RFR (M) 8207359: Make SymbolTable increment_refcount disallow zero
coleen.phillimore at oracle.com
coleen.phillimore at oracle.com
Thu Jul 19 12:34:56 UTC 2018
Please review the revision to this change. Summary:
* made decrement_refcount() use CAS loop.
* fixed duplicated logic in try_increment_refcount() thanks to Kim
* added gtest case for decrement_refcount.
* fixed SA code.
* added a bunch of comments
open webrev at http://cr.openjdk.java.net/~coleenp/8207359.02/webrev
Retested with hs-tier1-3.
Thanks,
Coleen
On 7/18/18 10:50 PM, coleen.phillimore at oracle.com wrote:
>
>
> On 7/18/18 6:35 PM, Ioi Lam wrote:
>>
>>
>> On 7/18/18 2:45 PM, coleen.phillimore at oracle.com wrote:
>>>
>>>
>>> On 7/18/18 5:14 PM, Ioi Lam wrote:
>>>> Hi Coleen,
>>>>
>>>> The changes look good! The new operations on _length_and_refcount
>>>> are much cleaner than my old ATOMIC_SHORT_PAIR hack.
>>>
>>> Yes, this makes more sense to me.
>>>>
>>>> symbolTable.cpp:
>>>>
>>>> SymbolTable::lookup_dynamic() {
>>>> ...
>>>> 214 Symbol* sym = e->literal();
>>>> 215 if (sym->equals(name, len) &&
>>>> sym->try_increment_refcount()) {
>>>> 216 // something is referencing this symbol now.
>>>> 217 return sym;
>>>> 218 }
>>>>
>>>>
>>>> symbol.cpp:
>>>>
>>>> 221 void Symbol::increment_refcount() {
>>>> 222 if (refcount() != PERM_REFCOUNT) { // not a permanent symbol
>>>> 223 if (!try_increment_refcount()) {
>>>> 224 #ifdef ASSERT
>>>> 225 print();
>>>> 226 #endif
>>>> 227 fatal("refcount has gone to zero");
>>>> 228 }
>>>> 229 NOT_PRODUCT(Atomic::inc(&_total_count);)
>>>> 230 }
>>>> 231 }
>>>>
>>>> 246 // Atomically increment while checking for zero, zero is bad.
>>>> 247 bool Symbol::try_increment_refcount() {
>>>> 248 uint32_t old_value = _length_and_refcount; // fetch once
>>>> 249 int refc = extract_refcount(old_value);
>>>> 250
>>>> 251 if (refc == PERM_REFCOUNT) {
>>>> 252 return true;
>>>> 253 } else if (refc == 0) {
>>>> 254 return false; // effectively dead, can't revive
>>>> 255 }
>>>> 256
>>>> 257 uint32_t now;
>>>> 258 while ((now = Atomic::cmpxchg(old_value + 1,
>>>> &_length_and_refcount, old_value)) != old_value) {
>>>> 259 // failed to increment, check refcount again.
>>>> 260 refc = extract_refcount(now);
>>>> 261 if (refc == 0) {
>>>> 262 return false; // just died
>>>> 263 } else if (refc == PERM_REFCOUNT) {
>>>> 264 return true; // just became permanent
>>>> 265 }
>>>> 266 old_value = now; // refcount changed, try again
>>>> 267 }
>>>> 268 return true;
>>>> 269 }
>>>>
>>>>
>>>> So is it valid for Symbol::try_increment_refcount() to return
>>>> false? SymbolTable::lookup_dynamic() seems to suggest YES, but
>>>> Symbol::increment_refcount() seems to suggest NO.
>>>
>>> True. If you are looking up a symbol and someone other thread has
>>> decremented the refcount to zero, this symbol should not be
>>> returned. My test exercises this code even without the concurrent
>>> hashtable. When the hashtable is concurrent, a zero-ed Symbol could
>>> be deallocated so we don't want to return it.
>>>
>> I think the following should be added as a comment in
>> increment_refcount().
>>> In the case where you call increment_refcount() not during lookup,
>>> it is assumed that you have a symbol with a non-zero refcount and it
>>> can't go away while you are holding it.
>
> Ok, added.
>>
>>>>
>>>> If it's always an invalid condition, I think the fatal() should be
>>>> moved inside try_increment_refcount.
>>>>
>>>
>>> It isn't fatal at lookup. The lookup must skip a zero-ed entry.
>>>> Otherwise, I think you need to add comments in all 3 places, to say
>>>> when it's possible to get a 0 refcount, and when it's not. And, it
>>>> might be worth expanding on why "zero is bad" :-)
>>>
>>> How about this comment to try_increment_refcount:
>>>
>>> // Increment refcount while checking for zero. If the Symbol's
>>> refcount becomes zero
>>> // a thread could be concurrently removing the Symbol. This is used
>>> during SymbolTable
>>> // lookup to avoid reviving a dead Symbol.
>> Sounds good.
>
> Thanks, Ioi.
> Coleen
>>
>> Thanks
>> - Ioi
>>
>>>>
>>>> My guess is:
>>>> + if you're doing a lookup, you might be seeing Symbols that have
>>>> already been marked for deletion, which is indicated by a 0
>>>> refcount. You want to skip such Symbols.
>>>>
>>>> + if you're incrementing the refcount, that means you're holding a
>>>> valid Symbol, which means this Symbol should have never been marked
>>>> for deletion.
>>>>
>>>> Is this correct?
>>>
>>> Yes, both true.
