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SubjectRe: [RFC] Is it correctly that the usage for spin_{lock|unlock}_irq in clear_page_dirty_for_io
On Mon 02-04-18 19:50:50, Wang Long wrote:
>
> Hi,  Johannes Weiner and Tejun Heo
>
> I use linux-4.4.y to test the new cgroup controller io and the current
> stable kernel linux-4.4.y has the follow logic
>
>
> int clear_page_dirty_for_io(struct page *page){
> ...
> ...
>                 memcg = mem_cgroup_begin_page_stat(page); ----------(a)
>                 wb = unlocked_inode_to_wb_begin(inode, &locked); ---------(b)
>                 if (TestClearPageDirty(page)) {
>                         mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_DIRTY);
>                         dec_zone_page_state(page, NR_FILE_DIRTY);
>                         dec_wb_stat(wb, WB_RECLAIMABLE);
>                         ret =1;
>                 }
>                 unlocked_inode_to_wb_end(inode, locked); -----------(c)
>                 mem_cgroup_end_page_stat(memcg); -------------(d)
>                 return ret;
> ...
> ...
> }
>
>
> when memcg is moving, and I_WB_SWITCH flags for inode is set. the logic
> is the following:
>
>
> spin_lock_irqsave(&memcg->move_lock, flags); -------------(a)
>         spin_lock_irq(&inode->i_mapping->tree_lock); ------------(b)
>         spin_unlock_irq(&inode->i_mapping->tree_lock); -----------(c)
> spin_unlock_irqrestore(&memcg->move_lock, flags); -----------(d)
>
>
> after (c) , the local irq is enabled. I think it is not correct.
>
> We get a deadlock backtrace after (c), the cpu get an softirq and in the
> irq it also call mem_cgroup_begin_page_stat to lock the same
> memcg->move_lock.
>
> Since the conditions are too harsh, this scenario is difficult to
> reproduce.  But it really exists.
>
> So how about change (b) (c) to spin_lock_irqsave/spin_lock_irqrestore?

Yes, it seems we really need this even for the current tree. Please note
that At least clear_page_dirty_for_io doesn't lock memcg anymore.
__cancel_dirty_page still uses lock_page_memcg though (former
mem_cgroup_begin_page_stat).
--
Michal Hocko
SUSE Labs

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