From ad576e63e0c8b274a8558b8e05a6d0526e804dc0 Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Thu, 5 May 2005 16:15:46 -0700 Subject: [PATCH] [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 Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/buffer.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/fs/buffer.c b/fs/buffer.c index af7c51d..bc75f2e 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -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; } - put_bh(bh); 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; } - put_bh(bh); bh = next; } while (bh != head); + bh = head; goto done; } -- 2.7.4