Re: [PATCH] locking/Documentation: Revise Documentation/locking/crossrelease.txt

Byungchul Park
2017-11-11T13:45:30Z

This is the big one including all of version 3.

You can take only this.

Thanks,
Byungchul

On Sat, Nov 11, 2017 at 10:33:34PM +0900, Byungchul Park wrote:
> Revise Documentation/locking/crossrelease.txt to improve its readability.
>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
> Documentation/locking/crossrelease.txt | 329 ++++++++++++++++-----------------
> 1 file changed, 155 insertions(+), 174 deletions(-)
>
> diff --git a/Documentation/locking/crossrelease.txt b/Documentation/locking/crossrelease.txt
> index bdf1423..c6d628b 100644
> --- a/Documentation/locking/crossrelease.txt
> +++ b/Documentation/locking/crossrelease.txt
> @@ -12,10 +12,10 @@ Contents:
>
> (*) Limitation
>
> - - Limit lockdep
> + - Limiting lockdep
> - Pros from the limitation
> - Cons from the limitation
> - - Relax the limitation
> + - Relaxing the limitation
>
> (*) Crossrelease
>
> @@ -30,9 +30,9 @@ Contents:
> (*) Optimizations
>
> - Avoid duplication
> - - Lockless for hot paths
> + - Make hot paths lockless
>
> - (*) APPENDIX A: What lockdep does to work aggresively
> + (*) APPENDIX A: What lockdep does to work aggressively
>
> (*) APPENDIX B: How to avoid adding false dependencies
>
> @@ -55,21 +55,21 @@ For example:
> 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,
> +A deadlock occurs when these three waiters 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:
> +A dependency might exist between two waiters and a deadlock happens due
> +to an incorrect relationship between dependencies. Thus, we must define
> +what a dependency is first. 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.
>
> -Each wait in the example creates its dependency like:
> +Each waiter in the example creates its dependency like:
>
> Event C depends on event A.
> Event A depends on event B.
> @@ -77,7 +77,7 @@ Each wait in the example creates its dependency like:
>
> 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
> + 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.
>
> @@ -109,9 +109,9 @@ event in the previous section.
> In short, lockdep does:
>
> 1. Detect a new dependency.
> - 2. Add the dependency into a global graph.
> + 2. Add the dependency to a global graph.
> 3. Check if that makes dependencies circular.
> - 4. Report a deadlock or its possibility if so.
> + 4. Report the deadlock or its possibility if so.
>
> For example, consider a graph built by lockdep that looks like:
>
> @@ -123,7 +123,7 @@ For example, consider a graph built by lockdep that looks like:
>
> where A, B,..., E are different lock classes.
>
> -Lockdep will add a dependency into the graph on detection of a new
> +Lockdep will add a dependency to 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:
>
> @@ -147,7 +147,7 @@ This graph contains a subgraph which demonstrates circular dependencies:
> \ /
> ------------------
>
> - where C, D and E are different lock classes.
> + 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
> @@ -163,13 +163,13 @@ dependencies were created after adding each new dependency.
> Limitation
> ==========
>
> -Limit lockdep
> --------------
> +Limiting lockdep
> +----------------
>
> Limiting lockdep to work on only typical locks e.g. spin locks and
> -mutexes, which are released within the acquire context, the
> +mutexes, which are released within their acquire contexts, the
> implementation becomes simple but its capacity for detection becomes
> -limited. Let's check pros and cons in next section.
> +limited. Let's check pros and cons in the next two sections.
>
>
> Pros from the limitation
> @@ -179,7 +179,7 @@ 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.
> +the held_locks and the lock to acquire at the moment.
>
> For example:
>
> @@ -195,22 +195,22 @@ For example:
> 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.
> +Dependencies can be simply added this way, 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.
> +Furthermore, since data required to create a dependency can be kept in
> +local task_struct, lockdep can avoid racy problems without explicit
> +protection by forcing to access the data within the context.
>
> 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
> +the dependency graph. However, relaxing the limitation, it needs to keep
> +even locks already released, because the 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.
> + 2. Races are avoidable without explicit protection while accessing
> + local locks in a held_locks.
> 3. Lockdep only needs to keep locks currently being held.
>
> CONCLUSION
> @@ -265,8 +265,8 @@ Given the limitation, lockdep cannot detect a deadlock or its
> possibility caused by page locks or completions.
>
>
> -Relax the limitation
> ---------------------
> +Relaxing the limitation
> +-----------------------
>
> Under the limitation, things to create dependencies are limited to
> typical locks. However, synchronization primitives like page locks and
> @@ -278,37 +278,36 @@ 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.
> +thoroughly works. Thus, lockdep has to do its best to detect and add as
> +many true dependencies to the graph as possible.
>
> -For example, considering only typical locks, lockdep builds a graph like:
> +For example:
>
> - A -> B -
> - \
> - -> E
> - /
> - C -> D -
> + CONTEXT X CONTEXT Y
> + --------- ---------
> + acquire A
> + acquire B /* A dependency 'A -> B' exists */
> + release B
> + release A held by Y
>
> - where A, B,..., E are different lock classes.
