| From: | Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com> |
|---|---|
| To: | Amul Sul <sulamul(at)gmail(dot)com> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Prabhat Sahu <prabhat(dot)sahu(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com> |
| Subject: | Re: [Patch] ALTER SYSTEM READ ONLY |
| Date: | 2021-09-10 16:20:38 |
| Message-ID: | 764084C9-4FA4-4730-A42B-CC0E0EFCF3B0@enterprisedb.com |
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| Lists: | pgsql-hackers |
> On Sep 10, 2021, at 8:42 AM, Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com> wrote:
>
> Take for example a code stanza from heapam.c:
>
> if (needwal)
> CheckWALPermitted();
>
> /* NO EREPORT(ERROR) from here till changes are logged */
> START_CRIT_SECTION();
>
> Now, I know that interrupts won't be processed after starting the critical section, but I can see plain as day that an interrupt might get processed *during* CheckWALPermitted, since that function isn't atomic.
A better example may be found in ginmetapage.c:
needwal = RelationNeedsWAL(indexrel);
if (needwal)
{
CheckWALPermitted();
computeLeafRecompressWALData(leaf);
}
/* Apply changes to page */
START_CRIT_SECTION();
Even if CheckWALPermitted is assumed to be close enough to atomic to not be a problem (I don't agree), that argument can't be made here, as computeLeafRecompressWALData is not trivial and signals could easily be processed while it is running.
—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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