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

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Date:	Tue, 26 Mar 2013 10:59:27 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Emmanuel Benisty <benisty.e@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.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 Mon, 2013-03-25 at 20:47 +0700, Emmanuel Benisty wrote:
> On Mon, Mar 25, 2013 at 12:10 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> 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@...com>
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@...com>
---
 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);
-- 
1.7.11.7



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