RFR(s): 8212173: Thread._stack_base/_stack_size initialized too late for new threads

Thomas Stüfe thomas.stuefe at gmail.com
Mon Oct 22 16:56:15 UTC 2018

Dear all,

may I have please reviews for the following fix:

Bug: https://bugs.openjdk.java.net/browse/JDK-8212173
cr: http://cr.openjdk.java.net/~stuefe/webrevs/8212173-thread-stack-and-base-initialized-too-late/webrev.02/webrev/

See also prior mail discussions: [1]

Wwhen iterating the thread list, threads may be visible which have
stack base/size not yet set. Calling stack_base() will yield NULL,
stack_size() 0 or it will assert. The reason for this is that there
exist a race: Thread::_stack_base and ::_stack_size are initialized
too late, when the Thread has already been added to the threads list.

My solution to this problem is basically to move the initialization of
Thread::_stack_base and ::_stack_size to an earlier point at which the
thread has not yet been added to the threads list.

Unfortunately, this is more difficult that it sounds. Retrieving
thread stack base and size needs to be done from within the thread in
question, and only on Linux and BSD we have the opportunity to execute
code in the new child thread before its Thread object is added to the
thread list. So, my patch solves the problem completely only on Linux
and BSD. On Windows, Aix and Solaris it only minimizes the chance for
it to happen.

For more details, please see bug description and the discussion in [1].


My changes in detail:

- Thread::record_stack_base_and_size(): I removed all code which needs
the surrounding Thread object being initialized, or which needs
Thread::current() to be set. That makes the function safe to be called
from the very start of the child thread's life (start of
thread_native_entry()), where nothing is initialized yet.

- I moved the call to Thread::record_stack_base_and_size() out of all
<ChildThreadClass>::run() functions into the very start of the
thread_native_entry() on every platform.

- The parts I moved out of Thread::record_stack_base_and_size() are:
NMT thread stack initialization and Logging. I needed to find a new
place for them. I wanted them to run at the end of the child thread
initialization, when the child Thread is already initialized and
Thread::current is initialized too. So I moved them to just before
<ChildThreadCLass>::run() is called.

-  To prevent code duplication, I changed:

public virtual Thread::run()


public (nonvirtual) Thread::call_run();
protected virtual Thread::run() = 0;

Thread::call_run() now is the place to put common,
platform-independent and <ChildThreadClass>-independent
initializations. There I put the NMT stuff and logging. After
<ChildThreadClass>::run() returns, I put common cleanup checks.

Other changes:

- We had a function os::initialize_thread() which only existed for the
sole benefit of Solaris, where if gives the platform a stab at
correcting the stack base, size for a certain corner case when we run
on primordial thread. On all other platform-cpu-combinations, this
function was empty. I removed all empty implementations and renamed
the function to the much more fitting, Solaris only

- When <ChildClassThread>::run() returns, the Thread object may have
been deleted by the child class run() function. To avoid errors I set
the thread pointer to zero (where possible) to prevent accessing it to
reference members.

- Since Thread::record_stack_base_and_size() does not register the
thread stack with NMT anymore, I needed to fix those places where the
former is called for attached threads and add that NMT registration

- I made Thread::clear_thread_current() a static function since it
needs no members of Thread (unlike Thread::initialize_thread_current()
which needs the this pointer to hook it into TLS). That makes it
possible to use clear_thread_current() without having a valid Thread
object, which is the case if <ChildClassThread>::run() deleted the


That's about it.

The change ran through jdk-submit tests without errors.

Thank you for reviewing.


[1] http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2018-October/030516.html

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