Re: [PATCH v2] fs/fs-writeback: wait isw_nr_in_flight to be zero when umount

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Hello,

On Wed, Apr 17, 2019 at 09:04:48AM +0800, Jiufei Xue wrote:
> Yes, it can be fixed if we replace synchronize_rcu() with rcu_barrier().
> However, I'm worried that rcu_barrier() is too heavyweight and we have
> encountered some hung tasks that rcu_barrier() waiting for callbacks that
> other drivers queued but not handled correctly.

rcu_barrier() wait for the pending callbacks to finish and none of the
callbacks can block, so I don't think it'd be much worse than
synchronize_rcu().  Also, it'd probably make sense to inc
isw_nr_in_flight after call_rcu() in inode_switch_wbs().  Given that
all inodes must be gone by umount, the actual race window isn't there
but that ordering still makes a lot more sense.

Thanks.

-- 
tejun




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