linux-kernel - Re: [PATCH] ipc/sem.c: prevent ENOMEM in semop() w/ SEM

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Date:	Mon, 06 Aug 2012 08:16:34 +0900
From:	Seiichi Ikarashi <s.ikarashi@...fujitsu.com>
To:	Manfred Spraul <manfred@...orfullife.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipc/sem.c: prevent ENOMEM in semop() w/ SEM_UNDO flag

Hi Manfred,

(2012-08-04 02:39), Manfred Spraul wrote:
> Hi Seiichi,
> 
> On 08/03/2012 02:49 PM, Seiichi Ikarashi wrote:
>> semop() with SEM_UNDO sem_flg can result in ENOMEM even after
>> succeeding semget() with large nsems.
> How large is nsems, what is the use case?
> Which kind of operations are performed?
> Only simple semop(,,1) calls?

A real case was as follows.
  semget(IPC_PRIVATE, 70000, IPC_CREAT | IPC_EXCL);
  sops[0].sem_num = 0;
  sops[0].sem_op  = 1;
  sops[0].sem_flg = SEM_UNDO;
  semop(semid, sops, 1);

> 
> <linux/sem.h> still documents ~8000 as the upper limit, I'm not sure if

Ah, I did not know it.

#define SEMMSL  250             /* <= 8 000 max num of semaphores per id */

Thanks,
Seiichi

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