Date: Mon, 03 Apr 2006 15:57:43 -0400 From: Tom Lane <tgl@sss.pgh.pa.us> To: Stephen Frost <sfrost@snowman.net> Cc: freebsd-stable@FreeBSD.org, "Marc G. Fournier" <scrappy@postgresql.org>, pgsql-hackers@postgresql.org, Robert Watson <rwatson@FreeBSD.org>, Kris Kennaway <kris@obsecurity.org> Subject: Re: [HACKERS] semaphore usage "port based"? Message-ID: <16158.1144094263@sss.pgh.pa.us> In-Reply-To: <20060403194251.GF4474@ns.snowman.net> References: <26796.1144028094@sss.pgh.pa.us> <20060402225204.U947@ganymede.hub.org> <26985.1144029657@sss.pgh.pa.us> <20060402231232.C947@ganymede.hub.org> <27148.1144030940@sss.pgh.pa.us> <20060402232832.M947@ganymede.hub.org> <20060402234459.Y947@ganymede.hub.org> <27417.1144033691@sss.pgh.pa.us> <20060403164139.D36756@fledge.watson.org> <14654.1144082224@sss.pgh.pa.us> <20060403194251.GF4474@ns.snowman.net>
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Stephen Frost <sfrost@snowman.net> writes: > Could this be handled sensibly by using SEM_UNDO? Just a thought. Interesting thought, but I think it doesn't work for the special case where the semaphore's "previous owner" is actually our same PID --- which is actually the more commonly exercised path, since true postmaster crashes are pretty rare. More commonly we're trying to detach from and recreate our own shmem and semas following a backend crash. We can special-case that pretty easily with the GETPID solution (pid == ours is obviously not some other process's sema), but with SEM_UNDO it wouldn't work right. I'm also concerned about the portability risks of depending on SEM_UNDO. I think a lot of systems set the semaphore-undo limits pretty small, maybe even zero. BTW, as long as we're annoying the freebsd-stable list with discussions of workarounds, I'm wondering whether our shared memory code might have similar risks. Does FBSD 6 also lie about the existence of other-jail processes connected to a shared memory segment --- ie, in shmctl(IPC_STAT)'s result, does shm_nattch count only processes in our own jail? regards, tom lane
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