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    SubjectRe: [Patch] cpusets: fix race in cpuset_add_file()
    On Thu, 16 Sep 2004, Paul Jackson wrote:

    > Color me confused - this cpuset_sem down/up should not be needed,
    > and should deadlock. In the call chain:
    >
    > cpuset_mkdir -> cpuset_create -> cpuset_populate_dir -> cpuset_add_file
    >
    > cpuset_create() already holds the cpuset_sem for the duration, and you're
    > adding another cpuset_sem down in cpuset_add_file(), which should deadlock.
    >
    > If you are seeing the duplicate invalid cpuset entries, then must be
    > something else going on, unfortunately.
    >
    > That, or I'm confused.
    Neither.
    You've just read the patch too quickly:

    + down(&dir->d_inode->i_sem);

    NOT down(&cpuset_sem);

    However, your remark is welcome, since there is indeed a slight chance of
    deadlock with my patch, but it needs 2 mkdirs racing.

    imagine:

    mkdir a/b mkdir a/b/c

    sys_mkdir(): down(a->i_sem);
    cpuset_create(): down(cpuset_sem);
    sys_mkdir(): down(b->i_sem);
    cpuset_add_file(): down(b->i_sem);
    cpuset_create(): down(cpuset_sem);


    -> deadlock.


    So we should release cpuset_sem a bit earlier in cpuset_create(), before
    calling cpuset_populate_dir().





    This updated patch should be better (hopefully).

    Signed-off-by: Simon Derr <simon.derr@bull.net>

    Index: mm4/kernel/cpuset.c
    ===================================================================
    --- mm4.orig/kernel/cpuset.c 2004-09-13 09:43:02.000000000 +0200
    +++ mm4/kernel/cpuset.c 2004-09-16 17:23:48.764321923 +0200
    @@ -956,13 +956,12 @@ static int cpuset_create_dir(struct cpus
    return error;
    }

    -/* MUST be called with dir->d_inode->i_sem held */
    -
    static int cpuset_add_file(struct dentry *dir, const struct cftype *cft)
    {
    struct dentry *dentry;
    int error;

    + down(&dir->d_inode->i_sem);
    dentry = cpuset_get_dentry(dir, cft->name);
    if (!IS_ERR(dentry)) {
    error = cpuset_create_file(dentry, 0644 | S_IFREG);
    @@ -971,6 +970,7 @@ static int cpuset_add_file(struct dentry
    dput(dentry);
    } else
    error = PTR_ERR(dentry);
    + up(&dir->d_inode->i_sem);
    return error;
    }

    @@ -1162,7 +1162,6 @@ static struct cftype cft_notify_on_relea
    .private = FILE_NOTIFY_ON_RELEASE,
    };

    -/* MUST be called with ->d_inode->i_sem held */
    static int cpuset_populate_dir(struct dentry *cs_dentry)
    {
    int err;
    @@ -1219,9 +1218,16 @@ static long cpuset_create(struct cpuset
    err = cpuset_create_dir(cs, name, mode);
    if (err < 0)
    goto err;
    +
    + /* release cpuset_sem before cpuset_populate_dir()
    + * because it will down() this new directory's i_sem
    + * and if we race with another mkdir,
    + * we might deadlock
    + */
    + up(&cpuset_sem);
    +
    err = cpuset_populate_dir(cs->dentry);
    /* If err < 0, we have a half-filled directory - oh well ;) */
    - up(&cpuset_sem);
    return 0;
    err:
    list_del(&cs->sibling);
    -
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