nfsd: Fix stable writes

Strictly speaking, a stable write error needs to reflect the
write + the commit of that write (and only that write). To
ensure that we don't pick up the write errors from other
writebacks, add a rw_semaphore to provide exclusion.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/filecache.c

@@ -195,6 +195,7 @@ nfsd_file_alloc(struct inode *inode, unsigned int may, unsigned int hashval,
 				__set_bit(NFSD_FILE_BREAK_READ, &nf->nf_flags);
 		}
 		nf->nf_mark = NULL;
+		init_rwsem(&nf->nf_rwsem);
 		trace_nfsd_file_alloc(nf);
 	}
 	return nf;

fs/nfsd/filecache.h

@@ -46,6 +46,7 @@ struct nfsd_file {
 	atomic_t		nf_ref;
 	unsigned char		nf_may;
 	struct nfsd_file_mark	*nf_mark;
+	struct rw_semaphore	nf_rwsem;
 };
 
 int nfsd_file_cache_init(void);

fs/nfsd/vfs.c

@@ -982,7 +982,18 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 		flags |= RWF_SYNC;
 
 	iov_iter_kvec(&iter, WRITE, vec, vlen, *cnt);
+	if (flags & RWF_SYNC) {
+		down_write(&nf->nf_rwsem);
 		host_err = vfs_iter_write(file, &iter, &pos, flags);
+		if (host_err < 0)
+			nfsd_reset_boot_verifier(net_generic(SVC_NET(rqstp),
+						 nfsd_net_id));
+		up_write(&nf->nf_rwsem);
+	} else {
+		down_read(&nf->nf_rwsem);
+		host_err = vfs_iter_write(file, &iter, &pos, flags);
+		up_read(&nf->nf_rwsem);
+	}
 	if (host_err < 0)
 		goto out_nfserr;
 	*cnt = host_err;
@@ -1097,8 +1108,10 @@ nfsd_commit(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (err)
 		goto out;
 	if (EX_ISSYNC(fhp->fh_export)) {
-		int err2 = vfs_fsync_range(nf->nf_file, offset, end, 0);
+		int err2;
 
+		down_write(&nf->nf_rwsem);
+		err2 = vfs_fsync_range(nf->nf_file, offset, end, 0);
 		switch (err2) {
 		case 0:
 			break;
@@ -1110,6 +1123,7 @@ nfsd_commit(struct svc_rqst *rqstp, struct svc_fh *fhp,
 			nfsd_reset_boot_verifier(net_generic(nf->nf_net,
 						 nfsd_net_id));
 		}
+		up_write(&nf->nf_rwsem);
 	}
 
 	nfsd_file_put(nf);