xfs: make xfs_*read_agf return EAGAIN to ALLOC_FLAG_TRYLOCK callers

Refactor xfs_read_agf and xfs_alloc_read_agf to return EAGAIN if the
caller passed TRYLOCK and we weren't able to get the lock; and change
the callers to recognize this.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/libxfs/xfs_alloc.c

@@ -2502,12 +2502,11 @@ xfs_alloc_fix_freelist(
 
 	if (!pag->pagf_init) {
 		error = xfs_alloc_read_agf(mp, tp, args->agno, flags, &agbp);
-		if (error)
+		if (error) {
+			/* Couldn't lock the AGF so skip this AG. */
+			if (error == -EAGAIN)
+				error = 0;
 			goto out_no_agbp;
-		if (!pag->pagf_init) {
-			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
-			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
-			goto out_agbp_relse;
 		}
 	}
 
@@ -2533,11 +2532,10 @@ xfs_alloc_fix_freelist(
 	 */
 	if (!agbp) {
 		error = xfs_alloc_read_agf(mp, tp, args->agno, flags, &agbp);
-		if (error)
-			goto out_no_agbp;
-		if (!agbp) {
-			ASSERT(flags & XFS_ALLOC_FLAG_TRYLOCK);
-			ASSERT(!(flags & XFS_ALLOC_FLAG_FREEING));
+		if (error) {
+			/* Couldn't lock the AGF so skip this AG. */
+			if (error == -EAGAIN)
+				error = 0;
 			goto out_no_agbp;
 		}
 	}
@@ -2768,11 +2766,10 @@ xfs_alloc_pagf_init(
 	xfs_buf_t		*bp;
 	int			error;
 
-	if ((error = xfs_alloc_read_agf(mp, tp, agno, flags, &bp)))
-		return error;
-	if (bp)
+	error = xfs_alloc_read_agf(mp, tp, agno, flags, &bp);
+	if (!error)
 		xfs_trans_brelse(tp, bp);
-	return 0;
+	return error;
 }
 
 /*
@@ -2961,12 +2958,6 @@ xfs_read_agf(
 	error = xfs_trans_read_buf(mp, tp, mp->m_ddev_targp,
 			XFS_AG_DADDR(mp, agno, XFS_AGF_DADDR(mp)),
 			XFS_FSS_TO_BB(mp, 1), flags, bpp, &xfs_agf_buf_ops);
-	/*
-	 * Callers of xfs_read_agf() currently interpret a NULL bpp as EAGAIN
-	 * and need to be converted to check for EAGAIN specifically.
-	 */
-	if (error == -EAGAIN)
-		return 0;
 	if (error)
 		return error;
 
@@ -2992,14 +2983,15 @@ xfs_alloc_read_agf(
 
 	trace_xfs_alloc_read_agf(mp, agno);
 
+	/* We don't support trylock when freeing. */
+	ASSERT((flags & (XFS_ALLOC_FLAG_FREEING | XFS_ALLOC_FLAG_TRYLOCK)) !=
+			(XFS_ALLOC_FLAG_FREEING | XFS_ALLOC_FLAG_TRYLOCK));
 	ASSERT(agno != NULLAGNUMBER);
 	error = xfs_read_agf(mp, tp, agno,
 			(flags & XFS_ALLOC_FLAG_TRYLOCK) ? XBF_TRYLOCK : 0,
 			bpp);
 	if (error)
 		return error;
-	if (!*bpp)
-		return 0;
 	ASSERT(!(*bpp)->b_error);
 
 	agf = XFS_BUF_TO_AGF(*bpp);

fs/xfs/libxfs/xfs_bmap.c

@@ -3311,11 +3311,12 @@ xfs_bmap_longest_free_extent(
 	pag = xfs_perag_get(mp, ag);
 	if (!pag->pagf_init) {
 		error = xfs_alloc_pagf_init(mp, tp, ag, XFS_ALLOC_FLAG_TRYLOCK);
-		if (error)
-			goto out;
-
-		if (!pag->pagf_init) {
+		if (error) {
+			/* Couldn't lock the AGF, so skip this AG. */
+			if (error == -EAGAIN) {
 				*notinit = 1;
+				error = 0;
+			}
 			goto out;
 		}
 	}

fs/xfs/xfs_filestream.c

@@ -159,16 +159,15 @@ xfs_filestream_pick_ag(
 
 		if (!pag->pagf_init) {
 			err = xfs_alloc_pagf_init(mp, NULL, ag, trylock);
-			if (err && !trylock) {
+			if (err) {
 				xfs_perag_put(pag);
+				if (err != -EAGAIN)
 					return err;
+				/* Couldn't lock the AGF, skip this AG. */
+				continue;
 			}
 		}
 
-		/* Might fail sometimes during the 1st pass with trylock set. */
-		if (!pag->pagf_init)
-			goto next_ag;
-
 		/* Keep track of the AG with the most free blocks. */
 		if (pag->pagf_freeblks > maxfree) {
 			maxfree = pag->pagf_freeblks;