RFR: 8214302: Allow safely calling is_unloading() on zombie nmethods
erik.osterlund at oracle.com
Fri Nov 30 11:19:52 UTC 2018
On 2018-11-27 01:40, Vladimir Kozlov wrote:
> Hi Erik,
> Can you tell if there is any concurrency window where you check
> is_zombie and store new unloading state - can other thread change
> nmethod to zombie?
Good question. Short version is: no.
Here is the longer version to explain why.
There are 2 modes of importance. Let's start with the first one - STW
unloading. With STW unloading, all is_alive && is_unloading() nmethods
are unlinked and unloaded in the safepoint. That makes races where an
nmethod is first observed as is_alive() && is_unloading() and
subsequently observed as is_zombie() impossible.
The more tricky case is with concurrent unloading. The way I have
structured this code is to unlink all references to is_unloading()
nmethods (IC caches and dependency contexts) when the mark end safepoint
is released, and then perform a global handshake operation with all
JavaThreads before unloading them. The sweeper never converts is_alive
&& is_unloading() nmethods to zombies; it waits for them to become
is_unloaded(). So before the global handshake, it is impossible for
is_unloading() nmethods to racingly become is_zombie(). And
is_unloading() is calculated for all is_alive() nmethods before taking
that global handshake, meaning that it will never be recalculated after
After that global handshake, is_unloading() nmethods are only observable
to the iterators, and they will never trigger recomputation of the
cached is_unloading_state, and hence may not suffer from such races.
> I also noticed that CodeCache::unloading_cycle() is called twice in this
> code. Can we cache it in local?
> On 11/26/18 7:30 AM, Erik Österlund wrote:
>> It is currently not safe to call is_unloading on zombie nmethods,
>> unless it has been observed to be alive. It should be supported to
>> make the code less fragile. When encountering a !is_alive() nmethod
>> that has not had its unloading epoch updated, and ask if it
>> is_unloading(), the answer is always false. So by adding that,
>> is_unloading() can always be safely called.
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