From: Boqun Feng <email@example.com> To: firstname.lastname@example.org Cc: Peter Zijlstra <email@example.com>, Ingo Molnar <firstname.lastname@example.org>, Andrea Parri <email@example.com>, "Paul E. McKenney" <firstname.lastname@example.org>, Boqun Feng <email@example.com>, Jonathan Corbet <firstname.lastname@example.org>, email@example.com (open list:DOCUMENTATION) Subject: [RFC tip/locking/lockdep v6 01/20] lockdep/Documention: Recursive read lock detection reasoning Date: Wed, 11 Apr 2018 21:50:51 +0800 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> This patch add the documentation piece for the reasoning of deadlock detection related to recursive read lock. The following sections are added: * Explain what is a recursive read lock, and what deadlock cases they could introduce. * Introduce the notations for different types of dependencies, and the definition of strong paths. * Proof for a closed strong path is both sufficient and necessary for deadlock detections with recursive read locks involved. The proof could also explain why we call the path "strong" Signed-off-by: Boqun Feng <firstname.lastname@example.org> --- Documentation/locking/lockdep-design.txt | 178 +++++++++++++++++++++++++++++++ 1 file changed, 178 insertions(+) diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt index 9de1c158d44c..6bb9e90e2c4f 100644 --- a/Documentation/locking/lockdep-design.txt +++ b/Documentation/locking/lockdep-design.txt @@ -284,3 +284,181 @@ Run the command and save the output, then compare against the output from a later run of this command to identify the leakers. This same output can also help you find situations where runtime lock initialization has been omitted. + +Recursive read locks: +--------------------- + +Lockdep now is equipped with deadlock detection for recursive read locks. + +Recursive read locks, as their name indicates, are the locks able to be +acquired recursively. Unlike non-recursive read locks, recursive read locks +only get blocked by current write lock *holders* other than write lock +*waiters*, for example: + + TASK A: TASK B: + + read_lock(X); + + write_lock(X); + + read_lock(X); + +is not a deadlock for recursive read locks, as while the task B is waiting for +the lock X, the second read_lock() doesn't need to wait because it's a recursive +read lock. However if the read_lock() is non-recursive read lock, then the above +case is a deadlock, because even if the write_lock() in TASK B can not get the +lock, but it can block the second read_lock() in TASK A. + +Note that a lock can be a write lock (exclusive lock), a non-recursive read +lock (non-recursive shared lock) or a recursive read lock (recursive shared +lock), depending on the lock operations used to acquire it (more specifically, +the value of the 'read' parameter for lock_acquire()). In other words, a single +lock instance has three types of acquisition depending on the acquisition +functions: exclusive, non-recursive read, and recursive read. + +To be concise, we call that write locks and non-recursive read locks as +"non-recursive" locks and recursive read locks as "recursive" locks. + +Recursive locks don't block each other, while non-recursive locks do (this is +even true for two non-recursive read locks). A non-recursive lock can block the +corresponding recursive lock, and vice versa. + +A deadlock case with recursive locks involved is as follow: + + TASK A: TASK B: + + read_lock(X); + read_lock(Y); + write_lock(Y); + write_lock(X); + +Task A is waiting for task B to read_unlock() Y and task B is waiting for task +A to read_unlock() X. + +Dependency types and strong dependency paths: +--------------------------------------------- +In order to detect deadlocks as above, lockdep needs to track different dependencies. +There are 4 categories for dependency edges in the lockdep graph: + +1) -(NN)->: non-recursive to non-recursive dependency. "X -(NN)-> Y" means + X -> Y and both X and Y are non-recursive locks. + +2) -(RN)->: recursive to non-recursive dependency. "X -(RN)-> Y" means + X -> Y and X is recursive read lock and Y is non-recursive lock. + +3) -(NR)->: non-recursive to recursive dependency, "X -(NR)-> Y" means + X -> Y and X is non-recursive lock and Y is recursive lock. + +4) -(RR)->: recursive to recursive dependency, "X -(RR)-> Y" means + X -> Y and both X and Y are recursive locks. + +Note that given two locks, they may have multiple dependencies between them, for example: + + TASK A: + + read_lock(X); + write_lock(Y); + ... + + TASK B: + + write_lock(X); + write_lock(Y); + +, we have both X -(RN)-> Y and X -(NN)-> Y in the dependency graph. + +We use -(*N)-> for edges that is either -(RN)-> or -(NN)->, the similar for -(N*)->, +-(*R)-> and -(R*)-> + +A "path" is a series of conjunct dependency edges in the graph. And we define a +"strong" path, which indicates the strong dependency throughout each dependency +in the path, as the path that doesn't have two conjunct edges (dependencies) as +-(*R)-> and -(R*)->. In other words, a "strong" path is a path from a lock +walking to another through the lock dependencies, and if X -> Y -> Z in the +path (where X, Y, Z are locks), if the walk from X to Y is through a -(NR)-> or +-(RR)-> dependency, the walk from Y to Z must not be through a -(RN)-> or +-(RR)-> dependency, otherwise it's not a strong path. + +We will see why the path is called "strong" in next section. + +Recursive Read Deadlock Detection: +---------------------------------- + +We now prove two things: + +Lemma 1: + +If there is a closed strong path (i.e. a strong cirle), then there is a +combination of locking sequences that causes deadlock. I.e. a strong circle is +sufficient for deadlock detection. + +Lemma 2: + +If there is no closed strong path (i.e. strong cirle), then