data required by lockdep exists in a local structure, held_locks
-embedded in task_struct. Forcing to access the data within the context,
-lockdep can avoid racy problems without explicit locks while handling
-the local data.
-
-Lastly, lockdep only needs to keep locks currently being held, to build
-a dependency graph. However, relaxing the limitation, it needs to keep
-even locks already released, because a decision whether they created
-dependencies might be long-deferred.
-
-To sum up, we can expect several advantages from the limitation:
-
- 1. Lockdep can easily identify a dependency when acquiring a lock.
- 2. Races are avoidable while accessing local locks in a held_locks.
- 3. Lockdep only needs to keep locks currently being held.
-
-CONCLUSION
-
-Given the limitation, the implementation becomes simple and efficient.
-
-
-Cons from the limitation
-------------------------
-
-Given the limitation, lockdep is applicable only to typical locks. For
-example, page locks for page access or completions for synchronization
-cannot work with lockdep.
-
-Can we detect deadlocks below, under the limitation?
-
-Example 1:
-
- CONTEXT X CONTEXT Y CONTEXT Z
- --------- --------- ----------
- mutex_lock A
- lock_page B
- lock_page B
- mutex_lock A /* DEADLOCK */
- unlock_page B held by X
- unlock_page B
- mutex_unlock A
- mutex_unlock A
-
- where A and B are different lock classes.
-
-No, we cannot.
-
-Example 2:
-
- CONTEXT X CONTEXT Y
- --------- ---------
- mutex_lock A
- mutex_lock A
- wait_for_complete B /* DEADLOCK */
- complete B
- mutex_unlock A
- mutex_unlock A
-
- where A is a lock class and B is a completion variable.
-
-No, we cannot.
-
-CONCLUSION
-
-Given the limitation, lockdep cannot detect a deadlock or its
-possibility caused by page locks or completions.
-
-
-Relax the limitation
---------------------
-
-Under the limitation, things to create dependencies are limited to
-typical locks. However, synchronization primitives like page locks and
-completions, which are allowed to be released in any context, also
-create dependencies and can cause a deadlock. So lockdep should track
-these locks to do a better job. We have to relax the limitation for
-these locks to work with lockdep.
-
-Detecting dependencies is very important for lockdep to work because
-adding a dependency means adding an opportunity to check whether it
-causes a deadlock. The more lockdep adds dependencies, the more it
-thoroughly works. Thus Lockdep has to do its best to detect and add as
-many true dependencies into a graph as possible.
-
-For example, considering only typical locks, lockdep builds a graph like:
-
- A -> B -
- \
- -> E
- /
- C -> D -
-
- where A, B,..., E are different lock classes.
-
-On the other hand, under the relaxation, additional dependencies might
-be created and added. Assuming additional 'FX -> C' and 'E -> GX' are
-added thanks to the relaxation, the graph will be:
-
- A -> B -
- \
- -> E -> GX
- /
- FX -> C -> D -
-
- where A, B,..., E, FX and GX are different lock classes, and a suffix
- 'X' is added on non-typical locks.
-
-The latter graph gives us more chances to check circular dependencies
-than the former. However, it might suffer performance degradation since
-relaxing the limitation, with which design and implementation of lockdep
-can be efficient, might introduce inefficiency inevitably. So lockdep
-should provide two options, strong detection and efficient detection.
-
-Choosing efficient detection:
-
- Lockdep works with only locks restricted to be released within the
- acquire context. However, lockdep works efficiently.
-
-Choosing strong detection:
-
- Lockdep works with all synchronization primitives. However, lockdep
- suffers performance degradation.
-
-CONCLUSION
-
-Relaxing the limitation, lockdep can add additional dependencies giving
-additional opportunities to check circular dependencies.
-
-
-============
-Crossrelease
-============
-
-Introduce crossrelease
-----------------------
-
-In order to allow lockdep to handle additional dependencies by what
-might be released in any context, namely 'crosslock', we have to be able
-to identify those created by crosslocks. The proposed 'crossrelease'
-feature provoides a way to do that.
