Re: 2.0, 2.2, 2.4, 2.5: fsync buffer race

[Andrew Morton <akpm@digeo.com>]

Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> wrote:
>
> Hi
> 
> there's a race condition in filesystem
> 
> let's have a two inodes that are placed in the same buffer.
> 
> call fsync on inode 1
> it goes down to ext2_update_inode [update == 1]
> it calls ll_rw_block at the end
> ll_rw_block starts to write buffer
> ext2_update_inode waits on buffer
> 
> while the buffer is writing, another process calls fsync on inode 2
> it goes again to ext2_update_inode
> it calls ll_rw_block
> ll_rw_block sees buffer locked and exits immediatelly
> ext2_update_inode waits for buffer
> the first write finished, ext2_update_inode exits and changes made by
> second proces to inode 2 ARE NOT WRITTEN TO DISK.
> 

hmm, yes.  This is a general weakness in the ll_rw_block() interface.  It is
not suitable for data-integrity writeouts, as you've pointed out.

A suitable fix would be do create a new

void wait_and_rw_block(...)
{
	wait_on_buffer(bh);
	ll_rw_block(...);
}

and go use that in all the appropriate places.

I shall make that change for 2.5, thanks.