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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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