Re: ipc,sem: sysv semaphore scalability

[Andrew Morton]

On Tue, 26 Mar 2013 10:59:27 -0700 Davidlohr Bueso <davidlohr.bueso@xxxxxx> wrote:

> On Mon, 2013-03-25 at 20:47 +0700, Emmanuel Benisty wrote:
> > On Mon, Mar 25, 2013 at 12:10 AM, Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > And you never see this problem without Rik's patches?
> > 
> > No, never.
> > 
> > > Could you bisect
> > > *which* patch it starts with? Are the first four ones ok (the moving
> > > of the locking around, but without the fine-grained ones), for
> > > example?
> > 
> > With the first four patches only, I got some X server freeze (just tried once).
> 
> Going over the code again, I found a potential recursive spinlock scenario. 
> Andrew, if you have no objections, please queue this up.
> 
> Thanks.
> 
> ---8<---
> 
> From: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> Subject: [PATCH] ipc, sem: prevent possible deadlock
> 
> In semctl_main(), when cmd == GETALL, we're locking
> sma->sem_perm.lock (through sem_lock_and_putref), yet
> after the conditional, we lock it again.
> Unlock sma right after exiting the conditional.
> 
> Signed-off-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> ---
>  ipc/sem.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 1a2913d..f257afe 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -1243,6 +1243,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
>  				err = -EIDRM;
>  				goto out_free;
>  			}
> +			sem_unlock(sma, -1);
>  		}
>  
>  		sem_lock(sma, NULL, -1);

Looks right. 

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?

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