From: Boqun Feng <email@example.com> To: Randy Dunlap <firstname.lastname@example.org> Cc: email@example.com, Peter Zijlstra <firstname.lastname@example.org>, Ingo Molnar <email@example.com>, Andrea Parri <firstname.lastname@example.org>, "Paul E. McKenney" <email@example.com>, Jonathan Corbet <firstname.lastname@example.org>, "open list:DOCUMENTATION" <email@example.com> Subject: Re: [RFC tip/locking/lockdep v6 01/20] lockdep/Documention: Recursive read lock detection reasoning Date: Mon, 16 Apr 2018 14:29:56 +0800 [thread overview] Message-ID: <20180416062956.fwoz5snuwcbwaueh@tardis> (raw) In-Reply-To: <firstname.lastname@example.org> [-- Attachment #1: Type: text/plain, Size: 9317 bytes --] On Sat, Apr 14, 2018 at 05:38:54PM -0700, Randy Dunlap wrote: > Hi, > Hello Randy, > Just a few typos etc. below... > Thanks! I fixed those typos according to your comments. > On 04/11/2018 06:50 AM, Boqun Feng wrote: > > Signed-off-by: Boqun Feng <email@example.com> > > --- > > 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 > > ?? circle > > > +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 > > ?? circle > > > +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: > > circle: > > > + > > + 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 > > prove > > > +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 > > > I would also change all /can not/ to /cannot/... Agreed. I will use 'cannot' for any future version, thanks a lot! Regards, Boqun > > -- > ~Randy [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 488 bytes --]
next prev parent reply other threads:[~2018-04-16 6:25 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 ` [RFC tip/locking/lockdep v6 01/20] lockdep/Documention: Recursive read lock detection reasoning Boqun Feng 2018-04-15 0:38 ` Randy Dunlap 2018-04-16 6:29 ` Boqun Feng [this message] 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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