RFR(S): 8170548: VM may crash at startup because StdoutLog/StderrLog logging stream can be badly aligned

[Volker Simonis]

On Thu, Dec 1, 2016 at 2:23 PM, David Holmes <david.holmes at oracle.com> wrote:
> Hi Volker,
>
>
> On 1/12/2016 8:35 PM, Volker Simonis wrote:
>>
>> Hi,
>>
>> can I please have a review and sponsor for the following fix:
>>
>> http://cr.openjdk.java.net/~simonis/webrevs/2016/8170548/
>> https://bugs.openjdk.java.net/browse/JDK-8170548
>>
>> Change "8146009: "pure virtual method called" with using new GC
>> logging mechanism" introduced a sophisticated initialization mechanism
>> for the logging stream. In order to avoid deconstruction of the
>> streams before the VM exits, it creates them with a placement new into
>> statically allocated memory:
>>
>> static bool initialized;
>> static char stdoutmem[sizeof(LogStdoutOutput)];
>> static char stderrmem[sizeof(LogStderrOutput)];
>>
>> LogStdoutOutput &StdoutLog =
>> reinterpret_cast<LogStdoutOutput&>(stdoutmem);
>> LogStderrOutput &StderrLog =
>> reinterpret_cast<LogStderrOutput&>(stderrmem);
>>
>> LogFileStreamInitializer::LogFileStreamInitializer() {
>>   if (!initialized) {
>>     ::new (&StdoutLog) LogStdoutOutput();
>>     ::new (&StderrLog) LogStderrOutput();
>>     initialized = true;
>>   }
>> }
>>
>> Unfortunately it is not guaranteed, that the static memory (which is a
>> char array) is well-aligned for the stream objects. Actually, the C++
>> standard only defines that it has to be at least 'char' aligned which
>> is obviously not enough for a stream object.
>> When building 'slowdebug' on Solaris with SS12u4 we indeed observed
>> reproducible crashes during VM initialization because of this issue.
>>
>> The fix is easy - just wrap the character arrays into unions to align
>> them appropriately.
>
>
> Seems reasonable - though I don't know the C++ alignment rules for static
> unions to know whether this is guaranteed to be correct, or just very likely
> to be correct.
>

The rule is that the union is aligned such that all of its members are
correctly aligned. With the fix, the union should be correctly aligned
for jlong which should be 'good enough' for
LogStdoutOutput/LogStderrOutput.

> I'm also wondering if anyone knows exactly why only the Solaris slowdebug
> build detected the problem?
>

Probably because the Sun Studio compiler takes -O0 very seriously and
places the character arrays at the next available address while other
compilers still aligned at least 'good enough' for the object
placement.

> Thanks,
> David
>
>
>> Thank you and best regards,
>> Volker
>>
>