Hi Jeremy,
Okay I have read:
https://sourceware.org/glibc/wiki/TLSandSignals
and the tree of mail postings referenced therefrom - great reading! :)
So basic problem: access to __thread variables is not async-signal-safe
Exacerbator to problem: first attempt to even read a __thread variable
can lead to allocation which is the real problem in relation to
async-signal-safety
I mention the exacerbator because pthread_getspecific and
pthread_setSpecific are also not async-signal-safe but we already use
them. However, pthread_getspecific is in fact (per email threads linked
above) effectively async-signal-safe, and further a call to
pthread_getspecific never results in a call to pthread_setspecific or an
allocation. Hence the pthread functions are almost, if not completely,
safe in practice with reasonable uses (ie only read from signal
handler). Which explain this code in existing Thread::current()
#ifdef PARANOID
// Signal handler should call ThreadLocalStorage::get_thread_slow()
Thread* t = ThreadLocalStorage::get_thread_slow();
assert(t != NULL && !t->is_inside_signal_handler(),
"Don't use Thread::current() inside signal handler");
#endif
So problem scenario is: use of __thread variable (that belongs to the
shared-library) in a signal handler.
Solution 0: don't do that. Seriously - like any other
async-signal-unsafe stuff we should not be using it in real signal
handlers. The crash handler is a different matter - we try all sorts
there because it might work and you can't die twice.
Otherwise: narrow the window of exposure.
1. We ensure we initialize thread_current (even with a dummy value) as
early as possible in the thread that loads libjvm. As we have no signal
handlers installed at that point that might use the same variable, we
can not hit the problem scenario.
2. We ensure we initialize thread_current in a new thread with all
signals blocked. This again avoids the problem scenario.
3. We initialize thread_current in an attaching thread as soon as
possible and we again first block all signals.
That still leaves the problem of an unattached native thread taking a
signal whilst in async-signal-unsafe code, and executing a signal
handler which in turns tries to access thread_current for the first
time. This signal handler need not be an actual JVM handler, but one
attached by other native code eg an agent. I'm not clear in the latter
case how reasonable it is for an agent's handler to try and do things
from an unattached thread - and we don't claim any JNI interfaces can,
or should, be called from a signal handler - but it is something you can
probably get away with today.
Let me also point out that we already effectively have this code in
Solaris already (at the ThreadLocalStorage class level). So if there is
something here that will prevent the current proposal we already have a
problem on Solaris. :(
Thoughts/comments/suggestions?
Thanks,
David
On 6/11/2015 1:09 PM, David Holmes wrote:
Hi Jeremy,
I was going to ask you to elaborate :)
On 6/11/2015 12:24 PM, Jeremy Manson wrote:
I should probably elaborate on this. With glibc + ELF, the first time a
thread accesses a variable declared __thread, if that variable is in a
shared library (as opposed to the main executable), the system calls
malloc() to allocate the space for it. If that happens in a signal that
is being delivered during a call to malloc(), then you usually get a
crash.
My understanding of the ELF ABI for thread-locals - which I read about
in the Solaris 11.1 Linkers and libraries guide - does require use of
the dynamic TLS model for any dynamically loaded shared object which
defines a thread-local, but that is what we use as I understand it. The
docs state:
"A shared object containing only dynamic TLS can be loaded following
process startup without limitations. The runtime linker extends the list
of initialization records to include the initialization template of the
new object. The new object is given an index of m = M + 1. The
counter M is incremented by 1. However, the allocation of new TLS blocks
is deferred until the blocks are actually referenced."
Now I guess "extends the list" might be implemented using malloc ... but
this will only occur in the main thread (the one started by the launcher
to load the JVM and become the main thread), at the time libjvm is
loaded - which will all be over before any agent etc can run and do
anything. But "allocation ... is deferred" suggests we may have a
problem until either the first call to Thread::current or the call to
Thread::initialize_thread_current. If it is the former then that should
occur well before any agent etc can be loaded. And I can easily inject
an initial dummy call to initialize_thread_current(null) to force the
TLS allocation.
This may bite you if AsyncGetCallTrace uses Thread::current(), and you
use system timers to do profiling. If a thread doing a malloc() prior
to the first time it accesses Thread::current(), and it gets delivered a
signal, it might die. This is especially likely for pure native threads
started by native code.
I believe that this is a use case you support, so you might want to make
sure it is okay.
For a VM embedded in a process, which already contains native threads,
that will later attach to the VM, this may indeed be a problem. One
would have hoped however that the implementation of TLS would be
completely robust, at least for something as simple as getting a signal
whilst in the allocator.
I'm unclear how to test for or check for this kind of problem. Arguably
there could be many things that are async-unsafe in this way.
Need to think more about this and do some more research. Would also
appreciate any insight from any glibc and/or ELF gurus.
Thanks.
David
Jeremy
On Thu, Nov 5, 2015 at 5:58 PM, Jeremy Manson <jeremyman...@google.com
<mailto:jeremyman...@google.com>> wrote:
Something that's bitten me with __thread: it isn't async-safe when
called from a shared object on Linux. Have you vetted to make sure
this doesn't make HS less async-safe?
Jeremy
On Sun, Nov 1, 2015 at 10:40 PM, David Holmes
<david.hol...@oracle.com <mailto:david.hol...@oracle.com>> wrote:
bug: https://bugs.openjdk.java.net/browse/JDK-8132510
Open webrev:
http://cr.openjdk.java.net/~dholmes/8132510/webrev.v2/
A simple (in principle) but wide-ranging change which should
appeal to our Code Deletion Engineer's. We implement
Thread::current() using a compiler/language-based thread-local
variable eg:
static __thread Thread *_thr_current;
inline Thread* Thread::current() {
return _thr_current;
}
with an appropriate setter of course. By doing this we can
completely remove the platform-specific ThreadLocalStorage
implementations, and the associated os::thread_local_storage*
calls, plus all the uses of ThreadLocalStorage::thread() and
ThreadLocalStorage::get_thread_slow(). This extends the previous
work done on Solaris to implement ThreadLocalStorage::thread()
using compiler-based thread-locals.
We can also consolidate nearly all the os_cpu versions of
MacroAssembler::get_thread on x86 into one cpu specific one ( a
special variant is still needed for 32-bit Windows).
As a result of this change we have further potential cleanups:
- all the src/os/<os>/vm/thread_<os>.inline.hpp files are now
completely empty and could also be removed
- the MINIMIZE_RAM_USAGE define (which avoids use of the linux
sp-map "cache" on 32-bit) now has no affect and so could be
completely removed from the build system
I plan to do the MINIMIZE_RAM_USAGE removal as a follow up CR,
but could add the removal of the "inline" files to this CR if
people think it worth removing them.
I have one missing piece on Aarch64 - I need to change
MacroAssembler::get_thread to simply call Thread::current() as
on other platforms, but I don't know how to write that. I would
appreciate it if someone could give me the right code for that.
I would also appreciate comments/testing by the AIX and PPC64
folk as well.
A concern about memory-leaks had previously been raised, but
experiments using simple C code on linux 86 and Solaris showed
no issues. Also note that Aarch64 already uses this kind of
thread-local.
Thanks,
David
