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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Dec 05, 2025 at 04:18:38PM +0900, Byungchul Park wrote: > Add documents describing the concept and APIs of dept. >=20 > Signed-off-by: Byungchul Park > --- > Documentation/dev-tools/dept.rst | 778 +++++++++++++++++++++++++++ > Documentation/dev-tools/dept_api.rst | 125 +++++ You forget to add toctree entries: ---- >8 ---- diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/in= dex.rst index 4b8425e348abd1..02c858f5ed1fa2 100644 --- a/Documentation/dev-tools/index.rst +++ b/Documentation/dev-tools/index.rst @@ -22,6 +22,8 @@ Documentation/process/debugging/index.rst clang-format coccinelle sparse + dept + dept_api kcov gcov kasan > +Lockdep detects a deadlock by checking lock acquisition order. For > +example, a graph to track acquisition order built by lockdep might look > +like: > + > +.. literal:: > + > + A -> B - > + \ > + -> E > + / > + C -> D - > + > + where 'A -> B' means that acquisition A is prior to acquisition B > + with A still held. Use code-block directive for literal code blocks: ---- >8 ---- diff --git a/Documentation/dev-tools/dept.rst b/Documentation/dev-tools/dep= t.rst index 333166464543d7..8394c4ea81bc2a 100644 --- a/Documentation/dev-tools/dept.rst +++ b/Documentation/dev-tools/dept.rst @@ -10,7 +10,7 @@ Lockdep detects a deadlock by checking lock acquisition o= rder. For example, a graph to track acquisition order built by lockdep might look like: =20 -.. literal:: +.. code-block:: =20 A -> B - \ @@ -25,7 +25,7 @@ Lockdep keeps adding each new acquisition order into the = graph at runtime. For example, 'E -> C' will be added when the two locks have been acquired in the order, E and then C. The graph will look like: =20 -.. literal:: +.. code-block:: =20 A -> B - \ @@ -41,7 +41,7 @@ been acquired in the order, E and then C. The graph will= look like: =20 This graph contains a subgraph that demonstrates a loop like: =20 -.. literal:: +.. code-block:: =20 -> E - / \ @@ -76,7 +76,7 @@ e.g. irq context, normal process context, wq worker conte= xt, or so on. =20 Can lockdep detect the following deadlock? =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -91,7 +91,7 @@ Can lockdep detect the following deadlock? =20 No. What about the following? =20 -.. literal:: +.. code-block:: =20 context X context Y =20 @@ -116,7 +116,7 @@ What leads a deadlock A deadlock occurs when one or multi contexts are waiting for events that will never happen. For example: =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -148,7 +148,7 @@ In terms of dependency: =20 Dependency graph reflecting this example will look like: =20 -.. literal:: +.. code-block:: =20 -> C -> A -> B - / \ @@ -171,7 +171,7 @@ Introduce DEPT DEPT(DEPendency Tracker) tracks wait and event instead of lock acquisition order so as to recognize the following situation: =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -186,7 +186,7 @@ acquisition order so as to recognize the following situ= ation: and builds up a dependency graph at runtime that is similar to lockdep. The graph might look like: =20 -.. literal:: +.. code-block:: =20 -> C -> A -> B - / \ @@ -199,7 +199,7 @@ DEPT keeps adding each new dependency into the graph at= runtime. For example, 'B -> D' will be added when event D occurrence is a prerequisite to reaching event B like: =20 -.. literal:: +.. code-block:: =20 context W =20 @@ -211,7 +211,7 @@ prerequisite to reaching event B like: =20 After the addition, the graph will look like: =20 -.. literal:: +.. code-block:: =20 -> D / @@ -236,7 +236,7 @@ How DEPT works Let's take a look how DEPT works with the 1st example in the section 'Limitation of lockdep'. =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -256,7 +256,7 @@ event. =20 Adding comments to describe DEPT's view in detail: =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -293,7 +293,7 @@ Adding comments to describe DEPT's view in detail: =20 Let's build up dependency graph with this example. Firstly, context X: =20 -.. literal:: +.. code-block:: =20 context X =20 @@ -304,7 +304,7 @@ Let's build up dependency graph with this example. Fir= stly, context X: =20 There are no events to create dependency. Next, context Y: =20 -.. literal:: +.. code-block:: =20 context Y =20 @@ -332,7 +332,7 @@ event A cannot be triggered if wait B cannot be awakene= d by event B. Therefore, we can say event A depends on event B, say, 'A -> B'. The graph