> + where A and B 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:
> +In this case, a dependency 'A -> B' exists since:
>
> - A -> B -
> - \
> - -> E -> GX
> - /
> - FX -> C -> D -
> + 1. A waiter for A and a waiter for B might exist when acquiring B.
> + 2. The 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. In other words, CONTEXT X cannot release A if it fails
> + to acquire B.
>
> - where A, B,..., E, FX and GX are different lock classes, and a suffix
> - 'X' is added on non-typical locks.
> +Considering only typical locks, lockdep builds nothing. However,
> +relaxing the limitation, a dependency 'A -> B' can be added, giving us
> +more chances to check circular dependencies.
>
> -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.
> +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:
>
> @@ -336,27 +335,27 @@ 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.
> +feature provides a way to do that.
>
> Crossrelease feature has to do:
>
> 1. Identify dependencies created by crosslocks.
> - 2. Add the dependencies into a dependency graph.
> + 2. Add the dependencies to the dependency graph.
>
> -That's all. Once a meaningful dependency is added into graph, then
> +That's all. Once a meaningful dependency is added to the 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.
> +dependencies to 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
> +released so that waiters for A can be woken up. That 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
> +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.
>
> @@ -375,10 +374,11 @@ Introduce commit
> ----------------
>
> Since crossrelease defers the work adding true dependencies of
> -crosslocks until they are actually released, crossrelease has to queue
> +crosslocks until they are eventually 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'.
> +Then lockdep can identify dependencies using the queued data in batches
> +at a proper time. We call the step adding true dependencies to the graph
> +in batches, 'commit'.
>
> There are four types of dependencies:
>
> @@ -404,10 +404,10 @@ There are four types of dependencies:
>
> 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.
> + 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
> +Lockdep can work without commit for typical locks, but the step is
> necessary once crosslocks are involved. Introducing commit, lockdep
> performs three steps. What lockdep does in each step is:
>
> @@ -416,7 +416,7 @@ performs three steps. What lockdep does in each step is:
> 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,
> +2. Commit: No action is required for typical locks. For crosslocks,
> lockdep adds CT type dependencies using the data saved at the
> acquisition step.
>
> @@ -442,9 +442,9 @@ Crossrelease introduces two main data structures.
>
> 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.
> + they are local data, they 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 a circular manner.
>
> 2. cross_lock
>
> @@ -456,29 +456,24 @@ 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.
> +appropriate point in time and perform the works at the commit step.
> +
> +Let's take a look at examples step by step, starting from how lockdep
> +works for typical locks, without crossrelease.
>
> - 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' */
> + acquire A /* Push A to held_locks */
> + acquire B /* Push B to held_locks and add 'A -> B' */
> + acquire C /* Push C to 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.
> + 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.
> +Lockdep adds 'the top of held_locks -> the lock to acquire' dependency
> +every time acquiring a lock.
>
> -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:
> +After adding 'A -> B', the dependency graph will be:
>
> A -> B
>
> @@ -488,15 +483,15 @@ And after adding 'B -> C', the graph will be:
>
> A -> B -> C
>
> - where A, B and C are different lock classes.
> + 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.
> +Let's build the graph using the commit step with the same example. Of
> +course, the step is not necessary for typical locks, however, it would
> +also work because this is a more general way.
>
> acquire A
> /*
> - * Queue A into hist_locks
> + * Queue A in hist_locks
> *
> * In hist_locks: A
> * In graph: Empty
> @@ -504,7 +499,7 @@ well because this is a more general way.
>
> acquire B
> /*
> - * Queue B into hist_locks
> + * Queue B in hist_locks
> *
> * In hist_locks: A, B
> * In graph: Empty
> @@ -512,7 +507,7 @@ well because this is a more general way.
>
> acquire C
> /*
> - * Queue C into hist_locks
> + * Queue C in hist_locks
> *
> * In hist_locks: A, B, C
> * In graph: Empty
> @@ -554,34 +549,32 @@ well because this is a more general way.
>
> release A
>
> - where A, B and C are different lock classes.
> -
> -In this case, dependencies are added at the commit step as described.
> + where A, B, and C are different lock classes.
>
> -After commits for A, B and C, the graph will be:
> +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.
> + 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.
> +We can see the former graph built without the commit step is same as the
> +latter graph. 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 preferred 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.
> +Lastly, let's look at how commit works for crosslocks in practice.
>
> BX RELEASE CONTEXT BX ACQUIRE CONTEXT
> ------------------ ------------------
> acquire A
> /*
> - * Push A onto held_locks
> - * Queue A into hist_locks
> + * Add 'the top of held_locks -> A'
