linux-kernel - Re: ipc,sem: sysv semaphore scalability

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Message-Id: <20130416163041.dfa3e82bfa6b10ae8ffe7608@linux-foundation.org>
Date:	Tue, 16 Apr 2013 16:30:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Emmanuel Benisty <benisty.e@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	hhuang@...hat.com, "Low, Jason" <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Larry Woodman <lwoodman@...hat.com>,
	"Vinod, Chegu" <chegu_vinod@...com>
Subject: Re: ipc,sem: sysv semaphore scalability

On Tue, 26 Mar 2013 11:35:33 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:

> Do we need the locking at all?  What does it actually do?
> 
> 			sem_lock_and_putref(sma);
> 			if (sma->sem_perm.deleted) {
> 				sem_unlock(sma, -1);
> 				err = -EIDRM;
> 				goto out_free;
> 			}
> 			sem_unlock(sma, -1);
> 
> We're taking the lock, testing an int and then dropping the lock. 
> What's the point in that?

Rikpoke.

The new semctl_main() is now taking a lock, testing
sma->sem_perm.deleted then dropping that lock.  It looks wrong.  What
is that lock testing against?  What prevents .deleted from changing
value 1ns after we dropped that lock?

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