RFR: 8014814 (str) StringBuffer "null" is not appended
david.holmes at oracle.com
Mon May 20 12:17:53 UTC 2013
On 20/05/2013 5:44 PM, David Holmes wrote:
> On 20/05/2013 4:25 PM, Martin Buchholz wrote:
>> Note that my pending change
>> does the same kind of thing, but without recursive lock acquisitions.
> I will take a look.
So it looks like I can do my push with nested sync with no concern
because once you do your getChars update the nested sync will be
removed. I like that outcome.
That said I'm concerned by your getChars method but I shall take that up
on another thread.
>> I'm curious why a recursive lock acquisition would be considered "very"
>> cheap. Is there some hotspot magic, or is it simply that we have
>> another write to a cache line that is already probably owned by the cpu
>> by virtue of the previous cas to acquire?
> Yes "hotspot magic". Acquiring a lock you already own doesn't require a
> CAS; and if it is locked via biased-locking then it is an even shorter
>> On Sun, May 19, 2013 at 10:48 PM, David Holmes <david.holmes at oracle.com
>> <mailto:david.holmes at oracle.com>> wrote:
>> The change put through for JDK-8013395 (StringBuffer toString cache)
>> has exposed a previously unnoticed bug in the
>> StringBuffer.append(__CharSequence cs) method. That method claimed
>> to achieve synchronization (and then correct toStringCache
>> behaviour) by the super.append method calling other StringBuffer
>> methods after narrowing of cs to a specific type. But that is
>> incorrect if cs==null as in that case the
>> AbstractStringBuilder.__appendNull method is called directly, with
>> no calls to an overridden StringBuffer method. (I have verified that
>> none of the other methods claiming to not need sync suffer from a
>> similar flaw - this is an isolated case.)
>> Consequently we started failing some existing append(null) tests.
>> The fix is quite simple: append(CharSequence) behaves as for other
>> append methods and is declared synchronized and clears the cache
>> explicitly. The existing test is extended to check append(null).
>> This fix does mean that recursive synchronization will now be used
>> for append(CharSequence) but recursive synchronization is very
>> cheap. An alternative fix suggested by Alan Bateman in the bug
>> report is to override appendNull and add the synchronization there.
>> That requires a change to the accessibility of
>> AbstractStringBuilder.__appendNull so I chose the more constrained
>> fix. Alan's fix will also introduce nested synchronization, but only
>> for the append(null) case. As I said I don't think performance will
>> be a concern here.
>> Testing (in progress): JPRT -testset core, SQE test that caught this
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