will look like after adding the dependency: =20 -.. literal:: +.. code-block:: =20 A -> B =20 @@ -340,7 +340,7 @@ graph will look like after adding the dependency: =20 Lastly, context Z: =20 -.. literal:: +.. code-block:: =20 context Z =20 @@ -362,7 +362,7 @@ triggered if wait A cannot be awakened by event A. The= refore, we can say event B depends on event A, say, 'B -> A'. The graph will look like after adding the dependency: =20 -.. literal:: +.. code-block:: =20 -> A -> B - / \ @@ -386,7 +386,7 @@ Interpret DEPT report =20 The following is the same example in the section 'How DEPT works'. =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -425,7 +425,7 @@ We can simplify this by labeling each waiting point wit= h [W], each point where its event's context starts with [S] and each event with [E]. This example will look like after the labeling: =20 -.. literal:: +.. code-block:: =20 context X context Y context Z =20 @@ -443,7 +443,7 @@ DEPT uses the symbols [W], [S] and [E] in its report as= described above. The following is an example reported by DEPT for a real problem in practice. =20 -.. literal:: +.. code-block:: =20 Link: https://lore.kernel.org/lkml/6383cde5-cf4b-facf-6e07-1378a485657d= @I-love.SAKURA.ne.jp/#t Link: https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-by= ungchul.park@lge.com/ @@ -646,7 +646,7 @@ practice. =20 Let's take a look at the summary that is the most important part. =20 -.. literal:: +.. code-block:: =20 --------------------------------------------------- summary @@ -669,7 +669,7 @@ Let's take a look at the summary that is the most impor= tant part. =20 The summary shows the following scenario: =20 -.. literal:: +.. code-block:: =20 context A context B context ?(unknown) =20 @@ -684,7 +684,7 @@ The summary shows the following scenario: =20 Adding comments to describe DEPT's view in detail: =20 -.. literal:: +.. code-block:: =20 context A context B context ?(unknown) =20 @@ -711,7 +711,7 @@ Adding comments to describe DEPT's view in detail: =20 Let's build up dependency graph with this report. Firstly, context A: =20 -.. literal:: +.. code-block:: =20 context A =20 @@ -735,7 +735,7 @@ unlock(&ni->ni_lock:0) depends on folio_unlock(&f1), sa= y, =20 The graph will look like after adding the dependency: =20 -.. literal:: +.. code-block:: =20 unlock(&ni->ni_lock:0) -> folio_unlock(&f1) =20 @@ -743,7 +743,7 @@ The graph will look like after adding the dependency: =20 Secondly, context B: =20 -.. literal:: +.. code-block:: =20 context B =20 @@ -762,7 +762,7 @@ folio_unlock(&f1) depends on unlock(&ni->ni_lock:0), sa= y, =20 The graph will look like after adding the dependency: =20 -.. literal:: +.. code-block:: =20 -> unlock(&ni->ni_lock:0) -> folio_unlock(&f1) - / \ > +Limitation of lockdep > +--------------------- > + > +Lockdep deals with a deadlock by typical lock e.g. spinlock and mutex, > +that are supposed to be released within the acquisition context. > +However, when it comes to a deadlock by folio lock that is not supposed > +to be released within the acquisition context or other general > +synchronization mechanisms, lockdep doesn't work. > + > +NOTE: In this document, 'context' refers to any type of unique context > +e.g. irq context, normal process context, wq worker context, or so on. > + > +Can lockdep detect the following deadlock? > + > +.. literal:: > + > + context X context Y context Z > + > + mutex_lock A > + folio_lock B > + folio_lock B <- DEADLOCK > + mutex_lock A <- DEADLOCK > + folio_unlock B > + folio_unlock B > + mutex_unlock A > + mutex_unlock A > + > +No. What about the following? > + > +.. literal:: > + > + context X context Y > + > + mutex_lock A > + mutex_lock A <- DEADLOCK > + wait_for_complete B <- DEADLOCK > + complete B > + mutex_unlock A > + mutex_unlock A > + > +No. One unanswered question from my v17 review [1]: You explain in "How DEPT wo= rks" section how DEPT detects deadlock in the first example (the former with thr= ee contexts). Can you do the same on the second example (the latter with two contexts)? Thanks. [1]: https://lore.kernel.org/linux-doc/aN84jKyrE1BumpLj@archie.me/ --=20 An old man doll... just what I always wanted! - Clara --tsFx90K4YzyOMDd5 Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iHUEABYKAB0WIQSSYQ6Cy7oyFNCHrUH2uYlJVVFOowUCaTN37QAKCRD2uYlJVVFO o40gAP9yWQe507aOQ9xG+y3WznUbz9K0gxVdcJgmBzyPkuLdOAD/SjStuxrT6yQi Wd1X9MlzPBf7sPwdNC1xXihj1C/n6go= =b9ga -----END PGP SIGNATURE----- --tsFx90K4YzyOMDd5--