Re: [PATCH] quota: fix race condition between dqput() and dquot_mark_dquot_dirty()

[Jan Kara]

Hello!

On Mon 26-06-23 21:55:49, Baokun Li wrote:
> On 2023/6/26 21:09, Jan Kara wrote:
> > On Sun 25-06-23 15:56:10, Baokun Li wrote:
> > > > > I think we can simply focus on the race between the DQ_ACTIVE_B flag and
> > > > > the DQ_MOD_B flag, which is the core problem, because the same quota
> > > > > should not have both flags. These two flags are protected by dq_list_lock
> > > > > and dquot->dq_lock respectively, so it makes sense to add a
> > > > > wait_on_dquot() to ensure the accuracy of DQ_ACTIVE_B.
> > > > But the fundamental problem is not only the race with DQ_MOD_B setting. The
> > > > dquot structure can be completely freed by the time
> > > > dquot_claim_space_nodirty() calls dquot_mark_dquot_dirty() on it. That's
> > > > why I think making __dquot_transfer() obey dquot_srcu rules is the right
> > > > solution.
> > > Yes, now I also think that making __dquot_transfer() obey dquot_srcu
> > > rules is a better solution. But with inode->i_lock protection, why would
> > > the dquot structure be completely freed?
> > Well, when dquot_claim_space_nodirty() calls mark_all_dquot_dirty() it does
> > not hold any locks (only dquot_srcu). So nothing prevents dquot_transfer()
> > to go, swap dquot structure pointers and drop dquot references and after
> > that mark_all_dquot_dirty() can use a stale pointer to call
> > mark_dquot_dirty() on already freed memory.
> > 
> No, this doesn't look like it's going to happen.  The
> mark_all_dquot_dirty() uses a pointer array pointer, the dquot in the
> array is dynamically changing, so after swap dquot structure pointers,
> mark_all_dquot_dirty() uses the new pointer, and the stale pointer is
> always destroyed after swap, so there is no case of using the stale
> pointer here.

There is a case - CPU0 can prefetch the values from dquots[] array into its
local cache, then CPU1 can update the dquots[] array (these writes can
happily stay in CPU1 store cache invisible to other CPUs) and free the
dquots via dqput(). Then CPU0 can pass the prefetched dquot pointers to
mark_dquot_dirty(). There are no locks or memory barries preventing CPUs
from ordering instructions and memory operations like this in the code...
You can read Documentation/memory-barriers.txt about all the perils current
CPU architecture brings wrt coordination of memory accesses among CPUs ;)

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR