Re: [RFC tip/locking/lockdep v5 16/17] lockdep: Documention for recursive read lock detection reasoning

[Andrea Parri <parri.andrea@gmail.com>]

On Thu, Feb 22, 2018 at 03:09:03PM +0800, Boqun Feng wrote:
> As now we support recursive read lock deadlock detection, add related
> explanation in the Documentation/lockdep/lockdep-desgin.txt:
> 
> *	Definition of recursive read locks, non-recursive locks, strong
> 	dependency path and notions of -(**)->.
> 
> *	Lockdep's assumption.
> 
> *	Informal proof of recursive read lock deadlock detection.

Once again... much appreciated!!, thanks for sharing.


> 
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> ---
>  Documentation/locking/lockdep-design.txt | 170 +++++++++++++++++++++++++++++++
>  1 file changed, 170 insertions(+)
> 
> diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
> index 382bc25589c2..fd8a8d97ce58 100644
> --- a/Documentation/locking/lockdep-design.txt
> +++ b/Documentation/locking/lockdep-design.txt
> @@ -284,3 +284,173 @@ 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 Deadlock Detection:
> +----------------------------------
> +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.
> +
> +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 API 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.
> +
> +That said, recursive read locks could introduce deadlocks too, considering the
> +following:
> +
> +	TASK A:			TASK B:
> +
> +	read_lock(X);
> +				read_lock(Y);
> +	write_lock(Y);
> +				write_lock(X);
> +
> +, neither task could get the write locks because the corresponding read locks
> +are held by each other.
> +
> +Lockdep could detect recursive read lock related deadlocks. The dependencies(edges)
> +in the lockdep graph are classified into four categories:
> +
> +1) -(NN)->: non-recursive to non-recursive dependency, non-recursive locks include
> +            non-recursive read locks, write locks and exclusive locks(e.g. spinlock_t).
> +	    They are treated equally in deadlock detection. "X -(NN)-> Y" means
> +            X -> Y and both X and Y are non-recursive locks.
> +
> +2) -(RN)->: recursive to non-recursive dependency, recursive locks means recursive read
> +	    locks. "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.
> +
> +And obviously a non-recursive lock can block the corresponding recursive lock,
> +and vice versa. Besides a non-recursive lock may block the other non-recursive
> +lock of the same instance(e.g. a write lock may block a corresponding
> +non-recursive read lock and vice versa).
> +
> +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*)->. IOW, 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 now prove that if a strong path forms a circle, then we have a potential deadlock.
> +By "forms a circle", it means for a set of locks A0,A1...An, there is a path from
> +A0 to An:
> +
> +	A0 -> A1 -> ... -> An
> +
> +and there is also a dependency An->A0. And as the circle is formed by a strong path,
> +so there are no two conjunct dependency edges as -(*R)-> and -(R*)->.
> +
> +
> +To understand the actual proof, let's look into lockdep's assumption:
> +
> +For each lockdep dependency A -> B, there may exist a case where someone is
> +trying to acquire B with A held, and the existence of such a case is
> +independent to the existences of cases for other lockdep dependencies.
> +
> +For example if we have two functions func1 and func2:
> +
> +	void func1(...) {
> +		lock(A);
> +		lock(B);
> +		unlock(A);
> +		unlock(B);
> +
> +		lock(C);
> +		lock(A);
> +		unlock(A);
> +		unlock(C);
> +	}
> +
> +	void func2(...) {
> +		lock(B);
> +		lock(C);
> +		unlock(C);
> +		unlock(B);
> +	}
> +
> +lockdep will generate dependencies: A->B, B->C and C->A, and assume that:
> +
> +	there may exist a case where someone is trying to acquire B with A held,
> +	there may exist a case where someone is trying to acquire C with B held,
> +	and there may exist a case where someone is trying to acquire A with C held,
> +
> +and those three cases exist *independently*, meaning they can happen at the
> +same time(which requires func1() being called simultaneously by two CPUs or
> +tasks, which may be impossible due to other constraints in the real life)
> +
> +
> +With this assumption, we can prove that if a strong dependency path forms a circle,
> +then it indicates a deadlock as far as lockdep is concerned.

As mentioned in a private communication, I would recommend the adoption of
a "more impersonal" style.  Here, for instance, the expression:

  "indicates a deadlock as far as lockdep is concerned"

would be replaced with:

  "indicates a deadlock as (informally) defined in Sect. ?.?";

similarly (from the proof):

  "For a strong dependency [...], lockdep assumes that [...]"

would be replaced with:

  "For a strong dependency [...], from assumption/notation ?.?,
   we have/we can conclude [...]".

This could mean that additional text/content would need to be added to the
present doc./.txt; OTOH, this could help to make this doc. self-contained/
more accessible to "non-lockdep-experts".

  Andrea


> +
> +For a strong dependency circle like:
> +
> +	A0 -> A1 -> ... -> An
> +	^                  |
> +	|                  |
> +	+------------------+
> +, lockdep assumes that
> +
> +	there may exist a case where someone is trying to acquire A1 with A0 held
> +	there may exist a case where someone is trying to acquire A2 with A1 held
> +	...
> +	there may exist a case where someone is trying to acquire An with An-1 held
> +	there may exist a case where someone is trying to acquire A0 with An held
> +
> +, and because it's a *strong* dependency circle for every Ai (0<=i<=n), Ai is either
> +held as a non-recursive lock or someone is trying to acquire Ai as a non-recursive lock,
> +which gives:
> +
> +*	If Ai is held as a non-recursive lock, then trying to acquire it,
> +	whether as a recursive lock or not, will get blocked.
> +
> +*	If Ai is held as a recursive lock, then there must be someone is trying to acquire
> +	it as a non-recursive lock, and because recursive locks blocks non-recursive locks,
> +	then it(the "someone") will get blocked.
> +
> +So all the holders of A0, A1...An are blocked with A0, A1...An held by each other,
> +no one can progress, therefore deadlock.
> -- 
> 2.16.1
>