-
-Crossrelease feature has to do:
-
- 1. Identify dependencies created by crosslocks.
- 2. Add the dependencies into a dependency graph.
-
-That's all. Once a meaningful dependency is added into graph, then
-lockdep would work with the graph as it did. The most important thing
-crossrelease feature has to do is to correctly identify and add true
-dependencies into the global graph.
-
-A dependency e.g. 'A -> B' can be identified only in the A's release
-context because a decision required to identify the dependency can be
-made only in the release context. That is to decide whether A can be
-released so that a waiter for A can be woken up. It cannot be made in
-other than the A's release context.
-
-It's no matter for typical locks because each acquire context is same as
-its release context, thus lockdep can decide whether a lock can be
-released in the acquire context. However for crosslocks, lockdep cannot
-make the decision in the acquire context but has to wait until the
-release context is identified.
-
-Therefore, deadlocks by crosslocks cannot be detected just when it
-happens, because those cannot be identified until the crosslocks are
-released. However, deadlock possibilities can be detected and it's very
-worth. See 'APPENDIX A' section to check why.
-
-CONCLUSION
-
-Using crossrelease feature, lockdep can work with what might be released
-in any context, namely crosslock.
-
-
-Introduce commit
-----------------
-
-Since crossrelease defers the work adding true dependencies of
-crosslocks until they are actually released, crossrelease has to queue
-all acquisitions which might create dependencies with the crosslocks.
-Then it identifies dependencies using the queued data in batches at a
-proper time. We call it 'commit'.
-
-There are four types of dependencies:
-
-1. TT type: 'typical lock A -> typical lock B'
-
- Just when acquiring B, lockdep can see it's in the A's release
- context. So the dependency between A and B can be identified
- immediately. Commit is unnecessary.
-
-2. TC type: 'typical lock A -> crosslock BX'
-
- Just when acquiring BX, lockdep can see it's in the A's release
- context. So the dependency between A and BX can be identified
- immediately. Commit is unnecessary, too.
-
-3. CT type: 'crosslock AX -> typical lock B'
-
- When acquiring B, lockdep cannot identify the dependency because
- there's no way to know if it's in the AX's release context. It has
- to wait until the decision can be made. Commit is necessary.
-
-4. CC type: 'crosslock AX -> crosslock BX'
-
- When acquiring BX, lockdep cannot identify the dependency because
- there's no way to know if it's in the AX's release context. It has
- to wait until the decision can be made. Commit is necessary.
- But, handling CC type is not implemented yet. It's a future work.
-
-Lockdep can work without commit for typical locks, but commit step is
-necessary once crosslocks are involved. Introducing commit, lockdep
-performs three steps. What lockdep does in each step is:
-
-1. Acquisition: For typical locks, lockdep does what it originally did
- and queues the lock so that CT type dependencies can be checked using
- it at the commit step. For crosslocks, it saves data which will be
- used at the commit step and increases a reference count for it.
-
-2. Commit: No action is reauired for typical locks. For crosslocks,
- lockdep adds CT type dependencies using the data saved at the
- acquisition step.
-
-3. Release: No changes are required for typical locks. When a crosslock
- is released, it decreases a reference count for it.
-
-CONCLUSION
-
-Crossrelease introduces commit step to handle dependencies of crosslocks
-in batches at a proper time.
-
-
-==============
-Implementation
-==============
-
-Data structures
----------------
-
-Crossrelease introduces two main data structures.
-
-1. hist_lock
-
- This is an array embedded in task_struct, for keeping lock history so
- that dependencies can be added using them at the commit step. Since
- it's local data, it can be accessed locklessly in the owner context.
- The array is filled at the acquisition step and consumed at the
- commit step. And it's managed in circular manner.
-
-2. cross_lock
-
- One per lockdep_map exists. This is for keeping data of crosslocks
- and used at the commit step.
