Re: How to shoot yourself in the foot: kill -9 postmaster

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alfred Perlstein <bright(at)wintelcom(dot)net>
Cc: Lamar Owen <lamar(dot)owen(at)wgcr(dot)org>, Hiroshi Inoue <Inoue(at)tpf(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: How to shoot yourself in the foot: kill -9 postmaster
Date: 2001-03-06 19:24:54
Message-ID: 6920.983906694@sss.pgh.pa.us
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Alfred Perlstein <bright(at)wintelcom(dot)net> writes:
> * Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> [010306 11:03] wrote:
>> I notice that our BeOS and QNX emulations of shmctl() don't support
>> IPC_STAT, but that could be dealt with, at least to the extent of
>> stubbing it out.

> Well since we already have spinlocks, I can't see why we can't
> keep the refcount and spinlock in a special place in the shm
> for all cases?

No, we mustn't go there. If the kernel isn't keeping the refcount
then it's worse than useless: as soon as some process crashes without
decrementing its refcount, you have a condition that you can't recover
from without reboot.

What I'm currently imagining is that the stub implementations will just
return a failure code for IPC_STAT, and the outer code will in turn fail
with a message along the lines of "It looks like there's a pre-existing
shmem block (id XXX) still in use. If you're sure there are no old
backends still running, remove the shmem block with ipcrm(1), or just
delete $PGDATA/postmaster.pid." I dunno what shmem management tools
exist on BeOS/QNX, but deleting the lockfile will definitely suppress
the startup interlock ;-).

> Yes, if possible a more meaningfull error message and pointer to
> some docco would be nice

Is the above good enough?

regards, tom lane

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