[patch] fix the 2nd buffer race properly

Nick Piggin
2005-04-27T13:16:34Z

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OK, so I found the exact cause of the 2nd buffer problem.

Surprisingly, the first patch I sent was exactly what is
needed. Surprising because I didn't have a full handle on
the problem so perhaps I got a bit lucky.

The bug (the reason I asked you to drop the patch just now)
was that the code previously did a get_bh on all bh's in a
page, but I changed it to only put_bh the ones to be written.

The minor fix for that was to only get_bh the buffer heads to
be written.

Exact problem is described in the patch changelog. Anyone who
is feeling brave please review because I'm tired and have a
headache from too much kernel debugging ;)

Tested and seems to work. Doesn't seem to leak memory.

Nick

--
SUSE Labs, Novell Inc.

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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 requests
rather than relying on testing all of them for buffer_async_write - because
by the time we submit the last buffer we have marked async_write, we no
longer own *any* of the buffers.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>

Index: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c 2005-04-27 22:43:05.000000000 +1000
+++ linux-2.6/fs/buffer.c 2005-04-27 22:45:03.000000000 +1000
@@ -1750,8 +1750,9 @@ static int __block_write_full_page(struc
int err;
sector_t block;
sector_t last_block;
- struct buffer_head *bh, *head;
- int nr_underway = 0;
+ struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+ int idx = 0;
+ int nr_underway;

BUG_ON(!PageLocked(page));

@@ -1808,7 +1809,6 @@ static int __block_write_full_page(struc
} while (bh != head);

do {
- get_bh(bh);
if (!buffer_mapped(bh))
continue;
/*
@@ -1826,6 +1826,8 @@ static int __block_write_full_page(struc
}
if (test_clear_buffer_dirty(bh)) {
mark_buffer_async_write(bh);
+ get_bh(bh);
+ arr[idx++] = bh;
} else {
unlock_buffer(bh);
}
@@ -1839,15 +1841,12 @@ static int __block_write_full_page(struc
set_page_writeback(page);
unlock_page(page);

- do {
- struct buffer_head *next = bh->b_this_page;
- if (buffer_async_write(bh)) {
- submit_bh(WRITE, bh);
- nr_underway++;
- }
+ for (nr_underway = 0; nr_underway < idx; nr_underway++) {
+ bh = arr[nr_underway];
+ BUG_ON(!buffer_async_write(bh));
+ submit_bh(WRITE, bh);
put_bh(bh);
- bh = next;
- } while (bh != head);
+ }

err = 0;
done:
@@ -1886,10 +1885,11 @@ 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);
+ arr[idx++] = bh;
} else {
/*
* The buffer may have been set dirty during
@@ -1902,16 +1902,13 @@ recover:
BUG_ON(PageWriteback(page));
set_page_writeback(page);
unlock_page(page);
- do {
- struct buffer_head *next = bh->b_this_page;
- if (buffer_async_write(bh)) {
- clear_buffer_dirty(bh);
- submit_bh(WRITE, bh);
- nr_underway++;
- }
+ for (nr_underway = 0; nr_underway < idx; nr_underway++) {
+ bh = arr[nr_underway];
+ BUG_ON(!buffer_async_write(bh));
+ clear_buffer_dirty(bh);
+ submit_bh(WRITE, bh);
put_bh(bh);
- bh = next;
- } while (bh != head);
+ }
goto done;
}

@@ -2741,6 +2738,7 @@ sector_t generic_block_bmap(struct addre
static int end_bio_bh_io_sync(struct bio *bio, unsigned int bytes_done, int err)
{
struct buffer_head *bh = bio->bi_private;
+ bh_end_io_t *end_fn;

if (bio->bi_size)
return 1;
@@ -2750,7 +2748,16 @@ static int end_bio_bh_io_sync(struct bio
set_bit(BH_Eopnotsupp, &bh->b_state);
}

- bh->b_end_io(bh, test_bit(BIO_UPTODATE, &bio->bi_flags));
+ end_fn = bh->b_end_io;
+
+ /*
+ * These two lines are debugging only - make sure b_end_io
+ * isn't run twice for the same io request.
+ */
+ BUG_ON(!end_fn);
+ bh->b_end_io = NULL;
+
+ end_fn(bh, test_bit(BIO_UPTODATE, &bio->bi_flags));
bio_put(bio);
return 0;
}