>>>
>>> Thanks,
>>> Coleen
>>>>
>>>> Thanks
>>>> - Ioi
>>>>
>>>>
>>>> On 7/17/18 2:08 PM, coleen.phillimore at oracle.com wrote:
>>>>>
>>>>> Gerard, thank you for the code review.
>>>>>
>>>>> On 7/17/18 4:13 PM, Gerard Ziemski wrote:
>>>>>> Thank you Coleen (and Kim)!
>>>>>>
>>>>>> #1 Need copyright year updates:
>>>>>>
>>>>>> src/hotspot/share/oops/symbol.cpp
>>>>>> src/hotspot/share/classfile/symbolTable.cpp
>>>>>> src/hotspot/share/classfile/compactHashtable.inline.hpp
>>>>>
>>>>> Yes, I'll update with my commit.
>>>>>>
>>>>>> #2 What’s the purpose of this code in
>>>>>> src/hotspot/share/oops/symbol.cpp
>>>>>>
>>>>>> 38 STATIC_ASSERT(max_symbol_length == ((1 << 16) - 1));
>>>>>>
>>>>>> when we have:
>>>>>>
>>>>>> 117 enum {
>>>>>> 118 // max_symbol_length is constrained by type of _length
>>>>>> 119 max_symbol_length = (1 << 16) -1
>>>>>> 120 };
>>>>>>
>>>>>> Wouldn’t that always be true? Is it to make sure that nobody
>>>>>> changes max_symbol_length, because the implementation needs it to
>>>>>> be that? If so, should we add comment to:
>>>>>>
>>>>>> 119 max_symbol_length = (1 << 16) -1
>>>>>>
>>>>>> with a big warning of some sorts?
>>>>>
>>>>> Yes, it's so that we can store the length of the symbol into 16 bits.
>>>>>
>>>>> How I change the comment above max_symbol_length from:
>>>>>
>>>>> // max_symbol_length is constrained by type of _length
>>>>>
>>>>> to
>>>>>
>>>>> // max_symbol_length must fit into the top 16 bits of
>>>>> _length_and_refcount
>>>>>
>>>>>>
>>>>>> #3 If we have:
>>>>>>
>>>>>> 39 STATIC_ASSERT(PERM_REFCOUNT == ((1 << 16) - 1));
>>>>>>
>>>>>> then why not
>>>>>>
>>>>>> 101 #define PERM_REFCOUNT ((1 << 16) - 1)) // 0xffff
>>>>>>
>>>>>> or
>>>>>> 39 STATIC_ASSERT(PERM_REFCOUNT == 0xffff;
>>>>>> 101 #define PERM_REFCOUNT 0xffff
>>>>>>
>>>>> I can change PERM_REFCOUNT to ((1 << 16)) -1) to be consistent.
>>>>>
>>>>>> #4 We have:
>>>>>>
>>>>>> 221 void Symbol::increment_refcount() {
>>>>>> 222 if (refcount() != PERM_REFCOUNT) { // not a permanent symbol
>>>>>> 223 if (!try_increment_refcount()) {
>>>>>> 224 #ifdef ASSERT
>>>>>> 225 print();
>>>>>> 226 #endif
>>>>>> 227 fatal("refcount has gone to zero");
>>>>>>
>>>>>> but
>>>>>>
>>>>>> 233 void Symbol::decrement_refcount() {
>>>>>> 234 if (refcount() != PERM_REFCOUNT) { // not a permanent symbol
>>>>>> 235 int new_value = Atomic::sub((uint32_t)1,
>>>>>> &_length_and_refcount);
>>>>>> 236 #ifdef ASSERT
>>>>>> 237 // Check if we have transitioned to 0xffff
>>>>>> 238 if (extract_refcount(new_value) == PERM_REFCOUNT) {
>>>>>> 239 print();
>>>>>> 240 fatal("refcount underflow");
>>>>>> 241 }
>>>>>> 242 #endif
>>>>>>
>>>>>> Where the line:
>>>>>>
>>>>>> 240 fatal("refcount underflow”);
>>>>>>
>>>>>> is inside #ifdef ASSERT, but:
>>>>>>
>>>>>> 227 fatal("refcount has gone to zero”);
>>>>>>
>>>>>> is outside. Shouldn't “fatal" be consistent in both?
>>>>>>
>>>>>
>>>>> I was thought that looked strange too. I'll move the #endif from
>>>>> 226 to after 227.
>>>>>
>>>>> Thank you for reviewing the code!
>>>>> Coleen
>>>>>
>>>>>> cheers
>>>>>>
>>>>>>
>>>>>>> On Jul 17, 2018, at 10:51 AM, coleen.phillimore at oracle.com wrote:
>>>>>>>
>>>>>>> Summary: Use cmpxchg for non permanent symbol refcounting, and
>>>>>>> pack refcount and length into an int.
>>>>>>>
>>>>>>> This is a precurser change to the concurrent SymbolTable change.
>>>>>>> Zeroed refcounted entries can be deleted at anytime so they
>>>>>>> cannot be allowed to be zero in runtime code. Thanks to Kim for
>>>>>>> writing the packing function and helping me avoid undefined
>>>>>>> behavior.
>>>>>>>
>>>>>>> open webrev at
>>>>>>> http://cr.openjdk.java.net/~coleenp/8207359.01/webrev
>>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8207359
>>>>>>>
>>>>>>> Tested with solaris ptrace helper, mach5 tier1-5 including
>>>>>>> solaris. Added multithreaded gtest which exercises the code.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Coleen
>>>>>
>>>>
>>>
>>
>
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