-
-
-How crossrelease works
-----------------------
-
-It's the key of how crossrelease works, to defer necessary works to an
-appropriate point in time and perform in at once at the commit step.
-Let's take a look with examples step by step, starting from how lockdep
-works without crossrelease for typical locks.
-
- acquire A /* Push A onto held_locks */
- acquire B /* Push B onto held_locks and add 'A -> B' */
- acquire C /* Push C onto held_locks and add 'B -> C' */
- release C /* Pop C from held_locks */
- release B /* Pop B from held_locks */
- release A /* Pop A from held_locks */
-
- where A, B and C are different lock classes.
-
- NOTE: This document assumes that readers already understand how
- lockdep works without crossrelease thus omits details. But there's
- one thing to note. Lockdep pretends to pop a lock from held_locks
- when releasing it. But it's subtly different from the original pop
- operation because lockdep allows other than the top to be poped.
-
-In this case, lockdep adds 'the top of held_locks -> the lock to acquire'
-dependency every time acquiring a lock.
-
-After adding 'A -> B', a dependency graph will be:
-
- A -> B
-
- where A and B are different lock classes.
-
-And after adding 'B -> C', the graph will be:
-
- A -> B -> C
-
- where A, B and C are different lock classes.
-
-Let's performs commit step even for typical locks to add dependencies.
-Of course, commit step is not necessary for them, however, it would work
-well because this is a more general way.
-
- acquire A
- /*
- * Queue A into hist_locks
- *
- * In hist_locks: A
- * In graph: Empty
- */
-
- acquire B
- /*
- * Queue B into hist_locks
- *
- * In hist_locks: A, B
- * In graph: Empty
- */
-
- acquire C
- /*
- * Queue C into hist_locks
- *
- * In hist_locks: A, B, C
- * In graph: Empty
- */
-
- commit C
- /*
- * Add 'C -> ?'
- * Answer the following to decide '?'
- * What has been queued since acquire C: Nothing
- *
- * In hist_locks: A, B, C
- * In graph: Empty
- */
-
- release C
-
- commit B
- /*
- * Add 'B -> ?'
- * Answer the following to decide '?'
- * What has been queued since acquire B: C
- *
- * In hist_locks: A, B, C
- * In graph: 'B -> C'
- */
-
- release B
-
- commit A
- /*
- * Add 'A -> ?'
- * Answer the following to decide '?'
- * What has been queued since acquire A: B, C
- *
- * In hist_locks: A, B, C
- * In graph: 'B -> C', 'A -> B', 'A -> C'
- */
-
- release A
-
- where A, B and C are different lock classes.
-
-In this case, dependencies are added at the commit step as described.
-
-After commits for A, B and C, the graph will be:
-
- A -> B -> C
-
- where A, B and C are different lock classes.
-
- NOTE: A dependency 'A -> C' is optimized out.
-
-We can see the former graph built without commit step is same as the
-latter graph built using commit steps. Of course the former way leads to
-earlier finish for building the graph, which means we can detect a
-deadlock or its possibility sooner. So the former way would be prefered
-when possible. But we cannot avoid using the latter way for crosslocks.
-
-Let's look at how commit steps work for crosslocks. In this case, the
-commit step is performed only on crosslock AX as real. And it assumes
-that the AX release context is different from the AX acquire context.
-
- BX RELEASE CONTEXT BX ACQUIRE CONTEXT
- ------------------ ------------------
- acquire A
- /*
- * Push A onto held_locks
- * Queue A into hist_locks
- *
- * In held_locks: A
- * In hist_locks: A
- * In graph: Empty
- */
-
- acquire BX
- /*
- * Add 'the top of held_locks -> BX'
- *
- * In held_locks: A
- * In hist_locks: A
- * In graph: 'A -> BX'
- */
-
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- It must be guaranteed that the following operations are seen after
- acquiring BX globally. It can be done by things like barrier.
