Re: Non-reproducible AIO failure

On 13/06/2025 11:20 pm, Andres Freund wrote:
> Attached is a patch that fixes the problem for me. Alexander, Konstantin,
> could you verify that it also fixes the problem for you?
>
> Given that it does address the problem for me, I'm inclined to push this
> fairly soon, the barrier is pretty obviously required.
>
Unfortunately I still able to reproduce assertion failure after 17450 
seconds (`--enable-debug --enable-cassert CFLAGS=-O0`):


   * frame #0: 0x0000000187248704 libsystem_kernel.dylib`__pthread_kill + 8
     frame #1: 0x000000018727fc28 libsystem_pthread.dylib`pthread_kill + 288
     frame #2: 0x000000018718dae8 libsystem_c.dylib`abort + 180
     frame #3: 0x00000001011a0fbc 
postgres`ExceptionalCondition(conditionName="ioh->op == 
PGAIO_OP_INVALID", fileName="aio_io.c", lineNumber=161) at assert.c:66:2
     frame #4: 0x0000000100ea9b40 
postgres`pgaio_io_before_start(ioh=0x000000010a6e22f0) at aio_io.c:161:2
     frame #5: 0x0000000100ea9a00 
postgres`pgaio_io_start_readv(ioh=0x000000010a6e22f0, fd=12, iovcnt=1, 
offset=85565440) at aio_io.c:81:2
     frame #6: 0x0000000100ec8798 
postgres`FileStartReadV(ioh=0x000000010a6e22f0, file=4, iovcnt=1, 
offset=85565440, wait_event_info=167772181) at fd.c:2241:2
     frame #7: 0x0000000100f1b7d0 
postgres`mdstartreadv(ioh=0x000000010a6e22f0, reln=0x000000010b0289c8, 
forknum=MAIN_FORKNUM, blocknum=10445, buffers=0x000000016f5ed998, 
nblocks=1) at md.c:1019:8
     frame #8: 0x0000000100f1ed88 
postgres`smgrstartreadv(ioh=0x000000010a6e22f0, reln=0x000000010b0289c8, 
forknum=MAIN_FORKNUM, blocknum=10445, buffers=0x000000016f5ed998, 
nblocks=1) at smgr.c:758:2
     frame #9: 0x0000000100eb1a48 
postgres`AsyncReadBuffers(operation=0x000000011d80d108, 
nblocks_progress=0x000000016f5edeb4) at bufmgr.c:1959:3
     frame #10: 0x0000000100eb0c24 
postgres`StartReadBuffersImpl(operation=0x000000011d80d108, 
buffers=0x000000011d80cdf4, blockNum=10445, nblocks=0x000000016f5edeb4, 
flags=0, allow_forwarding=true) at bufmgr.c:1428:18

I wonder if we also need second write barrier in pgaio_io_update_state:

     /*
      * Ensure the changes signified by the new state are visible before the
      * new state becomes visible.
      */
     pg_write_barrier();

     ioh->state = new_state;

     /*
      * Make sure that status is written
      */
     pg_write_barrier();


Otherwise is is not clear when status is written.
Also I think that replacing bitfields with `uint8` and may be even with 
`int`, is good idea at least to avoids false sharing.