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    SubjectRe: [PATCH RFC 1/1] kernfs: keep kernfs node alive for __kernfs_remove()
    Hello,

    On Wed, Apr 17, 2019 at 04:12:29PM +0000, Konstantin Khorenko wrote:
    > i don't know the full scenario unfortunately, but the idea is the following:
    >
    > __kernfs_remove() is called under kernfs_mutex and if
    > !(!kn || (kn->parent && RB_EMPTY_NODE(&kn->rb)))
    >
    > it assumes that nothing can change while we hold the mutex and
    > for each kernfs descendant should have kn->count > 0.
    >
    > =====
    > /* deactivate and unlink the subtree node-by-node */
    > do {
    > pos = kernfs_leftmost_descendant(kn);
    >
    > /*
    > * kernfs_drain() drops kernfs_mutex temporarily and @pos's
    > * base ref could have been put by someone else by the time
    > * the function returns. Make sure it doesn't go away
    > * underneath us.
    > */
    > kernfs_get(pos);
    > =====
    >
    > At the same time kernfs_notify_workfn() can do a kernfs_put() out of kernfs_mutex
    > which probably can be the last put and dec kn->count to 0 any moment.

    Yeah, but the caller of __kernfs_remove() should be holding the ref,
    so I don't see how it'd reach zero. Also, just putting that one
    kernfs_put() inside mutex can't possibly be the right solution given
    that the function is allowed to be called from any context. I think
    we need to understand what's going on better before making changes.

    Thanks.

    --
    tejun

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