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.
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.
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