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Commit30d8bad

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Be more careful about barriers when releasing BackgroundWorkerSlots.
ForgetBackgroundWorker lacked any memory barrier at all, whileBackgroundWorkerStateChange had one but unaccountably didadditional manipulation of the slot after the barrier. AFAICS,the rule must be that the barrier is immediately before settingor clearing slot->in_use.It looks like back in 9.6 when ForgetBackgroundWorker was firstwritten, there might have been some case for not needing abarrier there, but I'm not very convinced of that --- the factthat the load of bgw_notify_pid is in the caller doesn't seemto guarantee no memory ordering problem. So patch 9.6 too.It's likely that this doesn't fix any observable bug on Intelhardware, but machines with weaker memory ordering rules couldhave problems here.Discussion:https://postgr.es/m/4046084.1620244003@sss.pgh.pa.us
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‎src/backend/postmaster/bgworker.c

Lines changed: 12 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -327,9 +327,11 @@ BackgroundWorkerStateChange(bool allow_new_workers)
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notify_pid=slot->worker.bgw_notify_pid;
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if ((slot->worker.bgw_flags&BGWORKER_CLASS_PARALLEL)!=0)
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BackgroundWorkerData->parallel_terminate_count++;
330-
pg_memory_barrier();
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slot->pid=0;
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pg_memory_barrier();
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slot->in_use= false;
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if (notify_pid!=0)
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kill(notify_pid,SIGUSR1);
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@@ -416,6 +418,8 @@ BackgroundWorkerStateChange(bool allow_new_workers)
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* points to it. This convention allows deletion of workers during
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* searches of the worker list, and saves having to search the list again.
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*
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* Caller is responsible for notifying bgw_notify_pid, if appropriate.
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*
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* This function must be invoked only in the postmaster.
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*/
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void
@@ -428,9 +432,16 @@ ForgetBackgroundWorker(slist_mutable_iter *cur)
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Assert(rw->rw_shmem_slot<max_worker_processes);
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slot=&BackgroundWorkerData->slot[rw->rw_shmem_slot];
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Assert(slot->in_use);
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/*
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* We need a memory barrier here to make sure that the update of
439+
* parallel_terminate_count completes before the store to in_use.
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*/
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if ((rw->rw_worker.bgw_flags&BGWORKER_CLASS_PARALLEL)!=0)
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BackgroundWorkerData->parallel_terminate_count++;
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444+
pg_memory_barrier();
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slot->in_use= false;
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ereport(DEBUG1,

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