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    SubjectRe: What protection does sysfs_readdir have with SMP/Preemption?
    On Wed, Nov 23, 2005 at 09:15:44AM -0500, Steven Rostedt wrote:
    > On Wed, 2005-11-23 at 19:28 +0530, Maneesh Soni wrote:
    >
    > >
    > > hmm looks like we got some situation which is not desirable and could lead
    > > to bogus sysfs_dirent in the parent list. It may not be the exact problem
    > > in this case though, but needs fixing IMO.
    > >
    > > After sysfs_make_dirent(), the ref count for sysfs dirent will be 2.
    > > (one from allocation, and after linking the new dentry to it). On
    > > error from sysfs_create(), we do sysfs_put() once, decrementing the
    > > ref count to 1. And again when the new dentry for which we couldn't
    > > allocate the d_inode, is d_drop()'ed. In sysfs_d_iput() we again
    > > sysfs_put(), and decrement the sysfs dirent's ref count to 0, which will
    > > be the final sysfs_put(), and it will free the sysfs_dirent but never
    > > unlinks it from the parent list. So, parent list could still will having
    > > links to the freed sysfs_dirent in its s_children list.
    > >
    > > so basically list_del_init(&sd->s_sibling) should be done in error path
    > > in create_dir().
    > >
    > > Could you also put the appended patch in your trial runs..
    > >
    >
    > I'm already playing around with this. You might want this patch instead.
    > I noticed that if sysfs_make_dirent fails to allocate the sd, then a
    > null will be passed to sysfs_put.
    >
    > But this is not the end of the problems. I'll follow up on that comment
    > right after this.
    >
    > -- Steve
    >
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    >
    > Index: linux-2.6.15-rc2-git2/fs/sysfs/dir.c
    > ===================================================================
    > --- linux-2.6.15-rc2-git2.orig/fs/sysfs/dir.c 2005-11-23 08:40:33.000000000 -0500
    > +++ linux-2.6.15-rc2-git2/fs/sysfs/dir.c 2005-11-23 08:52:57.000000000 -0500
    > @@ -112,7 +112,11 @@
    > }
    > }
    > if (error && (error != -EEXIST)) {
    > - sysfs_put((*d)->d_fsdata);
    > + struct sysfs_dirent *sd = (*d)->d_fsdata;
    > + if (sd) {
    > + list_del_init(&sd->s_sibling);
    > + sysfs_put(sd);
    > + }
    > d_drop(*d);
    > }
    > dput(*d);
    >
    >

    Agreed. This makes more sense.


    Thanks
    Maneesh


    --
    Maneesh Soni
    Linux Technology Center,
    IBM India Software Labs,
    Bangalore, India
    email: maneesh@in.ibm.com
    Phone: 91-80-25044990
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