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    SubjectRe: Questions about the SYSVIPC share memory on NOMMU uClinuxarchitecture
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    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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