Hello!
On 2023/6/26 21:09, Jan Kara wrote:
Hello!
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.
Honza
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.
--
With Best Regards,
Baokun Li
.
