Re: [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Jens Axboe <axboe@kernel.dk>]

On 4/5/22 9:57 PM, Ming Lei wrote:
> On Tue, Apr 05, 2022 at 08:20:24PM -0600, Jens Axboe wrote:
>> On 4/3/22 5:45 AM, Ming Lei wrote:
>>> -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is
>>> set in io_complete_rw_iopoll(), but the req never gets chance to be handled.
>>> io_iopoll_check doesn't handle this situation, and io hang can be caused.
>>>
>>> Current dm io polling may return -EAGAIN after bio submission is
>>> returned, also blk-throttle might trigger this situation too.
>>
>> I don't think this is necessarily safe. Handling REQ_F_ISSUE from within
>> the issue path is fine, as the request hasn't been submitted yet and
>> hence we know that passed in structs are still stable. Once you hit it
>> when polling for it, the io_uring_enter() call to submit requests has
>> potentially already returned, and now we're in a second call where we
>> are polling for requests. If we're doing eg an IORING_OP_READV, the
>> original iovec may no longer be valid and we cannot safely re-import
>> data associated with it.
> 
> Yeah, this reissue is really not safe, thanks for the input.
> 
> I guess the only way is to complete the cqe for this situation.

At least if

io_op_defs[req->opcode].needs_async_setup

is true it isn't safe. But can't dm appropriately retry rather than
bubble up the -EAGAIN off ->iopoll?

-- 
Jens Axboe