Erik,
Thanks for chiming in on this thread...
On 7/7/19 4:48 AM, Erik Osterlund wrote:
Yeah that switch statement code and yet another plain non-volatile load/store
loop looks like complete nonsense unfortunately. It should at least use
Atomic::load/store.
Fortunately, on x86_64, I believe it will in practice yield word atomic copying
anyway by chance. But it should be fixed anyway. *sigh*
The real danger is SPARC though and its BIS instructions. I don’t have the code
in front of me, but I really hope not to see that switch statement and
non-volatile loop in that pd_disjoint_words_atomic() function.
And I agree that the atomic copying API should be used when we need atomic
copying. And if it turns out the implementation of that API is not atomic, it
should be fixed in that atomic copying API.
Okay, so I think we're on the same page w.r.t. this fix (8227338).
David, do you concur that this fix can move forward?
So I think this change looks good.
Thanks!
But it looks like we are not done yet. :c
So we need another bug:
JDK-8227369 pd_disjoint_words_atomic() needs to be atomic
https://bugs.openjdk.java.net/browse/JDK-8227369
However, I'll leave that for you (or someone else) to take.
Thanks for the review.
Dan
Thanks,
/Erik
On 7 Jul 2019, at 02:46, Daniel D. Daugherty <[email protected]>
wrote:
Added Erik O. to the "To:" list...
On 7/6/19 8:05 PM, Daniel D. Daugherty wrote:
On 7/6/19 6:06 PM, David Holmes wrote:
Hi Dan,
On 6/07/2019 11:53 pm, Daniel D. Daugherty wrote:
Greetings,
During the code review for the following fix:
JDK-8227117 normal interpreter table is not restored after single
stepping with TLH
https://bugs.openjdk.java.net/browse/JDK-8227117
Erik O. noticed a potential race with templateInterpreter.cpp: copy_table()
depending on C++ compiler optimizations. The following bug is being used
to fix this issue:
JDK-8227338 templateInterpreter.cpp: copy_table() needs to be safer
https://bugs.openjdk.java.net/browse/JDK-8227338
Here's the webrev URL:
http://cr.openjdk.java.net/~dcubed/8227338-webrev/0_for_jdk14/
So the original code uses a loop to copy, while the new code calls
Copy::disjoint_words_atomic, but the implementation of that on x64 is just a
loop same as the original AFAICS:
Yup. I figure Erik O. will jump in here with his reasoning... :-)
Thinking about it more... I think the answer is that we are switching to
calling code that specifies the type of behavior that we need:
Copy::disjoint_words_atomic()
is what we need when we're not at a safepoint. If, down the road, we find
that the compiler does something with pd_disjoint_words_atomic() that
breaks our expectation for pd_disjoint_words_atomic(), then we fix that
version of pd_disjoint_words_atomic() and all the callers will be good
again... Or something like that...
The version in src/hotspot/os_cpu/solaris_x86/copy_solaris_x86.inline.hpp
happens to be exactly our loop with no switch statement... which is
particularly funny given Erik's observations about what at least one
Solaris X64 compiler did to loops...
Still, I'm just guessing here on a Saturday night... hopefully Erik
will chime in here...
Dan
Dan
static void pd_disjoint_words_atomic(const HeapWord* from, HeapWord* to, size_t
count) {
#ifdef AMD64
switch (count) {
case 8: to[7] = from[7];
case 7: to[6] = from[6];
case 6: to[5] = from[5];
case 5: to[4] = from[4];
case 4: to[3] = from[3];
case 3: to[2] = from[2];
case 2: to[1] = from[1];
case 1: to[0] = from[0];
case 0: break;
default:
while (count-- > 0) {
*to++ = *from++;
}
break;
}
#else
David
-----
This fix has been tested via Mach5 Tier[1-3] on Oracle's usual platforms.
Mach5 tier[4-6] is running now. It has also been tested with the manual
jdb test from JDK-8227117 using 'release' and 'fastdebug' bits.
Thanks, in advance, for questions, comments or suggestions.
Dan
