[foreign-memaccess] Integrated: 8253025: Add support for registering a segment with a Cleaner

[Maurizio Cimadamore]

On Thu, 10 Sep 2020 17:58:01 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This patch adds an extra API point to register a segment against a cleaner.
> 
> The basic idea is that, when a segment is registered against a cleaner, the segment's scope is register, and the
> scope's cleanup action is passed to the cleaner.
> In practice, things are more subtle than it seems. First, we need to make sure that we cannot have races - which could
> result in cleanup action being called multiple times. The possible races are:
> * close vs. cleaner
> * cleaner vs. cleaner
> 
> We can get rid of the first type of race by adding reachability fences around the scope terminal operations. If we do
> that, we are guaranteed that the Cleaner cannot kick in before we are actually done with the scope.
> Now, even if reachability fences can give us some guarantee that close will never run concurrently with cleaner, we're
> not out of the woods yet. The cleaner can in fact still be called _after_ a segment has been closed explicitly.
> Now, the simplest thing to do on paper would be to check the scope liveness bit, and only call the cleanup action if
> the scope is still alive. But this is not possible, becaue the scope liveness bit is, well, inside the scope and, to
> access it, the Cleaner action must keep a strong reference to the scope being registered (which means the cleaner will
> never work).  To address this, I came up with a level of indirection: I replaced the references to Runnable with a new
> interface, called CleanupAction. This interface has methods to cleanup and to dup onto a new cleanup action, killing
> the previous one.  Now, segment factories create cleanup action, rather than simple Runnable. There is a static "dummy"
> cleanup action, which does nothing; and there is also a more complex and stateful cleanup action, which supports
> atomicity (this cleanup action has a boolean flag which is chcked with CAS).  So, all terminal operation in memory
> scope must either call cleanup() or dup() on the cleanup action, to ensure that the cleanup action attached to the
> scope is no longer usable upon return.  Since the logic inside the stateful cleanup action uses CAS, it is possible for
> multiple threads to race to cleanup the same action, and only one of them is guaranteed to win.  While the logic is
> tricky, and partly replicates the liveness bit in scope, the implementation strategy tries hard not to allocate more
> than previosly, especially when a new segment is created. The only drawback is that we need to create a new cleanup
> action every time we change scope ownership - but these operations are not performance critical, so I assume this is an
> acceptable cost.  Note that, given all our core memory access operation (in the newly define ScopeMemoryAccess) already
> take care of adding the require reachability fence on the scopes being accessed, it is not possible for a cleaner to
> kick in while a segment is being actively accessed.

This pull request has now been integrated.

Changeset: 0b0c1fa8
Author:    Maurizio Cimadamore <mcimadamore at openjdk.org>
URL:       https://git.openjdk.java.net/panama-foreign/commit/0b0c1fa8
Stats:     324 lines in 9 files changed: 4 ins; 291 del; 29 mod

8253025: Add support for registering a segment with a Cleaner

Reviewed-by: jvernee, psandoz

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PR: https://git.openjdk.java.net/panama-foreign/pull/320