Fix a race that can occur when nfs nfsiod threads are being created.

Without this patch it was possible for a different thread that calls
nfs_asyncio() to snitch a newly created nfsiod thread that was
intended for another caller of nfs_asyncio(), because the nfs_iod_mtx
mutex was unlocked while the new nfsiod thread was created. This patch
labels the newly created nfsiod, so that it is not taken by another
caller of nfs_asyncio(). This is believed to fix the problem reported
on the freebsd-stable email list under the subject:
FreeBSD NFS client/Linux NFS server issue.

Tested by:	to DOT my DOT trociny AT gmail DOT com
Reviewed by:	jhb
MFC after:	2 weeks

1 files changed, 13 insertions, 7 deletions

diff --git a/sys/nfsclient/nfs_nfsiod.c b/sys/nfsclient/nfs_nfsiod.c
index 3fafa1e..5302c56 100644
--- a/sys/nfsclient/nfs_nfsiod.c
+++ b/sys/nfsclient/nfs_nfsiod.c
@@ -113,7 +113,7 @@ sysctl_iodmin(SYSCTL_HANDLER_ARGS)
 	 * than the new minimum, create some more.
 	 */
 	for (i = nfs_iodmin - nfs_numasync; i > 0; i--)
-		nfs_nfsiodnew();
+		nfs_nfsiodnew(0);
 out:
 	mtx_unlock(&nfs_iod_mtx);	
 	return (0);
@@ -147,7 +147,7 @@ sysctl_iodmax(SYSCTL_HANDLER_ARGS)
 	 */
 	iod = nfs_numasync - 1;
 	for (i = 0; i < nfs_numasync - nfs_iodmax; i++) {
-		if (nfs_iodwant[iod])
+		if (nfs_iodwant[iod] == NFSIOD_AVAILABLE)
 			wakeup(&nfs_iodwant[iod]);
 		iod--;
 	}
@@ -160,7 +160,7 @@ SYSCTL_PROC(_vfs_nfs, OID_AUTO, iodmax, CTLTYPE_UINT | CTLFLAG_RW, 0,
     "Max number of nfsiod kthreads");
 
 int
-nfs_nfsiodnew(void)
+nfs_nfsiodnew(int set_iodwant)
 {
 	int error, i;
 	int newiod;
@@ -176,12 +176,17 @@ nfs_nfsiodnew(void)
 		}
 	if (newiod == -1)
 		return (-1);
+	if (set_iodwant > 0)
+		nfs_iodwant[i] = NFSIOD_CREATED_FOR_NFS_ASYNCIO;
 	mtx_unlock(&nfs_iod_mtx);
 	error = kproc_create(nfssvc_iod, nfs_asyncdaemon + i, NULL, RFHIGHPID,
 	    0, "nfsiod %d", newiod);
 	mtx_lock(&nfs_iod_mtx);
-	if (error)
+	if (error) {
+		if (set_iodwant > 0)
+			nfs_iodwant[i] = NFSIOD_NOT_AVAILABLE;
 		return (-1);
+	}
 	nfs_numasync++;
 	return (newiod);
 }
@@ -199,7 +204,7 @@ nfsiod_setup(void *dummy)
 		nfs_iodmin = NFS_MAXASYNCDAEMON;
 
 	for (i = 0; i < nfs_iodmin; i++) {
-		error = nfs_nfsiodnew();
+		error = nfs_nfsiodnew(0);
 		if (error == -1)
 			panic("nfsiod_setup: nfs_nfsiodnew failed");
 	}
@@ -236,7 +241,8 @@ nfssvc_iod(void *instance)
 			goto finish;
 		if (nmp)
 			nmp->nm_bufqiods--;
-		nfs_iodwant[myiod] = curthread->td_proc;
+		if (nfs_iodwant[myiod] == NFSIOD_NOT_AVAILABLE)
+			nfs_iodwant[myiod] = NFSIOD_AVAILABLE;
 		nfs_iodmount[myiod] = NULL;
 		/*
 		 * Always keep at least nfs_iodmin kthreads.
@@ -303,7 +309,7 @@ finish:
 	nfs_asyncdaemon[myiod] = 0;
 	if (nmp)
 	    nmp->nm_bufqiods--;
-	nfs_iodwant[myiod] = NULL;
+	nfs_iodwant[myiod] = NFSIOD_NOT_AVAILABLE;
 	nfs_iodmount[myiod] = NULL;
 	/* Someone may be waiting for the last nfsiod to terminate. */
 	if (--nfs_numasync == 0)