Bug#12560151 61132: infinite loop in buf_page_get_gen() when handling

compressed pages

After loading a compressed-only page in buf_page_get_gen() we allocate a new
block for decompression. The problem is that the compressed page is neither
buffer-fixed nor I/O-fixed by the time we call buf_LRU_get_free_block(),
so it may end up being evicted and returned back as a new block.

buf_page_get_gen(): Temporarily buffer-fix the compressed-only block
while allocating memory for an uncompressed page frame.
This should prevent this form of the infinite loop, which is more likely
with a small innodb_buffer_pool_size.

rb#2511 approved by Jimmy Yang, Sunny Bains

storage/innodb_plugin/ChangeLog

+2013-08-21	The InnoDB Team
+
+	* buf/buf0buf.c:
+	Fix Bug#12560151 61132: infinite loop in buf_page_get_gen()
+	when handling compressed pages
+
 2013-08-16	The InnoDB Team
 
 	* row/row0sel.c:

storage/innodb_plugin/buf/buf0buf.c

@@ -1615,7 +1615,6 @@ loop:
 		block = (buf_block_t*) buf_page_hash_get(space, offset);
 	}
 
-loop2:
 	if (block == NULL) {
 		/* Page not in buf_pool: needs to be read from file */
 
@@ -1706,6 +1705,11 @@ wait_until_unfixed:
 			goto loop;
 		}
 
+		/* Buffer-fix the block so that it cannot be evicted
+		or relocated while we are attempting to allocate an
+		uncompressed page. */
+		bpage->buf_fix_count++;
+
 		/* Allocate an uncompressed page. */
 		buf_pool_mutex_exit();
 		mutex_exit(&buf_pool_zip_mutex);
@@ -1715,32 +1719,19 @@ wait_until_unfixed:
 
 		buf_pool_mutex_enter();
 		mutex_enter(&block->mutex);
+		mutex_enter(&buf_pool_zip_mutex);
+		/* Buffer-fixing prevents the page_hash from changing. */
+		ut_ad(bpage == buf_page_hash_get(space, offset));
 
-		{
-			buf_page_t*	hash_bpage
-				= buf_page_hash_get(space, offset);
-
-			if (UNIV_UNLIKELY(bpage != hash_bpage)) {
-				/* The buf_pool->page_hash was modified
-				while buf_pool_mutex was released.
-				Free the block that was allocated. */
-
-				buf_LRU_block_free_non_file_page(block);
-				mutex_exit(&block->mutex);
-
-				block = (buf_block_t*) hash_bpage;
-				goto loop2;
-			}
-		}
-
-		if (UNIV_UNLIKELY
-		    (bpage->buf_fix_count
-		     || buf_page_get_io_fix(bpage) != BUF_IO_NONE)) {
+		if (--bpage->buf_fix_count
+		    || buf_page_get_io_fix(bpage) != BUF_IO_NONE) {
 
-			/* The block was buffer-fixed or I/O-fixed
-			while buf_pool_mutex was not held by this thread.
-			Free the block that was allocated and try again.
-			This should be extremely unlikely. */
+			mutex_exit(&buf_pool_zip_mutex);
+			/* The block was buffer-fixed or I/O-fixed while
+			buf_pool_mutex was not held by this thread.
+			Free the block that was allocated and retry.
+			This should be extremely unlikely, for example,
+			if buf_page_get_zip() was invoked. */
 
 			buf_LRU_block_free_non_file_page(block);
 			mutex_exit(&block->mutex);
@@ -1751,8 +1742,6 @@ wait_until_unfixed:
 		/* Move the compressed page from bpage to block,
 		and uncompress it. */
 
-		mutex_enter(&buf_pool_zip_mutex);
-
 		buf_relocate(bpage, &block->page);
 		buf_block_init_low(block);
 		block->lock_hash_val = lock_rec_hash(space, offset);