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From: Jens Axboe <axboe@kernel.dk>
To: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>,
	io-uring@vger.kernel.org
Cc: joseph.qi@linux.alibaba.com
Subject: Re: [PATCH] io_uring: refacor file register/unregister/update based on sequence
Date: Mon, 23 Mar 2020 10:05:58 -0600
Message-ID: <bf9c7c16-76bb-7fd5-7190-63d8c6bb430a@kernel.dk> (raw)
In-Reply-To: <20200323115036.6539-1-xiaoguang.wang@linux.alibaba.com>

On 3/23/20 5:50 AM, Xiaoguang Wang wrote:
> While diving into iouring fileset resigster/unregister/update codes,
> we found one bug in fileset update codes. Iouring fileset update codes
> use a percpu_ref variable to check whether can put previous registered
> file, only when the refcnt of the perfcpu_ref variable reachs zero, can
> we safely put these files, but this do not work well. If applications
> always issue requests continually, this perfcpu_ref will never have an
> chance to reach zero, and it'll always be in atomic mode, also will
> defeat the gains introduced by fileset register/unresiger/update feature,
> which are used to reduce the atomic operation overhead of fput/fget.
> 
> To fix this issue, we remove the percpu_ref related codes, and add two new
> counter: sq_seq and cq_seq to struct io_ring_ctx:
>     sq_seq: the most recent issued requset sequence number, which is
>             protected uring_lock.
>     cq_seq: the most recent completed request sequence number, which is
>             protected completion_lock.
> 
> When we update fileset(under uring_lock), we record the current sq_seq,
> and when cq_seq is greater or equal to recorded sq_seq, we know we can
> put previous registered file safely.

Maybe I'm misunderstanding the idea here, but what if you have the
following:

- sq_seq 200, cq_seq 100

We have 100 inflight, and an unregister request comes in. I then
issue 100 nops, which complete. cq_seq is now 200, but none of the
original requests that used the file have completed.

What am I missing?

-- 
Jens Axboe


  parent reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-23 11:50 Xiaoguang Wang
2020-03-23 12:02 ` Xiaoguang Wang
2020-03-23 16:05 ` Jens Axboe [this message]
2020-03-23 16:45   ` Xiaoguang Wang
2020-03-23 16:49     ` Jens Axboe

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