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    Subject[PATCH 5.1 023/115] fs/ocfs2: fix race in ocfs2_dentry_attach_lock()
    Date
    From: Wengang Wang <wen.gang.wang@oracle.com>

    commit be99ca2716972a712cde46092c54dee5e6192bf8 upstream.

    ocfs2_dentry_attach_lock() can be executed in parallel threads against the
    same dentry. Make that race safe. The race is like this:

    thread A thread B

    (A1) enter ocfs2_dentry_attach_lock,
    seeing dentry->d_fsdata is NULL,
    and no alias found by
    ocfs2_find_local_alias, so kmalloc
    a new ocfs2_dentry_lock structure
    to local variable "dl", dl1

    .....

    (B1) enter ocfs2_dentry_attach_lock,
    seeing dentry->d_fsdata is NULL,
    and no alias found by
    ocfs2_find_local_alias so kmalloc
    a new ocfs2_dentry_lock structure
    to local variable "dl", dl2.

    ......

    (A2) set dentry->d_fsdata with dl1,
    call ocfs2_dentry_lock() and increase
    dl1->dl_lockres.l_ro_holders to 1 on
    success.
    ......

    (B2) set dentry->d_fsdata with dl2
    call ocfs2_dentry_lock() and increase
    dl2->dl_lockres.l_ro_holders to 1 on
    success.

    ......

    (A3) call ocfs2_dentry_unlock()
    and decrease
    dl2->dl_lockres.l_ro_holders to 0
    on success.
    ....

    (B3) call ocfs2_dentry_unlock(),
    decreasing
    dl2->dl_lockres.l_ro_holders, but
    see it's zero now, panic

    Link: http://lkml.kernel.org/r/20190529174636.22364-1-wen.gang.wang@oracle.com
    Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
    Reported-by: Daniel Sobe <daniel.sobe@nxp.com>
    Tested-by: Daniel Sobe <daniel.sobe@nxp.com>
    Reviewed-by: Changwei Ge <gechangwei@live.cn>
    Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
    Cc: Mark Fasheh <mark@fasheh.com>
    Cc: Joel Becker <jlbec@evilplan.org>
    Cc: Junxiao Bi <junxiao.bi@oracle.com>
    Cc: Gang He <ghe@suse.com>
    Cc: Jun Piao <piaojun@huawei.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/ocfs2/dcache.c | 12 ++++++++++++
    1 file changed, 12 insertions(+)

    --- a/fs/ocfs2/dcache.c
    +++ b/fs/ocfs2/dcache.c
    @@ -310,6 +310,18 @@ int ocfs2_dentry_attach_lock(struct dent

    out_attach:
    spin_lock(&dentry_attach_lock);
    + if (unlikely(dentry->d_fsdata && !alias)) {
    + /* d_fsdata is set by a racing thread which is doing
    + * the same thing as this thread is doing. Leave the racing
    + * thread going ahead and we return here.
    + */
    + spin_unlock(&dentry_attach_lock);
    + iput(dl->dl_inode);
    + ocfs2_lock_res_free(&dl->dl_lockres);
    + kfree(dl);
    + return 0;
    + }
    +
    dentry->d_fsdata = dl;
    dl->dl_count++;
    spin_unlock(&dentry_attach_lock);

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