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[PATCH] __block_write_full_page race fix

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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>
This commit is contained in:
Nick Piggin 2005-05-05 16:15:46 -07:00 committed by Linus Torvalds
parent f3ddbdc626
commit ad576e63e0
1 changed files with 13 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
fs/buffer.c
View File

@ -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;
}