RFR: 8244416: Remove incorrect assert during inline cache cleaning

[Vladimir Kozlov]

On 5/15/20 1:10 AM, Erik Österlund wrote:
> Hi Vladimir,
> 
> On 2020-05-15 01:20, Vladimir Kozlov wrote:
>> Do we need to clean up IC in zombie nmethod 'from' here? Sweeper will do that later anyway as you said.
> 
> You are 100% right. We do not need to clean up IC in zombie nmethods. In fact we try
> to actively avoid it. The concurrent GC thread only initiates IC cleaning on is_alive()
> nmethods. The issue is that from checking if it is_alive() and then starting the IC cleaning,
> the state might flip over to zombie in a race with make_zombie from the sweeper. That is
> fine as I described, because the make_zombie() function will in this race wait for the GC to
> finish and then clear the ICs, releasing spurious IC stubs created by the GC mistakenly thinking
> it is_alive(), before IC finalization can run in a subsequent safepoint. The only issue is that we
> can't assert that from must not be zombie. It almost never is, but it is sometimes, and
> that is fine.

So you are saying that changing assert to from->is_zombie() check (to skip cleaning) will not help but only reduce 
concurrency window?

Vladimir

> 
> Thanks,
> /Erik
> 
>> Thanks,
>> Vladimir
>>
>> On 5/14/20 6:07 AM, Erik Österlund wrote:
>>> Hi,
>>>
>>> Months of stress testing triggered the following assert in inline cache cleaning:
>>>
>>> assert(!from->is_zombie(), "should not clean inline caches on zombies");
>>>
>>> It used to be assert(!from->is_alive()), but after finding a race that resulted in
>>> false positives for unloaded nmethods, the assert was relaxed to check only that
>>> we shouldn't have zombies here, with a fat comment explaining why.
>>>
>>> Unfortunately, the relaxed assert also gets false positives, so I have chosen to
>>> remove it, and further increase the size of that fat comment, explaning why we can't
>>> assert that it isn't a zombie either.
>>>
>>> What it boils down to is a race where inline cache cleaning is performed from
>>> a concurrent GC thread, concurrent to the sweeper making an nmethod zombie.
>>>
>>> The order of the race ingredients making the nmethod zombie from the sweeper is
>>> the following:
>>>
>>> 1. Transition state to zombie
>>> 2. Unregister nmethod form GC (importantly triggers a wait for concurrent code cache unloading to finish)
>>> 3. clear_ic_callsites() importantly releasing all IC stubs >
>>> The race happens when the GC finds an is_alive && !is_unloading nmethod and
>>> triggers inline cache cleaning on it before 1, but then when racingly checking
>>> again in the assert, we are past 1.
>>>
>>> Since the assert will spuriously fail even though there is no actual problem, and
>>> we are at a point where there isn't much else it could assert, I chose to remove it.
>>>
>>> Other than triggering the assert, it is worth noting that this racingly zombiefied
>>> nmethod will have its inline caches cleaned *with* IC stubs from the concurrent
>>> GC thread. Subsequent IC finalization in safepoint cleanup requires live IC stubs
>>> to belong to is_alive() nmethods, so the IC stubs must be released before the next
>>> safepoint for this race to not be dangerous. That is ensured, because the racing
>>> sweeper will get stuck waiting for concurrent code cache unloading to finish,
>>> and when it wakes up, it will clear_ic_callsites(), releasing the spurious ICStubs,
>>> before finishing the make_zombie() function. Then it will eventually hit a safepoint
>>> poll, when the nmethod has been purged from IC stubs.
>>>
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8244416
>>>
>>> Webrev:
>>> http://cr.openjdk.java.net/~eosterlund/8244416/webrev.00/
>>>
>>> Thanks,
>>> /Erik
>