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- acquire C
- /*
- * Push C onto held_locks
- * Queue C into hist_locks
- *
- * In held_locks: C
- * In hist_locks: C
- * In graph: 'A -> BX'
- */
-
- release C
- /*
- * Pop C from held_locks
- *
- * In held_locks: Empty
- * In hist_locks: C
- * In graph: 'A -> BX'
- */
- acquire D
- /*
- * Push D onto held_locks
- * Queue D into hist_locks
- * Add 'the top of held_locks -> D'
- *
- * In held_locks: A, D
- * In hist_locks: A, D
- * In graph: 'A -> BX', 'A -> D'
- */
- acquire E
- /*
- * Push E onto held_locks
- * Queue E into hist_locks
- *
- * In held_locks: E
- * In hist_locks: C, E
- * In graph: 'A -> BX', 'A -> D'
- */
-
- release E
- /*
- * Pop E from held_locks
- *
- * In held_locks: Empty
- * In hist_locks: D, E
- * In graph: 'A -> BX', 'A -> D'
- */
- release D
- /*
- * Pop D from held_locks
- *
- * In held_locks: A
- * In hist_locks: A, D
- * In graph: 'A -> BX', 'A -> D'
- */
- commit BX
- /*
- * Add 'BX -> ?'
- * What has been queued since acquire BX: C, E
- *
- * In held_locks: Empty
- * In hist_locks: D, E
- * In graph: 'A -> BX', 'A -> D',
- * 'BX -> C', 'BX -> E'
- */
-
- release BX
- /*
- * In held_locks: Empty
- * In hist_locks: D, E
- * In graph: 'A -> BX', 'A -> D',
- * 'BX -> C', 'BX -> E'
- */
- release A
- /*
- * Pop A from held_locks
- *
- * In held_locks: Empty
- * In hist_locks: A, D
- * In graph: 'A -> BX', 'A -> D',
- * 'BX -> C', 'BX -> E'
- */
-
- where A, BX, C,..., E are different lock classes, and a suffix 'X' is
- added on crosslocks.
-
-Crossrelease considers all acquisitions after acqiuring BX are
-candidates which might create dependencies with BX. True dependencies
-will be determined when identifying the release context of BX. Meanwhile,
-all typical locks are queued so that they can be used at the commit step.
-And then two dependencies 'BX -> C' and 'BX -> E' are added at the
-commit step when identifying the release context.
-
-The final graph will be, with crossrelease:
-
- -> C
- /
- -> BX -
- / \
- A - -> E
- \
- -> D
-
- where A, BX, C,..., E are different lock classes, and a suffix 'X' is
- added on crosslocks.
-
-However, the final graph will be, without crossrelease:
-
- A -> D
-
- where A and D are different lock classes.
-
-The former graph has three more dependencies, 'A -> BX', 'BX -> C' and
-'BX -> E' giving additional opportunities to check if they cause
-deadlocks. This way lockdep can detect a deadlock or its possibility
-caused by crosslocks.
-
-CONCLUSION
-
-We checked how crossrelease works with several examples.
-
-
-=============
-Optimizations
-=============
-
-Avoid duplication
------------------
-
-Crossrelease feature uses a cache like what lockdep already uses for
-dependency chains, but this time it's for caching CT type dependencies.
-Once that dependency is cached, the same will never be added again.
-
-
-Lockless for hot paths
-----------------------
-
-To keep all locks for later use at the commit step, crossrelease adopts
-a local array embedded in task_struct, which makes access to the data
-lockless by forcing it to happen only within the owner context. It's
-like how lockdep handles held_locks. Lockless implmentation is important
-since typical locks are very frequently acquired and released.
-
-
-=================================================
-APPENDIX A: What lockdep does to work aggresively
-=================================================
-
-A deadlock actually occurs when all wait operations creating circular
-dependencies run at the same time. Even though they don't, a potential
-deadlock exists if the problematic dependencies exist. Thus it's
-meaningful to detect not only an actual deadlock but also its potential
-possibility. The latter is rather valuable. When a deadlock occurs
-actually, we can identify what happens in the system by some means or
-other even without lockdep. However, there's no way to detect possiblity
-without lockdep unless the whole code is parsed in head. It's terrible.
