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

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

[yangerkun]

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;
+
+		if (tmp >= tmp_nxt)
 			break;
 	}
 	list_add(&req->list, entry);
-- 
2.17.2

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

[Jens Axboe]

On 10/15/19 7:59 AM, 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.

Thanks, this looks great. Applied for 5.4.

-- 
Jens Axboe

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

[yangerkun]

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.

> +
> +		if (tmp >= tmp_nxt)
>   			break;
>   	}
>   	list_add(&req->list, entry);
> 

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

[Jens Axboe]

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

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

[yangerkun]

On 2019/10/16 9:45, Jens Axboe wrote:
> 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.
> 
Good idea! I will try to add a testcase for this in liburing.

> Just send an incremental patch to fix it.

OK, will send the fix patch!

>