On 05/23/2018 06:21 AM, Ben Asher wrote:
Hi there! I believe I've found a SQLite bug that triggers TSAN. I'm hoping
to either confirm this bug or learn what I'm doing wrong. Some background:
in our code, we make sure to synchronize writes to our database such that
only one write can happen at any given time for the same database in our
application (i.e. there's one synchronized writer connection). However, we
allow any number of reads (readers create their own connections, which are
similarly not shared with other threads) to happen on the same database
(with a cap on the number of connections to respect resource limits).
Additionally, the database has PRAGMA journal_mode=WAL; set. At the moment
when TSAN is triggered, there is one reader and one writer each with their
own connection to the same db performing a read and a write:
- Reader is running: sqlite3WalFindFrame, specifically the line with this
code (line 59679 in my sqlite.c):
for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
- Writer is running walIndexAppend, specifically the line with this code
(line 57856 in my sqlite.c):
Thanks for the thread-testing. This one is a known condition. Tsan can't
see it, but we think it is safe. See the comment above the block in
sqlite3WalFindFrame() for details:
http://www.sqlite.org/src/artifact/aa9cffc7a2bad?ln=2859..2883
Basically the race condition is only on the append-only hash-table. But
the hash-table is only used to accelerate lookups on the aPgno[] array.
And since the hash-table is append-only, it can only produce
false-positives (which are filtered out when aPgno[] is inspected).
Dan.
aHash[iKey] = (ht_slot)idx;
I apologize if those line numbers aren't helpful, however I hope the
function names + code are unique enough to locate the lines in question.
This is sqlite3 version 3.23.1.
Please let me know if there's any other information I can provide. Thanks!
Ben
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