From: Matthew Dillon <dillon@apollo.backplane.com>
Date: Wed, 10 Dec 2008 18:27:32 +0000 (-0800)
Subject: Fix a buffer cache deadlock which can occur when simulated disk devices
X-Git-Tag: v2.3.0~244
X-Git-Url: http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff_plain/cb63d1bc9eab45185cdc14bad4d3f7d116ed9ffa

Fix a buffer cache deadlock which can occur when simulated disk devices
(VN) are backed by a HAMMER file.  Do not call bwillwrite() via the
VN->VOP_WRITE->HAMMER path.

Add a new IO_RECURSE flag to identify paths for which bwillwrite() should
never be called.

Reported-by: Rumko <rumcic@gmail.com>
---

diff --git a/sys/dev/disk/vn/vn.c b/sys/dev/disk/vn/vn.c
index 1a35ec0..e012a26 100644
--- a/sys/dev/disk/vn/vn.c
+++ b/sys/dev/disk/vn/vn.c
@@ -391,9 +391,9 @@ vnstrategy(struct dev_strategy_args *ap)
 		auio.uio_td = curthread;
 		vn_lock(vn->sc_vp, LK_EXCLUSIVE | LK_RETRY);
 		if (bp->b_cmd == BUF_CMD_READ)
-			error = VOP_READ(vn->sc_vp, &auio, IO_DIRECT, vn->sc_cred);
+			error = VOP_READ(vn->sc_vp, &auio, IO_DIRECT | IO_RECURSE, vn->sc_cred);
 		else
-			error = VOP_WRITE(vn->sc_vp, &auio, IO_DIRECT, vn->sc_cred);
+			error = VOP_WRITE(vn->sc_vp, &auio, IO_DIRECT | IO_RECURSE, vn->sc_cred);
 		vn_unlock(vn->sc_vp);
 		bp->b_resid = auio.uio_resid;
 		if (error) {
diff --git a/sys/sys/vnode.h b/sys/sys/vnode.h
index 0cbbb29..253577c 100644
--- a/sys/sys/vnode.h
+++ b/sys/sys/vnode.h
@@ -305,7 +305,7 @@ struct vnode {
 #define	IO_INVAL	0x0040		/* invalidate after I/O */
 #define IO_ASYNC	0x0080		/* bawrite rather then bdwrite */
 #define	IO_DIRECT	0x0100		/* attempt to bypass buffer cache */
-#define	IO_UNUSED0200	0x0200
+#define	IO_RECURSE	0x0200		/* possibly device-recursive (vn) */
 #define	IO_CORE		0x0400		/* I/O is part of core dump */
 
 #define	IO_SEQMAX	0x7F		/* seq heuristic max value */
diff --git a/sys/vfs/hammer/hammer_vnops.c b/sys/vfs/hammer/hammer_vnops.c
index 53443a0..7c443c8 100644
--- a/sys/vfs/hammer/hammer_vnops.c
+++ b/sys/vfs/hammer/hammer_vnops.c
@@ -388,9 +388,12 @@ hammer_vop_write(struct vop_write_args *ap)
 		 * with the buffer cache.  Even so, we cannot afford to
 		 * allow the pageout daemon to build up too many dirty buffer
 		 * cache buffers.
+		 *
+		 * Only call this if we aren't being recursively called from
+		 * a virtual disk device (vn), else we may deadlock.
 		 */
-		/*if (((int)uio->uio_offset & (blksize - 1)) == 0)*/
-		bwillwrite(blksize);
+		if ((ap->a_ioflag & IO_RECURSE) == 0)
+			bwillwrite(blksize);
 
 		/*
 		 * Do not allow HAMMER to blow out system memory by