Messages in this thread | | | Date | Fri, 3 Mar 2006 16:43:12 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 7/5] Optimise d_find_alias() |
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* 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. ]
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