Hi David, thanks for the feedback. I fear you are right, the current code is not safe.
During much of the development I actually used a global lock. But then I thought it would look strange, that more than one CompressionBackend would use the same. But apart from aesthetics, this would pose no actual problem I can think of. And I could move the code in ~ CompressionBackend to a deactivate() method, which would be called at the end of the VM operation. This way the mutex would only used during the actual VM operation, so there should be no problem even in theory. So the easiest way to fix this would be to use a global lock instead, again. What do you think? Best regards, Ralf -----Original Message----- From: David Holmes <david.hol...@oracle.com> Sent: Montag, 10. Februar 2020 23:20 To: Schmelter, Ralf <ralf.schmel...@sap.com>; Yasumasa Suenaga <suen...@oss.nttdata.com>; OpenJDK Serviceability <serviceability-dev@openjdk.java.net> Cc: yasue...@gmail.com Subject: Re: RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump Hi Ralf, One part of this caught my eye and now I look at the webrev I have some concerns. Introducing new threads to the VM is not something that should be done lightly and it has to be done very carefully - I need to look closer at this aspect. Further when using Mutexes/Monitors in such code you have to be extremely careful about how (or even if) those Mutex/Monitor get deleted. The code you have at present is not safe because you cannot know when other threads have completely exited the Monitor/Mutex code. The last thread to terminate will signal the destructing thread (blocked in wait) then release the monitor, allowing the destructing thread to acquire the monitor and then delete the _lock. But at the point at which the monitor becomes free and the destructor thread is unparked, the terminating thread may be context switched out and remain inside the Monitor code. The destructor thread then deletes the monitor and frees it. When the terminating thread resumes, if it touches any memory associated with the Monitor it could SEGV. To safely delete a Monitor/Mutex you have to know for certain that all threads using it have completely ceased to use it. You cannot use that Monitor/Mutex as the means for determining that. It is a non-trivial problem to solve. Cheers, David -----
