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When LockBufferForCleanup() has to wait for getting a cleanup lock on a buffer it does so by setting a flag in the buffer header and then wait for other backends to signal it using ProcWaitForSignal(). Unfortunately LockBufferForCleanup() missed that ProcWaitForSignal() can return for other reasons than the signal it is hoping for. If such a spurious signal arrives the wait flags on the buffer header will still be set. That then triggers "ERROR: multiple backends attempting to wait for pincount 1". The fix is simple, unset the flag if still set when retrying. That implies an additional spinlock acquisition/release, but that's unlikely to matter given the cost of waiting for a cleanup lock. Alternatively it'd have been possible to move responsibility for maintaining the relevant flag to the waiter all together, but that might have had negative consequences due to possible floods of signals. Besides being more invasive. This looks to be a very longstanding bug. The relevant code in LockBufferForCleanup() hasn't changed materially since its introduction and ProcWaitForSignal() was documented to return for unrelated reasons since 8.2. The master only patch series removing ImmediateInterruptOK made it much easier to hit though, as ProcSendSignal/ProcWaitForSignal now uses a latch shared with other tasks. Per discussion with Kevin Grittner, Tom Lane and me. Backpatch to all supported branches. Discussion: 11553.1423805224@sss.pgh.pa.us |
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.