> + * Push A to held_locks
> + * Queue A in hist_locks
> *
> * In held_locks: A
> * In hist_locks: A
> @@ -604,8 +597,9 @@ that the AX release context is different from the AX acquire context.
>
> acquire C
> /*
> - * Push C onto held_locks
> - * Queue C into hist_locks
> + * Add 'the top of held_locks -> C'
> + * Push C to held_locks
> + * Queue C in hist_locks
> *
> * In held_locks: C
> * In hist_locks: C
> @@ -622,9 +616,9 @@ that the AX release context is different from the AX acquire context.
> */
> acquire D
> /*
> - * Push D onto held_locks
> - * Queue D into hist_locks
> * Add 'the top of held_locks -> D'
> + * Push D to held_locks
> + * Queue D in hist_locks
> *
> * In held_locks: A, D
> * In hist_locks: A, D
> @@ -632,8 +626,9 @@ that the AX release context is different from the AX acquire context.
> */
> acquire E
> /*
> - * Push E onto held_locks
> - * Queue E into hist_locks
> + * Add 'the top of held_locks -> E'
> + * Push E to held_locks
> + * Queue E in hist_locks
> *
> * In held_locks: E
> * In hist_locks: C, E
> @@ -659,6 +654,7 @@ that the AX release context is different from the AX acquire context.
> commit BX
> /*
> * Add 'BX -> ?'
> + * Answer the following to decide '?'
> * What has been queued since acquire BX: C, E
> *
> * In held_locks: Empty
> @@ -684,15 +680,15 @@ that the AX release context is different from the AX acquire context.
> * 'BX -> C', 'BX -> E'
> */
>
> - where A, BX, C,..., E are different lock classes, and a suffix 'X' is
> - added on crosslocks.
> + where A, BX, C,..., E are different lock classes and a suffix 'X' is
> + added at 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.
> +Crossrelease considers all acquisitions following acquiring BX because
> +they can create dependencies with BX. The dependencies will be
> +determined in the release context of BX. Meanwhile, all typical locks
> +are queued so that they can be used at the commit step. Finally, two
> +dependencies 'BX -> C' and 'BX -> E' will be added at the commit step,
> +when identifying the release context.
>
> The final graph will be, with crossrelease:
>
> @@ -704,8 +700,8 @@ The final graph will be, with crossrelease:
> \
> -> D
>
> - where A, BX, C,..., E are different lock classes, and a suffix 'X' is
> - added on crosslocks.
> + where A, BX, C,..., E are different lock classes and a suffix 'X' is
> + added at crosslocks.
>
> However, the final graph will be, without crossrelease:
>
> @@ -732,39 +728,40 @@ 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.
> +Once a dependency is cached, the same will never be added again.
>
>
> -Lockless for hot paths
> -----------------------
> +Make hot paths lockless
> +-----------------------
>
> 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.
> +a local array embedded in task_struct, which makes the data locklessly
> +accessible by forcing it to happen only within the owner context. It's
> +like how lockdep handles held_locks. Lockless implementation 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
> +A deadlock actually occurs when all waiters 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
> +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.
> +possibility. The latter is rather valuable. When a deadlock actually
> +occurs, we can identify what happens in the system by some means or
> +other even without lockdep. However, there's no way to detect a
> +possibility without lockdep, unless the whole code is parsed in the 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
> +switched so that all waiters creating the dependencies run
> +simultaneously. Thus, to detect a deadlock possibility even in the case
> +that it has not occurred yet, lockdep should consider all possible
> combinations of dependencies, trying to:
>
> 1. Use a global dependency graph.
> @@ -776,7 +773,7 @@ combinations of dependencies, trying to:
>
> 2. Check dependencies between classes instead of instances.
>
> - What actually causes a deadlock are instances of lock. However,
> + What actually causes a deadlock are instances of locks. 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.
> @@ -805,44 +802,28 @@ Remind what a dependency is. A dependency exists if:
>
> 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
> - ------ ------
> + CONTEXT X CONTEXT 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.
> + where AX and B are different lock classes and a suffix 'X' is added
> + at crosslocks.
>
> -Even in this case involving crosslocks, the same rule can be applied. A
> -depedency 'AX -> B' exists since:
> +Here, a dependency '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.
> + 2. The 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.
> + other can. In other words, CONTEXT X cannot release AX if it fails
> + to acquire B.
>
> -Let's take a look at more complicated example:
> +Let's take a look at a more complicated example:
>
> - TASK X TASK Y
> - ------ ------
> + CONTEXT X CONTEXT Y
> + --------- ---------
> acquire B
> release B
> fork Y
> @@ -851,22 +832,22 @@ Let's take a look at more complicated example:
> release C
> release AX held by Y
>
> - where AX, B and C are different lock classes, and a suffix 'X' is
> - added on crosslocks.
> + where AX, B, and C are different lock classes and a suffix 'X' is
> + added at 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.
> +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.
> +for typical locks. There might be more true dependencies than lockdep
> +has detected. Lockdep has no choice but to rely on what actually happens.
> +Crossrelease also relies on it.
>
> CONCLUSION
>
> --
> 1.9.1