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Re: unsafe use of hash_search(... HASH_ENTER ...)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Qingqing Zhou" <zhouqq(at)cs(dot)toronto(dot)edu>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: unsafe use of hash_search(... HASH_ENTER ...)
Date: 2005-05-28 16:46:29
Message-ID: 16797.1117298789@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Qingqing Zhou" <zhouqq(at)cs(dot)toronto(dot)edu> writes:
> Consider the senario like this:

> Backends register some dirty segments in BgWriterShmem->requests; bgwrite
> will AbsorbFsyncRequests() asynchornously but failed to record some one in
> pendingOpsTable due to an "out of memory" error. All dirty segments
> remembered in "requests" after this one will not have chance be absorbed by
> bgwriter.

So really we have to PANIC if we fail to record a dirty segment.  That's
a bit nasty, but since the hashtable is so small (only 16 bytes per
gigabyte-sized dirty segment) it seems unlikely that the situation will
ever occur in practice.

I'll put a critical section around it --- seems the easiest way to
ensure a panic ...

			regards, tom lane

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