arc: avoid possible deadlock in arc_read

In l2arc_evict(), the config lock may be acquired in reverse order
(e.g., first the config lock (writer), then a hash lock) unlike in
arc_read() during scenarios like L2ARC device removal. To avoid
deadlocks, if the attempt to acquire the config lock (reader) fails
in arc_read(), release the hash lock, wait for the config lock, and
retry from the beginning.

Signed-off-by: Ameer Hamza <[email protected]>

Author: ixhamza

cmd/zdb/zdb.c

@@ -9043,7 +9043,7 @@ zdb_read_block(char *thing, spa_t *spa)
 		const blkptr_t *b = (const blkptr_t *)(void *)
 		    ((uintptr_t)buf + (uintptr_t)blkptr_offset);
 		if (zfs_blkptr_verify(spa, b,
-		    BLK_CONFIG_NEEDED, BLK_VERIFY_ONLY) == B_FALSE) {
+		    BLK_CONFIG_NEEDED, BLK_VERIFY_ONLY)) {
 			abd_return_buf_copy(pabd, buf, lsize);
 			borrowed = B_FALSE;
 			buf = lbuf;
@@ -9052,7 +9052,7 @@ zdb_read_block(char *thing, spa_t *spa)
 			b = (const blkptr_t *)(void *)
 			    ((uintptr_t)buf + (uintptr_t)blkptr_offset);
 			if (lsize == -1 || zfs_blkptr_verify(spa, b,
-			    BLK_CONFIG_NEEDED, BLK_VERIFY_LOG) == B_FALSE) {
+			    BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
 				printf("invalid block pointer at this DVA\n");
 				goto out;
 			}

include/sys/zio.h

@@ -546,6 +546,7 @@ enum blk_verify_flag {
 enum blk_config_flag {
 	BLK_CONFIG_HELD,   // SCL_VDEV held for writer
 	BLK_CONFIG_NEEDED, // SCL_VDEV should be obtained for reader
+	BLK_CONFIG_NEEDED_TRY, // Try with SCL_VDEV for reader
 	BLK_CONFIG_SKIP,   // skip checks which require SCL_VDEV
 };
 
@@ -663,7 +664,7 @@ extern void zio_suspend(spa_t *spa, zio_t *zio, zio_suspend_reason_t);
 extern int zio_resume(spa_t *spa);
 extern void zio_resume_wait(spa_t *spa);
 
-extern boolean_t zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
+extern int zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
     enum blk_config_flag blk_config, enum blk_verify_flag blk_verify);
 
 /*

module/zfs/arc.c

@@ -5683,6 +5683,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 	boolean_t no_buf = *arc_flags & ARC_FLAG_NO_BUF;
 	arc_buf_t *buf = NULL;
 	int rc = 0;
+	boolean_t bp_validation = B_FALSE;
 
 	ASSERT(!embedded_bp ||
 	    BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA);
@@ -5725,7 +5726,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 		 * should always be the case since the blkptr is protected by
 		 * a checksum.
 		 */
-		if (!zfs_blkptr_verify(spa, bp, BLK_CONFIG_SKIP,
+		if (zfs_blkptr_verify(spa, bp, BLK_CONFIG_SKIP,
 		    BLK_VERIFY_LOG)) {
 			mutex_exit(hash_lock);
 			rc = SET_ERROR(ECKSUM);
@@ -5877,6 +5878,8 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 		abd_t *hdr_abd;
 		int alloc_flags = encrypted_read ? ARC_HDR_ALLOC_RDATA : 0;
 		arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
+		int config_lock;
+		int error;
 
 		if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
 			if (hash_lock != NULL)
@@ -5885,16 +5888,31 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
 			goto done;
 		}
 