there is no +combination of locking sequences that could cause deadlock. I.e. strong +circles are necessary for deadlock detection. + +With these two Lemmas, we can easily say a closed strong path is both sufficient +and necessary for deadlocks, therefore a closed strong path is equivalent to +deadlock possibility. As a closed strong path stands for a dependency chain that +could cause deadlocks, so we call it "strong", considering there are dependency +circles that won't cause deadlocks. + +Proof for sufficiency (Lemma 1): + +Let's say we have a strong cirlce: + + L1 -> L2 ... -> Ln -> L1 + +, which means we have dependencies: + + L1 -> L2 + L2 -> L3 + ... + Ln-1 -> Ln + Ln -> L1 + +We now can construct a combination of locking sequences that cause deadlock: + +Firstly let's make one CPU/task get the L1 in L1 -> L2, and then another get +the L2 in L2 -> L3, and so on. After this, all of the Lx in Lx -> Lx+1 are +held by different CPU/tasks. + +And then because we have L1 -> L2, so the holder of L1 is going to acquire L2 +in L1 -> L2, however since L2 is already held by another CPU/task, plus L1 -> +L2 and L2 -> L3 are not *R and R* (the definition of strong), therefore the +holder of L1 can not get L2, it has to wait L2's holder to release. + +Moreover, we can have a similar conclusion for L2's holder: it has to wait L3's +holder to release, and so on. We now can proof that Lx's holder has to wait for +Lx+1's holder to release, and note that Ln+1 is L1, so we have a circular +waiting scenario and nobody can get progress, therefore a deadlock. + +Proof for necessary (Lemma 2): + +Lemma 2 is equivalent to: If there is a deadlock scenario, then there must be a +strong circle in the dependency graph. + +According to Wikipedia, if there is a deadlock, then there must be a circular +waiting scenario, means there are N CPU/tasks, where CPU/task P1 is waiting for +a lock held by P2, and P2 is waiting for a lock held by P3, ... and Pn is waiting +for a lock held by P1. Let's name the lock Px is waiting as Lx, so since P1 is waiting +for L1 and holding Ln, so we will have Ln -> L1 in the dependency graph. Similarly, +we have L1 -> L2, L2 -> L3, ..., Ln-1 -> Ln in the dependency graph, which means we +have a circle: + + Ln -> L1 -> L2 -> ... -> Ln + +, and now let's prove the circle is strong: + +For a lock Lx, Px contributes the dependency Lx-1 -> Lx and Px+1 contributes +the dependency Lx -> Lx+1, and since Px is waiting for Px+1 to release Lx, +so Lx can not be both recursive in Lx -> Lx+1 and Lx-1 -> Lx, because recursive +locks don't block each other, therefore Lx-1 -> Lx and Lx -> Lx+1 can not be a +-(*R)-> -(R*)-> pair, and this is true for any lock in the circle, therefore, +the circle is strong. + +References: +----------- +: https://en.wikipedia.org/wiki/Deadlock +: Shibu, K. (2009). Intro To Embedded Systems (1st ed.). Tata McGraw-Hill -- 2.16.2
next prev parent reply other threads:[~2018-04-11 13:47 UTC|newest] Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-04-11 13:50 [RFC tip/locking/lockdep v6 00/20] lockdep: Support deadlock detection for recursive read locks Boqun Feng 2018-04-11 13:50 ` Boqun Feng [this message] 2018-04-15 0:38 ` [RFC tip/locking/lockdep v6 01/20] lockdep/Documention: Recursive read lock detection reasoning Randy Dunlap 2018-04-16 6:29 ` Boqun Feng 2018-04-27 13:50 ` Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 02/20] lockdep: Demagic the return value of BFS Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 03/20] lockdep: Make __bfs() visit every dependency until a match Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 04/20] lockdep: Redefine LOCK_*_STATE* bits Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 05/20] lockdep: Reduce the size of lock_list::distance Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 06/20] lockdep: Introduce lock_list::dep Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 07/20] lockdep: Extend __bfs() to work with multiple types of dependencies Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 08/20] lockdep: Make __bfs(.match) return bool Boqun Feng 2018-04-11 13:50 ` [RFC tip/locking/lockdep v6 09/20] lockdep: Support deadlock detection for recursive read locks in check_noncircular() Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 10/20] lockdep: Adjust check_redundant() for recursive read change Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 11/20] lockdep: Fix recursive read lock related safe->unsafe detection Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 12/20] lockdep: Add recursive read locks into dependency graph Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 13/20] lockdep/selftest: Add a R-L/L-W test case specific to chain cache behavior Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 14/20] lockdep: Take read/write status in consideration when generate chainkey Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 15/20] lockdep/selftest: Unleash irq_read_recursion2 and add more Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 16/20] lockdep/selftest: Add more recursive read related test cases Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 17/20] Revert "locking/lockdep/selftests: Fix mixed read-write ABBA tests" Boqun Feng 2018-04-11 13:51 ` [RFC tip/locking/lockdep v6 18/20] MAINTAINERS: Add myself as a LOCKING PRIMITIVES reviewer Boqun Feng 2018-04-11 13:56 ` [RFC tip/locking/lockdep v6 19/20] rcu: Equip sleepable RCU with lockdep dependency graph checks Boqun Feng 2018-04-11 18:57 ` Paul E. McKenney 2018-04-12 2:12 ` Boqun Feng 2018-04-12 9:12 ` Peter Zijlstra 2018-04-13 13:24 ` Boqun Feng 2018-04-11 13:57 ` [RFC tip/locking/lockdep v6 20/20] lockdep/selftest: Add a test case for SRCU Boqun Feng
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