From: | Noah Misch <noah(at)leadboat(dot)com> |
---|---|
To: | David Johnston <polobo(at)yahoo(dot)com> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Weaker shmem interlock w/o postmaster.pid |
Date: | 2013-09-12 03:25:20 |
Message-ID: | 20130912032520.GA261339@tornado.leadboat.com |
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Thread: | |
Lists: | pgsql-hackers |
On Wed, Sep 11, 2013 at 07:57:22PM -0700, David Johnston wrote:
> Noah Misch-2 wrote
> > Making PGSharedMemoryCreate() pickier in all branches will greatly
> > diminish
> > the marginal value of preserving postmaster.pid, so I'm fine with dropping
> > the
> > postmaster.pid side of the proposal.
>
> Its probably still worth a fresh look at the immediate shutdown process to
> see whether the current location where postmaster.pid is removed is
> acceptable. It may not be necessary to leave it in place always but:
>
> 1) if there is a section of shared memory that can only be reached/found if
> one knows the pid, and
Similar: one needs a sysv shared memory key to find the segment, and
postmaster.pid records that key. The chosen key is almost always the same
from run to run, so a new postmaster typically does find the old segment even
without postmaster.pid.
> 2) postmaster.pid is removed before that area is "secured from future
> clobbering"
Clobbering shared memory is not the actual threat here. We use the shared
memory segment as a witness to the fact that PostgreSQL processes are active
in a data directory. If we let children of different postmasters operate in
the same directory simultaneously, they can corrupt data.
> then there may be a risk that can still be mitigated by moving its removal
> without having to go to the extreme.
--
Noah Misch
EnterpriseDB http://www.enterprisedb.com
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