-Lockdep does the both, and crossrelease only focuses on the latter.
-
-Whether or not a deadlock actually occurs depends on several factors.
-For example, what order contexts are switched in is a factor. Assuming
-circular dependencies exist, a deadlock would occur when contexts are
-switched so that all wait operations creating the dependencies run
-simultaneously. Thus to detect a deadlock possibility even in the case
-that it has not occured yet, lockdep should consider all possible
-combinations of dependencies, trying to:
-
-1. Use a global dependency graph.
-
- Lockdep combines all dependencies into one global graph and uses them,
- regardless of which context generates them or what order contexts are
- switched in. Aggregated dependencies are only considered so they are
- prone to be circular if a problem exists.
-
-2. Check dependencies between classes instead of instances.
-
- What actually causes a deadlock are instances of lock. However,
- lockdep checks dependencies between classes instead of instances.
- This way lockdep can detect a deadlock which has not happened but
- might happen in future by others but the same class.
-
-3. Assume all acquisitions lead to waiting.
-
- Although locks might be acquired without waiting which is essential
- to create dependencies, lockdep assumes all acquisitions lead to
- waiting since it might be true some time or another.
-
-CONCLUSION
-
-Lockdep detects not only an actual deadlock but also its possibility,
-and the latter is more valuable.
-
-
-==================================================
-APPENDIX B: How to avoid adding false dependencies
-==================================================
-
-Remind what a dependency is. A dependency exists if:
-
- 1. There are two waiters waiting for each event at a given time.
- 2. The only way to wake up each waiter is to trigger its event.
- 3. Whether one can be woken up depends on whether the other can.
-
-For example:
-
- acquire A
- acquire B /* A dependency 'A -> B' exists */
- release B
- release A
-
- where A and B are different lock classes.
-
-A depedency 'A -> B' exists since:
-
- 1. A waiter for A and a waiter for B might exist when acquiring B.
- 2. Only way to wake up each is to release what it waits for.
- 3. Whether the waiter for A can be woken up depends on whether the
- other can. IOW, TASK X cannot release A if it fails to acquire B.
-
-For another example:
-
- TASK X TASK Y
- ------ ------
- acquire AX
- acquire B /* A dependency 'AX -> B' exists */
- release B
- release AX held by Y
-
- where AX and B are different lock classes, and a suffix 'X' is added
- on crosslocks.
-
-Even in this case involving crosslocks, the same rule can be applied. A
-depedency 'AX -> B' exists since:
-
- 1. A waiter for AX and a waiter for B might exist when acquiring B.
- 2. Only way to wake up each is to release what it waits for.
- 3. Whether the waiter for AX can be woken up depends on whether the
- other can. IOW, TASK X cannot release AX if it fails to acquire B.
-
-Let's take a look at more complicated example:
-
- TASK X TASK Y
- ------ ------
- acquire B
- release B
- fork Y
- acquire AX
- acquire C /* A dependency 'AX -> C' exists */
- release C
- release AX held by Y
-
- where AX, B and C are different lock classes, and a suffix 'X' is
- added on crosslocks.
-
-Does a dependency 'AX -> B' exist? Nope.
-
-Two waiters are essential to create a dependency. However, waiters for
-AX and B to create 'AX -> B' cannot exist at the same time in this
-example. Thus the dependency 'AX -> B' cannot be created.
-
-It would be ideal if the full set of true ones can be considered. But
-we can ensure nothing but what actually happened. Relying on what
-actually happens at runtime, we can anyway add only true ones, though
-they might be a subset of true ones. It's similar to how lockdep works
-for typical locks. There might be more true dependencies than what
-lockdep has detected in runtime. Lockdep has no choice but to rely on
-what actually happens. Crossrelease also relies on it.
-
-CONCLUSION
-
-Relying on what actually happens, lockdep can avoid adding false
-dependencies.