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 <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Dave Chinner <[e-mail]>

fs/xfs/libxfs/xfs_alloc.c

···
 
 	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;
 		}
 	}
 
···
 	 */
 	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;
 		}
 	}
···
 	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;
 }
 
 /*
···
 	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;
 
···
 
 	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

···
 	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) {
-			*notinit = 1;
+		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

···
 
 		if (!pag->pagf_init) {
 			err = xfs_alloc_pagf_init(mp, NULL, ag, trylock);
-			if (err && !trylock) {
+			if (err) {
 				xfs_perag_put(pag);
-				return err;
+				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) {