Commit ad576e63 authored May 05, 2005 by Nick Piggin Committed by Linus Torvalds May 05, 2005

[PATCH] __block_write_full_page race fix



When running
	fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
on an ext2 filesystem with 1024 byte block size, on SMP i386 with 4096 byte
page size over loopback to an image file on a tmpfs filesystem, I would
very quickly hit
	BUG_ON(!buffer_async_write(bh));
in fs/buffer.c:end_buffer_async_write

It seems that more than one request would be submitted for a given bh
at a time.

What would happen is the following:
2 threads doing __mpage_writepages on the same page.
Thread 1 - lock the page first, and enter __block_write_full_page.
Thread 1 - (eg.) mark_buffer_async_write on the first 2 buffers.
Thread 1 - set page writeback, unlock page.
Thread 2 - lock page, wait on page writeback
Thread 1 - submit_bh on the first 2 buffers.
=> both requests complete, none of the page buffers are async_write,
   end_page_writeback is called.
Thread 2 - wakes up. enters __block_write_full_page.
Thread 2 - mark_buffer_async_write on (eg.) the last buffer
Thread 1 - finds the last buffer has async_write set, submit_bh on that.
Thread 2 - submit_bh on the last buffer.
=> oops.

So change __block_write_full_page to explicitly keep track of the last bh
we need to issue, so we don't touch anything after issuing the last
request.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent f3ddbdc6

fs/buffer.c

+13 −5

Original line number	Diff line number	Diff line
		@@ -1751,7 +1751,7 @@ static int __block_write_full_page(struct inode inode, struct page page,
		int err;
		sector_t block;
		sector_t last_block;
		struct buffer_head bh, head;
		struct buffer_head bh, head, *last_bh = NULL;
		int nr_underway = 0;

		BUG_ON(!PageLocked(page));
		@@ -1809,7 +1809,6 @@ static int __block_write_full_page(struct inode inode, struct page page,
		} while (bh != head);

		do {
		get_bh(bh);
		if (!buffer_mapped(bh))
		continue;
		/*
		@@ -1827,6 +1826,8 @@ static int __block_write_full_page(struct inode inode, struct page page,
		}
		if (test_clear_buffer_dirty(bh)) {
		mark_buffer_async_write(bh);
		get_bh(bh);
		last_bh = bh;
		} else {
		unlock_buffer(bh);
		}
		@@ -1845,10 +1846,13 @@ static int __block_write_full_page(struct inode inode, struct page page,
		if (buffer_async_write(bh)) {
		submit_bh(WRITE, bh);
		nr_underway++;
		}
		put_bh(bh);
		if (bh == last_bh)
		break;
		}
		bh = next;
		} while (bh != head);
		bh = head;

		err = 0;
		done:
		@@ -1887,10 +1891,11 @@ recover:
		bh = head;
		/* Recovery: lock and submit the mapped buffers */
		do {
		get_bh(bh);
		if (buffer_mapped(bh) && buffer_dirty(bh)) {
		lock_buffer(bh);
		mark_buffer_async_write(bh);
		get_bh(bh);
		last_bh = bh;
		} else {
		/*
		* The buffer may have been set dirty during
		@@ -1909,10 +1914,13 @@ recover:
		clear_buffer_dirty(bh);
		submit_bh(WRITE, bh);
		nr_underway++;
		}
		put_bh(bh);
		if (bh == last_bh)
		break;
		}
		bh = next;
		} while (bh != head);
		bh = head;
		goto done;
		}

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