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    SubjectRe: [PATCH] of: Rework and simplify phandle cache to use a fixed size
    On Thu, Dec 12, 2019 at 7:05 AM Sebastian Andrzej Siewior
    <bigeasy@linutronix.de> wrote:
    >
    > On 2019-12-11 17:48:54 [-0600], Rob Herring wrote:
    > > > - if (phandle_cache) {
    > > > - if (phandle_cache[masked_handle] &&
    > > > - handle == phandle_cache[masked_handle]->phandle)
    > > > - np = phandle_cache[masked_handle];
    > > > - if (np && of_node_check_flag(np, OF_DETACHED)) {
    > > > - WARN_ON(1); /* did not uncache np on node removal */
    > > > - of_node_put(np);
    > > > - phandle_cache[masked_handle] = NULL;
    > > > - np = NULL;
    > > > - }
    > > > + if (phandle_cache[handle_hash] &&
    > > > + handle == phandle_cache[handle_hash]->phandle)
    > > > + np = phandle_cache[handle_hash];
    > > > + if (np && of_node_check_flag(np, OF_DETACHED)) {
    > > > + WARN_ON(1); /* did not uncache np on node removal */
    > >
    > > BTW, I don't think this check is even valid. If we failed to detach
    > > and remove the node from the cache, then we could be accessing np
    > > after freeing it.
    >
    > this is kmalloc()ed memory which is always valid. If the memory is
    > already re-used then
    > handle == phandle_cache[handle_hash]->phandle
    >
    > will fail (the check, not the memory access itself).

    There's a 1 in 2^32 chance it won't.

    > If the check
    > remains valid then you can hope for the OF_DETACHED flag to trigger the
    > warning.

    Keyword is hope.

    To look at it another way. Do we need this check? It is in the "fast
    path". There's a single location where we set OF_DETACHED and the
    cache entry is removed at the same time. Also, if we do free the
    node's memory, it also checks for OF_DETACHED. Previously, a free
    wouldn't happen because we incremented the ref count on nodes in the
    cache.

    Rob

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