Details
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Type:
Bug
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Status: Resolved
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Priority:
Major - P3
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Resolution: Fixed
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Affects Version/s: WT2.8.0
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Fix Version/s: WT2.9.0
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Labels:None
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# Replies:9
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Last comment by Customer:true
Description
While testing MongoDB on PowerPC, we found a situation where reads were not repeatable in a transaction running at snapshot isolation. In particular, with multiple threads inserting and deleting records, we saw:
begin transaction with isolation=snapshot
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search for ID=X, found
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...
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search for ID=X, not found
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Issue Links
- is depended on by
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SERVER-23517
WiredTiger changes for MongoDB 3.3.5
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- Closed
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SERVER-23682
WiredTiger changes for MongoDB 3.2.6
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- Closed
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Activity
Author:
{u'username': u'michaelcahill', u'name': u'Michael Cahill', u'email': u'michael.cahill@mongodb.com'}Message: WT-2534 Use atomic add when allocating transaction IDs.
Branch: develop
https://github.com/wiredtiger/wiredtiger/commit/c0446cf3b7a6fa0e596f103458e53ef7302b17d5
Author:
{u'username': u'agorrod', u'name': u'Alex Gorrod', u'email': u'alexander.gorrod@mongodb.com'}Message: Merge pull request #2637 from wiredtiger/wt-2534
WT-2534 Use atomic add when allocating transaction IDs.
Branch: develop
https://github.com/wiredtiger/wiredtiger/commit/5535876ce00b27135a728dee4b5171ab0732481b
Author:
{u'username': u'michaelcahill', u'name': u'Michael Cahill', u'email': u'michael.cahill@mongodb.com'}Message: WT-2534 Use atomic add when allocating transaction IDs.
Branch: mongodb-3.4
https://github.com/wiredtiger/wiredtiger/commit/c0446cf3b7a6fa0e596f103458e53ef7302b17d5
Author:
{u'username': u'agorrod', u'name': u'Alex Gorrod', u'email': u'alexander.gorrod@mongodb.com'}Message: Merge pull request #2637 from wiredtiger/wt-2534
WT-2534 Use atomic add when allocating transaction IDs.
Branch: mongodb-3.4
https://github.com/wiredtiger/wiredtiger/commit/5535876ce00b27135a728dee4b5171ab0732481b
Author:
{u'username': u'michaelcahill', u'name': u'Michael Cahill', u'email': u'michael.cahill@mongodb.com'}Message: Import wiredtiger-wiredtiger-2.8.0-137-g94c171d.tar.gz from wiredtiger branch mongodb-3.4
ref: 5047aab..94c171d
WT-2534 Invalid transaction snapshots on PowerPC
SERVER-23517 WiredTiger changes for MongoDB 3.3.5
Branch: master
https://github.com/mongodb/mongo/commit/281700981dca1a43e4fc32364951b09c562f1720
Author:
{u'username': u'michaelcahill', u'name': u'Michael Cahill', u'email': u'michael.cahill@mongodb.com'}Message: WT-2534 Use atomic add when allocating transaction IDs.
Branch: mongodb-3.2
https://github.com/wiredtiger/wiredtiger/commit/c0446cf3b7a6fa0e596f103458e53ef7302b17d5
Author:
{u'username': u'agorrod', u'name': u'Alex Gorrod', u'email': u'alexander.gorrod@mongodb.com'}Message: Merge pull request #2637 from wiredtiger/wt-2534
WT-2534 Use atomic add when allocating transaction IDs.
Branch: mongodb-3.2
https://github.com/wiredtiger/wiredtiger/commit/5535876ce00b27135a728dee4b5171ab0732481b
Author:
{u'name': u'Ramon Fernandez', u'email': u'ramon@mongodb.com'}Message: Import wiredtiger-wiredtiger-2.8.0-201-g7ea2631.tar.gz from wiredtiger branch mongodb-3.2
ref: 43e885a..7ea2631
SERVER-23504 Coverity analysis defect 98177: Resource leak
SERVER-23526 Replication relies on storage engines reporting a non-zero size for correctness
SERVER-23588 mongod with WiredTiger won't start on Windows when built with --dbg=on --opt=off
SERVER-23682 WiredTiger changes for MongoDB 3.2.6
WT-2330 in-memory configurations should not create on-disk collection files
WT-2507 Add upgrading documentation in preparation for 2.8 release.
WT-2512 wtperf: MSVC complains about float conversion in throttle code
WT-2513 conversion from 'int64_t' to 'uint32_t'
WT-2517 wtperf uses setvbuf in a way that isn't supported on Windows
WT-2522 Incorrect format code in message
WT-2525 in-memory configurations: miscellaneous cleanups
WT-2527 OS X compile error, missing POSIX_FADV_WILLNEED #define
WT-2528 style error in WiredTiger build
WT-2529 The readonly test case is crashing with a stack overflow
WT-2531 in-memory tables are allocating unnecessary memory
WT-2532 WT_STREAM_APPEND and WT_STREAM_LINE_BUFFER flag overlap
WT-2533 Ensure that in-memory tables don't report a zero size
WT-2534 Invalid transaction snapshots on PowerPC
Branch: v3.2
https://github.com/mongodb/mongo/commit/7ee4e4e493c3785fea489ee3508ca18526709c16
The underlying issue here was assumptions in WiredTiger about atomic loads and stores of 64-bit values.
In particular, we maintain a "current transaction ID" in the WT_TXN_GLOBAL struct, usually referred to as txn_global->current. When establishing a snapshot, we read that value into a local variable and rely on getting an atomic read. The local copy is then used to establish the snapshot. It is critical for correctness that allocated snapshot IDs are visible in the global transaction state table before and thread sees txn_global->current incremented. Otherwise, the result is an invalid transaction snapshot where values are visible before their transaction commits.
We update txn_global->currrent when a new transaction ID is allocated, and that is done inside a spinlock to guarantee uniqueness. The ID is published with a write barrier before executing ++txn_global->current, then the spinlock is released.
On x86, this is sufficient, and readers only see the increment after the ID is published. On PowerPC, we have seen a variety of results where reading txn_global->current results in unexpected values and thus invalid snapshots. Replacing the ordinary increment with an atomic_fetch_and_add eliminates these errors in testing.