On Thu, Jun 30, 2011 at 3:30 AM, Vincent Ficet
> Merlin Moncure wrote:
>> On Wed, Jun 29, 2011 at 10:11 AM, Vincent Ficet
>> <jean-vincent(dot)ficet(at)bull(dot)net> wrote:
>>> I'm having some trouble trying to use postgresql locks. After trying
>>> several options and reading the postgresql online documentation, I still
>>> can't figure out what I'm doing wrong. Here's the use case:
>>> A multithreaded application collecting adapter firmwares on a network
>>> loads data into the following table:
>>> CREATE TABLE firmware (
>>> id SERIAL NOT NULL,
>>> type CHARACTER VARYING(32),
>>> version CHARACTER VARYING(30),
>>> build_id INTEGER,
>>> date CHARACTER VARYING(25),
>>> ps_id CHARACTER VARYING(25)
>>> Typically, there are a few hundred adapters, but only 5 firmwares (many
>>> adapters should have the same firmware if the sysadmins did their jobs
>>> properly ;-) ).
>>> Only a single entry is required per firmware (many separate adapters can
>>> share the same firmware by pointing to the appropriate firmware id field).
>>> To make sure that only one entry is created per firmware, I use the
>>> following trigger:
>>> CREATE TRIGGER firmware_pre_insert_trigger
>>> BEFORE INSERT ON firmware
>>> FOR EACH ROW
>>> EXECUTE PROCEDURE firmware_pre_insert_trigger_cb();
>>> CREATE FUNCTION firmware_pre_insert_trigger_cb() RETURNS TRIGGER
>>> AS $_$
>>> fw_id INT;
>>> SELECT fw.id FROM firmware fw INTO fw_id
>>> WHERE (fw.type = new.type AND
>>> fw.version = new.version AND
>>> fw.build_id = new.build_id AND
>>> fw.date = new.date AND
>>> fw.ps_id = new.ps_id);
>>> IF fw_id IS NULL THEN
>>> -- create the non-existing firmware
>>> RETURN new;
>>> -- skip firmware which already exists
>>> RETURN NULL;
>>> END IF;
>>> LANGUAGE PLPGSQL;
>>> When a thread wishes to add a firmware after discovering one adapter, it
>>> executes the following code:
>>> PERFORM pg_advisory_lock(1);
>>> INSERT INTO firmware (type, version, build_id, date, ps_id)
>>> VALUES (chip_type, firm_version, firm_build_id, firm_date, firm_ps_id);
>>> PERFORM pg_advisory_unlock(1);
>> Advisory lock is not going to work here. You are releasing the lock
>> before the transaction resolves and that leaves a window for second
>> transaction to do the 'select' and not see the data because it hasn't
>> committed yet.
>>> Unfortunately, I still get duplicated entries using advisory locks, and
>>> they don't seem to lock anything at all...
>>> On the other hand, If I use builtin locks as follows without the
>>> trigger, I get deadlocks in the server logs:
>>> LOCK TABLE firmware IN SHARE MODE;
>> well a sharelock certainly isn't going to work. share blocks row
>> exclusive (see http://www.postgresql.org/docs/current/static/explicit-locking.html#LOCKING-TABLES),
>> so two transactions can simultaneously get a share lock and wait for
>> each other to to resolve to get the exclusive lock on a row.
>> 'EXCLUSIVE' would be better (although that would effectively serialize
>> the transactions).
> Thanks for the tip.
> It now works fine using a SHARE UPDATE EXCLUSIVE lock in the PRE INSERT
> trigger. This does not conflict with the ROW EXCLUSIVE lock which is
> implicitely taken by the INSERT statement in add_firmware() function.
> BTW I think there might be a thread safety issue in postgres, as I often
> get a segfault when deadlocks occur (prior to applying the fix I just
> described). For example, the following deadlock situation:
> DETAIL: Process 7643 waits for RowExclusiveLock on relation 21060
> of database 20535; blocked by process 7593.
> Process 7593 waits for RowExclusiveLock on relation 21060 of
> database 20535; blocked by process 7643.
> Process 7643: SELECT
> Process 7593: SELECT
> HINT: See server log for query details.
> QUERY: INSERT INTO firmware (type, version, build_id, date, ps_id)
> VALUES (chip_type, firm_version, firm_build_id,
> firm_date, firm_ps_id)
> CONTEXT: PL/pgSQL function "add_firmware" line 31 at SQL statement
> #0 0x0000003c46725742 in __strncpy_ssse3 () from /lib64/libc.so.6
> #1 0x00007ffff5f1617f in pqParseInput3 (conn=0x7ffff0000da0) at
> #2 0x00007ffff5f0cae7 in parseInput (conn=0x7ffff0000da0) at fe-exec.c:1493
> #3 0x00007ffff5f0cc01 in PQgetResult (conn=0x7ffff0000da0) at
> #4 0x00007ffff5f0d26b in PQexecFinish (conn=0x7ffff0000da0) at
> #5 0x00007ffff5f0cee4 in PQexec (conn=0x7ffff0000da0,
> query=0x7ffff612d340 "SELECT 1") at fe-exec.c:1648
> #6 0x00007ffff612c344 in dbd_ping () from
> Stack traces often occur in different flavours, but can always be
> correlated with messages such as:
> DBI error -9: unexpected field count in "D" message
> DBI error -9: message contents do not agree with length in message type
> "T" server sent data ("D" message) without prior row description ("T"
> I'll try to set up a minimalist reproducer and see what can be done for
> Note that this happens with both postgresql 8.4.7 and 9.0.4.
is the segfault happening in the client? it looks like you have a
thread safety issue either in dbd or in your code...
In response to
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