linux-kernel - Re: Questions about the SYSVIPC share memory on NOMMU uClinuxarchitecture

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Date:	Mon, 05 Mar 2007 09:40:01 +0800
From:	"Wu, Bryan" <bryan.wu@...log.com>
To:	bryan.wu@...log.com, ebiederm@...ssion.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Questions about the SYSVIPC share memory on NOMMU
	uClinuxarchitecture

On Fri, 2007-03-02 at 05:33 -0500, Wu, Bryan wrote:
> Hi folks,
> 
> Recently, I was struggling in a bug about the shm->nattch. Actually,
> the 
> test case is from LTP kernel/syscall/ipc/shmctl/shmctl01.c code. We 
> ported it to the uClinux-blackfin platform.
> 

Sorry for dropping the kernel version inforamtion, I found this in
2.6.19 kernel and 2.6.20-mm2 kernel.

> The algorithm is very simple.  
> a) the parent process will create a share memory  
> b) parent will vfork/execlp 4 children process 
> c) children will call shmat() attach to the share memory (shm->nattch 
> should be increased), then children will pause() 
> d) parent call shmclt() to get the share memory nattch, if nattch !=
> 4, 
> then the testcase will fail.
> 
> In our uClinux-blackfin platform, nattch = 1.
> 
> So I dig into the source code ipc/shm.c, then there are some
> questions 
> about the code.
> 
> a) 
> in function do_shmat(), after nattch++ why nattch-- as following: 
> ================================================================================ 
>  user_addr = (void*) do_mmap (file, addr, size, prot, flags, 0);
> 
>         // here no return or goto valid place.
> 
> invalid: 
>         up_write(&current->mm->mmap_sem);
> 
>         mutex_lock(&shm_ids(ns).mutex); 
>         shp = shm_lock(ns, shmid); 
>         BUG_ON(!shp); 
>         shp->shm_nattch--; /* Why??? */ 
>         if(shp->shm_nattch == 0 && 
>            shp->shm_perm.mode & SHM_DEST) 
>                 shm_destroy(ns, shp); 
>         else 
>                 shm_unlock(shp); 
>         mutex_unlock(&shm_ids(ns).mutex);
> 
>         *raddr = (unsigned long) user_addr; 
>         err = 0; 
>         if (IS_ERR(user_addr)) 
>                 err = PTR_ERR(user_addr); 
> out: 
>         return err; 
> ================================================================================
> 
> b) do_mmap() -> mm/nommu.c do_mmap_pgoff() 
> When create a new vma structure, shm_open(), shm_inc() will be called.
> Then nattch++. 
> So the nattch counting is disordered.
> 
> Please give me some hint about it. Actually, this case can pass on X86
> platform
> 

Is there any help available? 
Thanks a lot
-Bryan
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