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    SubjectRe: [PATCH 7/5] Optimise d_find_alias()

    * David Howells <dhowells@redhat.com> wrote:

    > Andrew Morton <akpm@osdl.org> wrote:
    >
    > > How can we get away without a barrier?
    >
    > Is this what you're thinking of:
    >
    > struct dentry * d_find_alias(struct inode *inode)
    > {
    > struct dentry *de = NULL;
    >
    > smp_rmb();
    > if (!list_empty(&inode->i_dentry)) {
    > spin_lock(&dcache_lock);

    no barrier is needed i think. This code just picks an alias, with no
    guarantee of any ordering wrt. add/removal ops from the alias list. In
    other words, d_find_alias() depends on external serialization - when
    used standalone it is fundamentally nondeterministic anyway.

    it is true that the list_empty() might observe an intermediate state of
    the list [e.g. backlink not modified yet but forward link modified - or
    the other way around], but that's not a problem: it either returns true
    or false, both of which are correct results when the modification is
    underway in parallel on another CPU. When d_find_alias() returns
    non-NULL it will only do it by taking dcache_lock, so the correct use of
    the list is always guaranteed.

    [ a barrier would have no useful effect here anyway - it only guarantees
    instruction ordering locally, and has no deterministic effect on the
    order of memory values being written from another CPU. A barrier makes
    sense when ordering of writes to two different memory values is
    deterministic - e.g. a flag and a resource pointer. But in the case of
    list_empty(), the order of modifications to the two pointers in the
    list is not guaranteed. ]

    Ingo
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