Re: [PATCH 2/2] io_uring: fix the judging condition in io_sequence_defer

[Jens Axboe <axboe@kernel.dk>]

On 7/14/19 8:34 PM, Zhengyuan Liu wrote:
> 
> On 7/14/19 5:43 AM, Jens Axboe wrote:
>> On 7/12/19 9:58 PM, Zhengyuan Liu wrote:
>>> sq->cached_sq_head and cq->cached_cq_tail are both unsigned int.
>>> if cached_sq_head gets overflowed before cached_cq_tail, then we
>>> may miss a barrier req. As cached_cq_tail moved always following
>>> cached_sq_head, the NQ should be enough.
>> This (and the previous patch) looks fine to me, just wondering if
>> you had a test case showing this issue?
>>
> Hi.
> 
> Actually I don't have a real test case,  but I have emulated a test case
> and showed the issue.
> 
> Let's preset those count to close overflowed status at the begin:
> 
>         @@ -2979,6 +2987,10 @@ static int io_allocate_scq_urings(struct
> io_ring_ctx *ctx,
>             cq_ring->ring_entries = p->cq_entries;
>             ctx->cq_mask = cq_ring->ring_mask;
>             ctx->cq_entries = cq_ring->ring_entries;
> +
> +          ctx->cached_sq_head = ctx->sq_ring->r.head =
> ctx->sq_ring->r.tail = 0xfffffff8;
> +          ctx->cached_cq_tail = ctx->cq_ring->r.head =
> ctx->cq_ring->r.tail = 0xfffffff8
> +
>             return 0;
>      }
> 
> And queue serveral sqes  following a io_uring_enter like this:
> 
>       write1 write2 ... write8  barrier write9 write10
> 
> Then we can miss the barrier almost every time.

No problem, just curious if you had a test case, since then I'd add
it to the arsenal.

I've applied this one, thank you.

-- 
Jens Axboe