Re: [PATCH V3] io_uring: consider the overflow of sequence for timeout req

[Jens Axboe <axboe@kernel.dk>]

On 10/15/19 7:35 PM, yangerkun wrote:
> 
> 
> On 2019/10/15 21:59, yangerkun wrote:
>> Now we recalculate the sequence of timeout with 'req->sequence =
>> ctx->cached_sq_head + count - 1', judge the right place to insert
>> for timeout_list by compare the number of request we still expected for
>> completion. But we have not consider about the situation of overflow:
>>
>> 1. ctx->cached_sq_head + count - 1 may overflow. And a bigger count for
>> the new timeout req can have a small req->sequence.
>>
>> 2. cached_sq_head of now may overflow compare with before req. And it
>> will lead the timeout req with small req->sequence.
>>
>> This overflow will lead to the misorder of timeout_list, which can lead
>> to the wrong order of the completion of timeout_list. Fix it by reuse
>> req->submit.sequence to store the count, and change the logic of
>> inserting sort in io_timeout.
>>
>> Signed-off-by: yangerkun <yangerkun@huawei.com>
>> ---
>>    fs/io_uring.c | 27 +++++++++++++++++++++------
>>    1 file changed, 21 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 76fdbe84aff5..c9512da06973 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -1884,7 +1884,7 @@ static enum hrtimer_restart io_timeout_fn(struct hrtimer *timer)
>>    
>>    static int io_timeout(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>    {
>> -	unsigned count, req_dist, tail_index;
>> +	unsigned count;
>>    	struct io_ring_ctx *ctx = req->ctx;
>>    	struct list_head *entry;
>>    	struct timespec64 ts;
>> @@ -1907,21 +1907,36 @@ static int io_timeout(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>    		count = 1;
>>    
>>    	req->sequence = ctx->cached_sq_head + count - 1;
>> +	/* reuse it to store the count */
>> +	req->submit.sequence = count;
>>    	req->flags |= REQ_F_TIMEOUT;
>>    
>>    	/*
>>    	 * Insertion sort, ensuring the first entry in the list is always
>>    	 * the one we need first.
>>    	 */
>> -	tail_index = ctx->cached_cq_tail - ctx->rings->sq_dropped;
>> -	req_dist = req->sequence - tail_index;
>>    	spin_lock_irq(&ctx->completion_lock);
>>    	list_for_each_prev(entry, &ctx->timeout_list) {
>>    		struct io_kiocb *nxt = list_entry(entry, struct io_kiocb, list);
>> -		unsigned dist;
>> +		unsigned nxt_sq_head;
>> +		long long tmp, tmp_nxt;
>>    
>> -		dist = nxt->sequence - tail_index;
>> -		if (req_dist >= dist)
>> +		/*
>> +		 * Since cached_sq_head + count - 1 can overflow, use type long
>> +		 * long to store it.
>> +		 */
>> +		tmp = (long long)ctx->cached_sq_head + count - 1;
>> +		nxt_sq_head = nxt->sequence - nxt->submit.sequence + 1;
>> +		tmp_nxt = (long long)nxt_sq_head + nxt->submit.sequence - 1;
>> +
>> +		/*
>> +		 * cached_sq_head may overflow, and it will never overflow twice
>> +		 * once there is some timeout req still be valid.
>> +		 */
>> +		if (ctx->cached_sq_head < nxt_sq_head)
>> +			tmp_nxt += UINT_MAX;
> 
> Maybe there is a mistake, it should be tmp. So sorry about this.

I ran it through the basic testing, but I guess it doesn't catch overflow
cases. Maybe we can come up with one? Should be pretty simple to setup a
io_uring, post UINT_MAX - 10 nops (or something like that), then do some
timeout testing.

Just send an incremental patch to fix it.

-- 
Jens Axboe