Broken order-of-operations in parallel query latch manipulation

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Broken order-of-operations in parallel query latch manipulation
Date: 2016-07-31 20:28:31
Message-ID: 1661.1469996911@sss.pgh.pa.us
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Both shm_mq.c and nodeGather.c contain instances of this coding pattern:

WaitLatch(MyLatch, WL_LATCH_SET, 0);
CHECK_FOR_INTERRUPTS();
ResetLatch(MyLatch);

I believe this is wrong and the CHECK_FOR_INTERRUPTS needs to be before
or after the two latch operations. As-is, if the reason somebody set
our latch was to get us to notice that a CHECK_FOR_INTERRUPTS condition
happened, there's a race condition where we'd fail to realize that.
Other places such as ProcWaitForSignal() do it that way; only recently
introduced (and unproven in the field) code has it like this.

Anyone want to argue it's okay as-is?

regards, tom lane

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