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I somehow had assumed that in the spinlock (in turn possibly using semaphores) based fallback atomics implementation 32 bit writes could be done without a lock. As far as the write goes that's correct, since postgres supports only platforms with single-copy atomicity for aligned 32bit writes. But writing without holding the spinlock breaks read-modify-write operations like pg_atomic_compare_exchange_u32(), since they'll potentially "miss" a concurrent write, which can't happen in actual hardware implementations. In 9.6+ when using the fallback atomics implementation this could lead to buffer header locks not being properly marked as released, and potentially some related state corruption. I don't see a related danger in 9.5 (earliest release with the API), because pg_atomic_write_u32() wasn't used in a concurrent manner there. The state variable of local buffers, before this change, were manipulated using pg_atomic_write_u32(), to avoid unnecessary synchronization overhead. As that'd not be the case anymore, introduce and use pg_atomic_unlocked_write_u32(), which does not correctly interact with RMW operations. This bug only caused issues when postgres is compiled on platforms without atomics support (i.e. no common new platform), or when compiled with --disable-atomics, which explains why this wasn't noticed in testing. Reported-By: Tom Lane Discussion: <14947.1475690465@sss.pgh.pa.us> Backpatch: 9.5-, where the atomic operations API was introduced. |
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