[Lustre-discuss] question about dcache revalidate

[tao.peng at emc.com]

> On Jan 12, 2012, at 3:52 AM, Lai Siyao wrote:
>
> > No, to add a dentry to hash client needs holding LOOKUP lock, but lustre client unhash (see
> ll_unhash_aliases()) doesn't really remove dentry from hash, but set LUSTRE_DCACHE_INVALID flag. So in
> the race you mentioned, another process may add the dentry but later the lock is canceled soon, so at
> the time of revalidate, it needs to do a real lookup.
>
> In fact there's more to it.
> Certain operations like open, for example, don't actually get a lock from server so if there was no
> lock to begin with, we add
> the "known invalid" entry to the hash and immediately mark it as invalid, no later lock cancel needed
> to trigger this.
>
> Are you referring to ll_lookup_it_finish() that flag negative dentry DCACHE_LUSTRE_INVALID and hash/unhash it immediately if client doesn't have UDATE lock? Looks like such dentry is skipped during link path walk as ll_dcompare checks DCACHE_LUSTRE_INVALID. Or am I looking at wrong place?
 
> Normally lustre client doesn't fetch any child lock for lookup(IT_OPEN), but a child dentry will be inserted into hash, however this dentry is marked INVALID, and it will be freed after file->release(), that means, no other processes will use the dentry (in this case other processes will create another dentry for this file and add to hash too). So Oleg means a dentry may be inserted into hash without LOOKUP lock, but this dentry won't be used by others, so it will not cause the race you mentioned.

I see. mdc_intent_open_pack() only asks for child OPEN and parent UPDATE lock. Thanks very much!

Best,
Tao