On Tue 27-06-23 17:08:27, Baokun Li wrote: > Hello! > > On 2023/6/27 16:34, Jan Kara wrote: > > 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 > > Got it! > > Sorry for misunderstanding you (I thought "completely freed" meant > dquot_destroy(), but you should have meant dquot_release()). Well, the dquot can even get to dquot_destroy(). There's nothing really preventing CPU2 going into memory reclaim and free the dquot in dqcache_shrink_scan() still before CPU0 even calls mark_dquot_dirty() on it. Sure such timing on real hardware is very unlikely but in a VM where a virtual CPU can get starved for a significant amount of time this could happen. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
