Serialize NFS vinvalbuf operations by acquiring/upgrading to the

vnode EXCLUSIVE lock. This prevents threads from adding pages to
the vnode while an invalidation is in progress, closing potential
races. In the bioread() path, callers acquire the SHARED vnode lock
- so while an invalidate was in progress, it was possible to fault
in new pages onto the vnode causing the invalidation to take a while
or fail. We saw these races at Yahoo! with very large files+heavy
concurrent access. Forcing an upgrade to EXCLUSIVE lock before doing
the invalidation closes all these races.

Submitted by:	Mohan Srinivasan mohans at yahoo-inc dot com

1 files changed, 23 insertions, 26 deletions

diff --git a/sys/nfsclient/nfs_bio.c b/sys/nfsclient/nfs_bio.c
index 56e2cb6..9fd12a1 100644
--- a/sys/nfsclient/nfs_bio.c
+++ b/sys/nfsclient/nfs_bio.c
@@ -1066,6 +1066,7 @@ nfs_vinvalbuf(struct vnode *vp, int flags, struct ucred *cred,
 	struct nfsnode *np = VTONFS(vp);
 	struct nfsmount *nmp = VFSTONFS(vp->v_mount);
 	int error = 0, slpflag, slptimeo;
+ 	int old_lock = 0;
 
 	ASSERT_VOP_LOCKED(vp, "nfs_vinvalbuf");
 
@@ -1086,40 +1087,36 @@ nfs_vinvalbuf(struct vnode *vp, int flags, struct ucred *cred,
 		slpflag = 0;
 		slptimeo = 0;
 	}
-	/*
-	 * First wait for any other process doing a flush to complete.
-	 */
-	while (np->n_flag & NFLUSHINPROG) {
-		np->n_flag |= NFLUSHWANT;
-		error = tsleep(&np->n_flag, PRIBIO + 2, "nfsvinval",
-			slptimeo);
-		if (error && intrflg &&
-		    nfs_sigintr(nmp, NULL, td))
-			return (EINTR);
-	}
+
+ 	if ((old_lock = VOP_ISLOCKED(vp, td)) != LK_EXCLUSIVE) {
+ 		if (old_lock == LK_SHARED) {
+ 			/* Upgrade to exclusive lock, this might block */
+ 			vn_lock(vp, LK_UPGRADE | LK_RETRY, td);
+ 		} else {
+ 			vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
+ 		}
+  	}
 
 	/*
 	 * Now, flush as required.
 	 */
-	np->n_flag |= NFLUSHINPROG;
 	error = vinvalbuf(vp, flags, cred, td, slpflag, 0);
 	while (error) {
-		if (intrflg && (error = nfs_sigintr(nmp, NULL, td))) {
-			np->n_flag &= ~NFLUSHINPROG;
-			if (np->n_flag & NFLUSHWANT) {
-				np->n_flag &= ~NFLUSHWANT;
-				wakeup(&np->n_flag);
-			}
-			return (error);
-		}
+		if (intrflg && (error = nfs_sigintr(nmp, NULL, td)))
+			goto out;
 		error = vinvalbuf(vp, flags, cred, td, 0, slptimeo);
 	}
-	np->n_flag &= ~(NMODIFIED | NFLUSHINPROG);
-	if (np->n_flag & NFLUSHWANT) {
-		np->n_flag &= ~NFLUSHWANT;
-		wakeup(&np->n_flag);
-	}
-	return (0);
+	np->n_flag &= ~NMODIFIED;
+out:
+ 	if (old_lock != LK_EXCLUSIVE) {
+ 		if (old_lock == LK_SHARED) {
+ 			/* Downgrade from exclusive lock, this might block */
+ 			vn_lock(vp, LK_DOWNGRADE, td);
+ 		} else {
+ 			VOP_UNLOCK(vp, 0, td);
+ 		}
+  	}
+	return error;
 }
 
 /*