Re: Remove header lock BufferGetLSNAtomic() on architectures with 64 bit atomic operations

On 11/24/25 12:10 AM, Andreas Karlsson wrote:
> Andres pointed out this possible optimization on Discord so I hacked up 
> a quick patch which avoids taking a lock when reading the LSN from a 
> page on architectures where we can be sure to not get a torn value. It 
> is always nice to remove a lock from a reasonably hot code path.
> 
> I thought about using our functions for atomics but did not do so since 
> I did not want to introduce any extra overhead on platforms which do not 
> support 64-bit atomic operations.
> 
> I decided to just remove the struct to make the code simpler and more 
> consistent but I can also see an argument for keeping it to get some 
> degree of type safety.

Noticed that the pageinspect tests in the CI did not like that I 
increased the alignment requirement of PageHeaderData from 4 to 8 bytes 
presumably due to a bytea's data being 4 byte aligned. I will need to 
think how to best fix this. Maybe copying the struct is fine in the the 
pageinspect code because being able to check for this alignment seems 
like a nice property for code which is not in pageinspect.

It gives us the following backtrace:

../src/include/storage/bufpage.h:397:16: runtime error: member access 
within misaligned address 0x55d6721e69bc for type 'const struct 
PageHeaderData', which requires 8 byte alignment
0x55d6721e69bc: note: pointer points here
   10 80 00 00 00 00 00 00  00 00 00 00 00 00 04 00  1c 00 e0 1f 00 20 
04 20  00 00 00 00 e0 9f 40 00
               ^
     #0 0x7fb39deb9b07 in PageGetMaxOffsetNumber 
../src/include/storage/bufpage.h:397
     #1 0x7fb39debac3e in heap_page_items 
../contrib/pageinspect/heapfuncs.c:168
     #2 0x55d63ff127c7 in ExecMakeTableFunctionResult 
../src/backend/executor/execSRF.c:234
     #3 0x55d63ff42aa7 in FunctionNext 
../src/backend/executor/nodeFunctionscan.c:94
     #4 0x55d63ff14157 in ExecScanFetch 
../src/include/executor/execScan.h:134
     #5 0x55d63ff14157 in ExecScanExtended 
../src/include/executor/execScan.h:195
     #6 0x55d63ff14157 in ExecScan ../src/backend/executor/execScan.c:59
     #7 0x55d63ff427b6 in ExecFunctionScan 
../src/backend/executor/nodeFunctionscan.c:269
     #8 0x55d63ff0c38c in ExecProcNodeFirst 
../src/backend/executor/execProcnode.c:469
     #9 0x55d63fef5cee in ExecProcNode 
../src/include/executor/executor.h:319
     #10 0x55d63fef6465 in ExecutePlan 
../src/backend/executor/execMain.c:1707
     #11 0x55d63fef7c00 in standard_ExecutorRun 
../src/backend/executor/execMain.c:366
     #12 0x55d63fef7c60 in ExecutorRun 
../src/backend/executor/execMain.c:303
     #13 0x55d640322295 in PortalRunSelect ../src/backend/tcop/pquery.c:916
     #14 0x55d64032534e in PortalRun ../src/backend/tcop/pquery.c:760
     #15 0x55d64031e34a in exec_simple_query 
../src/backend/tcop/postgres.c:1279
     #16 0x55d6403215ce in PostgresMain ../src/backend/tcop/postgres.c:4775
     #17 0x55d640316c6c in BackendMain 
../src/backend/tcop/backend_startup.c:124
     #18 0x55d6401b2711 in postmaster_child_launch 
../src/backend/postmaster/launch_backend.c:268
     #19 0x55d6401b7b19 in BackendStartup 
../src/backend/postmaster/postmaster.c:3598
     #20 0x55d6401ba061 in ServerLoop 
../src/backend/postmaster/postmaster.c:1713
     #21 0x55d6401bb972 in PostmasterMain 
../src/backend/postmaster/postmaster.c:1403
     #22 0x55d63ffde07b in main ../src/backend/main/main.c:231
     #23 0x7fb39e033ca7  (/lib/x86_64-linux-gnu/libc.so.6+0x29ca7) 
(BuildId: def5460e3cee00bfee25b429c97bcc4853e5b3a8)
     #24 0x7fb39e033d64 in __libc_start_main 
(/lib/x86_64-linux-gnu/libc.so.6+0x29d64) (BuildId: 
def5460e3cee00bfee25b429c97bcc4853e5b3a8)
     #25 0x55d63fb70260 in _start 
(/tmp/cirrus-ci-build/build/tmp_install/usr/local/pgsql/bin/postgres+0x8b0260) 
(BuildId: d2e585ffc320b6de4bfce11bb801bb923e7d3484)

Links

- https://cirrus-ci.com/task/5353983260229632
- 

https://api.cirrus-ci.com/v1/artifact/task/5353983260229632/testrun/build/testrun/pageinspect/regress/log/postmaster.log

Andreas