Make recursive getblk() slightly more useful. Keep the buffer state

intact if getblk() is done on the already owned buffer.  Exit from
brelse() early when the lock recursion is detected, otherwise brelse()
might prematurely destroy the buffer under some circumstances.

Sponsored by:	The FreeBSD Foundation
Noted by:	mckusick
Tested by:	pho
MFC after:	2 weeks

1 files changed, 13 insertions, 6 deletions

diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index 6393399..f9b4dbb 100644
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -1268,6 +1268,15 @@ brelse(struct buf *bp)
 	KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
 	    ("brelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));
 
+	if (BUF_LOCKRECURSED(bp)) {
+		/*
+		 * Do not process, in particular, do not handle the
+		 * B_INVAL/B_RELBUF and do not release to free list.
+		 */
+		BUF_UNLOCK(bp);
+		return;
+	}
+
 	if (bp->b_flags & B_MANAGED) {
 		bqrelse(bp);
 		return;
@@ -1444,12 +1453,6 @@ brelse(struct buf *bp)
 			brelvp(bp);
 	}
 			
-	if (BUF_LOCKRECURSED(bp)) {
-		/* do not release to free list */
-		BUF_UNLOCK(bp);
-		return;
-	}
-
 	/* enqueue */
 	mtx_lock(&bqlock);
 	/* Handle delayed bremfree() processing. */
@@ -2681,6 +2684,9 @@ loop:
 		/* We timed out or were interrupted. */
 		else if (error)
 			return (NULL);
+		/* If recursed, assume caller knows the rules. */
+		else if (BUF_LOCKRECURSED(bp))
+			goto end;
 
 		/*
 		 * The buffer is locked.  B_CACHE is cleared if the buffer is 
@@ -2864,6 +2870,7 @@ loop:
 	}
 	CTR4(KTR_BUF, "getblk(%p, %ld, %d) = %p", vp, (long)blkno, size, bp);
 	BUF_ASSERT_HELD(bp);
+end:
 	KASSERT(bp->b_bufobj == bo,
 	    ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
 	return (bp);