On Wed, 2016-06-08 at 12:10 -0400, Oleg Drokin wrote:
> On Jun 8, 2016, at 6:58 AM, Jeff Layton wrote:
>
> > A simple way to confirm that might be to convert all of the read locks
> > on the st_rwsem to write locks. That will serialize all of the open
> > operations and should prevent that particular race from occurring.
> >
> > If that works, we'd probably want to fix it in a less heavy-handed way,
> > but I'd have to think about how best to do that.
>
> So I looked at the call sites for nfs4_get_vfs_file(), how about something like this:
>
> after we grab the fp->fi_lock, we can do test_access(open->op_share_access, stp);
>
> If that returns true - just drop the spinlock and return EAGAIN.
>
> The callsite in nfs4_upgrade_open() would handle that by retesting the access map
> again and either coming back in or more likely reusing the now updated stateid
> (synchronised by the fi_lock again).
> We probably need to convert the whole access map testing there to be under
> fi_lock.
> Something like:
> nfs4_upgrade_open(struct svc_rqst *rqstp, struct nfs4_file *fp, struct svc_fh *cur_fh, struct nfs4_ol_stateid *stp, struct nfsd4_open *open)
> {
> __be32 status;
> unsigned char old_deny_bmap = stp->st_deny_bmap;
>
> again:
> + spin_lock(&fp->fi_lock);
> if (!test_access(open->op_share_access, stp)) {
> + spin_unlock(&fp->fi_lock);
> + status = nfs4_get_vfs_file(rqstp, fp, cur_fh, stp, open);
> + if (status == -EAGAIN)
> + goto again;
> + return status;
> + }
>
> /* test and set deny mode */
> - spin_lock(&fp->fi_lock);
> status = nfs4_file_check_deny(fp, open->op_share_deny);
>
>
> The call in nfsd4_process_open2() I think cannot hit this condition, right?
> probably can add a WARN_ON there? BUG_ON? more sensible approach?
>
> Alternatively we can probably always call nfs4_get_vfs_file() under this spinlock,
> just have it drop that for the open and then reobtain (already done), not as transparent I guess.
>
Yeah, I think that might be best. It looks like things could change
after you drop the spinlock with the patch above. Since we have to
retake it anyway in nfs4_get_vfs_file, we can just do it there.
> Or the fi_lock might be converted to say a mutex, so we can sleep with it held and
> then we can hold it across whole invocation of nfs4_get_vfs_file() and access testing and stuff.
I think we'd be better off taking the st_rwsem for write (maybe just
turning it into a mutex). That would at least be per-stateid instead of
per-inode. That's a fine fix for now.
It might slow down a client slightly that is sending two stateid
morphing operations in parallel, but they shouldn't affect each other.
I'm liking that solution more and more here.
Longer term, I think we need to further simplify OPEN handling. It has
gotten better, but it's still really hard to follow currently (and is
obviously error-prone).
--
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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