Both NFS clients can deadlock when using the "rdirplus" mount

option. This can occur when an nfsiod thread that already holds
a buffer lock attempts to acquire a vnode lock on an entry in
the directory (a LOR) when another thread holding the vnode lock
is waiting on an nfsiod thread. This patch avoids the deadlock by disabling
readahead for this case, so the nfsiod threads never do readdirplus.
Since readaheads for directories need the directory offset cookie
from the previous read, they cannot normally happen in parallel.
As such, testing by jhb@ and myself didn't find any performance
degredation when this patch is applied. If there is a case where
this results in a significant performance degradation, mounting
without the "rdirplus" option can be done to re-enable readahead
for directories.

Reported and tested by:	jhb
Reviewed by:	jhb
MFC after:	2 weeks

1 files changed, 10 insertions, 2 deletions

diff --git a/sys/nfsclient/nfs_bio.c b/sys/nfsclient/nfs_bio.c
index b7fe832..630a7ff 100644
--- a/sys/nfsclient/nfs_bio.c
+++ b/sys/nfsclient/nfs_bio.c
@@ -1345,10 +1345,18 @@ nfs_asyncio(struct nfsmount *nmp, struct buf *bp, struct ucred *cred, struct thr
 	 * Commits are usually short and sweet so lets save some cpu and
 	 * leave the async daemons for more important rpc's (such as reads
 	 * and writes).
+	 *
+	 * Readdirplus RPCs do vget()s to acquire the vnodes for entries
+	 * in the directory in order to update attributes. This can deadlock
+	 * with another thread that is waiting for async I/O to be done by
+	 * an nfsiod thread while holding a lock on one of these vnodes.
+	 * To avoid this deadlock, don't allow the async nfsiod threads to
+	 * perform Readdirplus RPCs.
 	 */
 	mtx_lock(&nfs_iod_mtx);
-	if (bp->b_iocmd == BIO_WRITE && (bp->b_flags & B_NEEDCOMMIT) &&
-	    (nmp->nm_bufqiods > nfs_numasync / 2)) {
+	if ((bp->b_iocmd == BIO_WRITE && (bp->b_flags & B_NEEDCOMMIT) &&
+	     (nmp->nm_bufqiods > nfs_numasync / 2)) ||
+	    (bp->b_vp->v_type == VDIR && (nmp->nm_flag & NFSMNT_RDIRPLUS))) {
 		mtx_unlock(&nfs_iod_mtx);
 		return(EIO);
 	}