+		if (zio_flags & ZIO_FLAG_CONFIG_WRITER) {
+			config_lock = BLK_CONFIG_HELD;
+		} else if (hash_lock != NULL) {
+			/*
+			 * Prevent lock order reversal
+			 */
+			config_lock = BLK_CONFIG_NEEDED_TRY;
+		} else {
+			config_lock = BLK_CONFIG_NEEDED;
+		}
+
 		/*
 		 * Verify the block pointer contents are reasonable.  This
 		 * should always be the case since the blkptr is protected by
 		 * a checksum.
 		 */
-		if (!zfs_blkptr_verify(spa, bp,
-		    (zio_flags & ZIO_FLAG_CONFIG_WRITER) ?
-		    BLK_CONFIG_HELD : BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
+		if (!bp_validation && (error = zfs_blkptr_verify(spa, bp,
+		    config_lock, BLK_VERIFY_LOG))) {
 			if (hash_lock != NULL)
 				mutex_exit(hash_lock);
+			if (error == EBUSY && !zfs_blkptr_verify(spa, bp,
+			    BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
+				bp_validation = B_TRUE;
+				goto top;
+			}
 			rc = SET_ERROR(ECKSUM);
 			goto done;
 		}

module/zfs/dsl_scan.c

@@ -2305,7 +2305,7 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
 			    DMU_USERUSED_OBJECT, tx);
 		}
 		arc_buf_destroy(buf, &buf);
-	} else if (!zfs_blkptr_verify(spa, bp,
+	} else if (zfs_blkptr_verify(spa, bp,
 	    BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
 		/*
 		 * Sanity check the block pointer contents, this is handled

module/zfs/spa.c

@@ -2778,7 +2778,7 @@ spa_load_verify_cb(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
 	 * When damaged consider it to be a metadata error since we cannot
 	 * trust the BP_GET_TYPE and BP_GET_LEVEL values.
 	 */
-	if (!zfs_blkptr_verify(spa, bp, BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
+	if (zfs_blkptr_verify(spa, bp, BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
 		atomic_inc_64(&sle->sle_meta_count);
 		return (0);
 	}

module/zfs/zio.c

@@ -1164,7 +1164,7 @@ zfs_blkptr_verify_log(spa_t *spa, const blkptr_t *bp,
  * it only contains known object types, checksum/compression identifiers,
  * block sizes within the maximum allowed limits, valid DVAs, etc.
  *
- * If everything checks out B_TRUE is returned.  The zfs_blkptr_verify
+ * If everything checks out 0 is returned.  The zfs_blkptr_verify
  * argument controls the behavior when an invalid field is detected.
  *
  * Values for blk_verify_flag:
@@ -1179,7 +1179,7 @@ zfs_blkptr_verify_log(spa_t *spa, const blkptr_t *bp,
  *   BLK_CONFIG_SKIP: skip checks which require SCL_VDEV, for better
  *   performance
  */
-boolean_t
+int
 zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
     enum blk_config_flag blk_config, enum blk_verify_flag blk_verify)
 {
@@ -1211,7 +1211,7 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
 			    "blkptr at %px has invalid PSIZE %llu",
 			    bp, (longlong_t)BPE_GET_PSIZE(bp));
 		}
-		return (errors == 0);
+		return ((errors) ? ECKSUM : 0);
 	}
 	if (unlikely(BP_GET_CHECKSUM(bp) >= ZIO_CHECKSUM_FUNCTIONS)) {
 		errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
@@ -1229,7 +1229,7 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
 	 * will be done once the zio is executed in vdev_mirror_map_alloc.
 	 */
 	if (unlikely(!spa->spa_trust_config))
-		return (errors == 0);
+		return ((errors) ? ECKSUM : 0);
 
 	switch (blk_config) {
 	case BLK_CONFIG_HELD:
@@ -1238,8 +1238,12 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
 	case BLK_CONFIG_NEEDED:
 		spa_config_enter(spa, SCL_VDEV, bp, RW_READER);
 		break;
+	case BLK_CONFIG_NEEDED_TRY:
+		if (!spa_config_tryenter(spa, SCL_VDEV, bp, RW_READER))
+			return (EBUSY);
+		break;
 	case BLK_CONFIG_SKIP:
-		return (errors == 0);
+		return ((errors) ? ECKSUM : 0);
 	default:
 		panic("invalid blk_config %u", blk_config);
 	}
@@ -1294,10 +1298,11 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp,
 			    bp, i, (longlong_t)offset);
 		}
 	}
-	if (blk_config == BLK_CONFIG_NEEDED)
+	if (blk_config == BLK_CONFIG_NEEDED || blk_config ==
+	    BLK_CONFIG_NEEDED_TRY)
 		spa_config_exit(spa, SCL_VDEV, bp);
 
-	return (errors == 0);
+	return ((errors) ? ECKSUM : 0);
 }
 
 boolean_t