Messages in this thread | | | Subject | Re: [RFC] atomic create+open | From | Trond Myklebust <> | Date | Thu, 06 Oct 2005 15:17:38 -0400 |
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to den 06.10.2005 Klokka 20:49 (+0200) skreiv Miklos Szeredi:
> For simplicity case let's omit the creation of simlink, just say, the > file is removed. > > So NFS calls have_submounts(), which returns true. > > Then the bind is umounted. Nothing prevents this happening > concurrently with the lookup. > > Then the file is removed on the server. > > When open_namei() gets around to following the mounts, it is not there > any more, so the dentry for /mnt/foo (the NFS one is returned) and > NFS's ->open is called on the file, which returns -ENOENT. But > open(..., O_CREAT, ...) should never return -ENOENT.
...and so the VFS can recognise the case, and be made to retry the operation. A more difficult race to deal with occurs if you allow a mount while inside d_revalidate(). In that case NFS can end up opening the wrong file. Both these two races could, however, be fixed by moving the __follow_mount() in open_namei() inside the section that is protected by the parent directory i_sem.
In any case, all you are doing here is showing that the situation w.r.t. mount races and lookup+create+open is difficult. I see nothing that convinces me that a special atomic create+open will help to resolve those races. Nor do I see that adding a special atomic create+open will help me avoid intents for the case of atomic lookup+open(). As far as I'm concerned, the case of lookup+create+open is just a special case of lookup+open.
Cheers, Trond
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