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Date:   Mon, 12 Jun 2023 21:46:33 +0800
MIME-Version: 1.0
Subject: Re: [PATCH 07/11] io_uring: add new api to register fixed workers
To:     Ammar Faizi <ammarfaizi2@gnuweeb.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Linux Fsdevel Mailing List <linux-fsdevel@vger.kernel.org>,
        io-uring Mailing List <io-uring@vger.kernel.org>
References: <20230609122031.183730-1-hao.xu@linux.dev>
 <20230609122031.183730-8-hao.xu@linux.dev>
 <ZIMkGh3kI0tfoxxp@biznet-home.integral.gnuweeb.org>
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From:   Hao Xu <hao.xu@linux.dev>
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Hi Ammar,

On 6/9/23 21:07, Ammar Faizi wrote:
> On Fri, Jun 09, 2023 at 08:20:27PM +0800, Hao Xu wrote:
>> +	rcu_read_lock();
>> +
>> +	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
>> +		unsigned int nr = count[i].nr_workers;
>> +
>> +		acct = &wq->acct[i];
>> +		acct->fixed_nr = nr;
>> +		acct->fixed_workers = kcalloc(nr, sizeof(struct io_worker *),
>> +					      GFP_KERNEL);
>> +		if (!acct->fixed_workers) {
>> +			ret = -ENOMEM;
>> +			break;
>> +		}
>> +
>> +		for (j = 0; j < nr; j++) {
>> +			struct io_worker *worker =
>> +				io_wq_create_worker(wq, acct, true);
>> +			if (IS_ERR(worker)) {
>> +				ret = PTR_ERR(worker);
>> +				break;
>> +			}
>> +			acct->fixed_workers[j] = worker;
>> +		}
>> +		if (j < nr)
>> +			break;
>> +	}
>> +	rcu_read_unlock();
> 
> This looks wrong. kcalloc() with GFP_KERNEL may sleep. Note that you're
> not allowed to sleep inside the RCU read lock critical section.
> 
> Using GFP_KERNEL implies GFP_RECLAIM, which means that direct reclaim
> may be triggered under memory pressure; the calling context must be
> allowed to sleep.
> 

I think you are right, I'll fix it in v2.

Hi Jens, ask a question about this: I saw same rcu_read_lock() in 
io_wq_max_workers(), but what is it really protect?

Regards,
Hao