Re: [PATCH 1/3] io_uring: add remote task_work execution helper

[Jens Axboe <axboe@kernel.dk>]

On 3/29/24 6:51 AM, Pavel Begunkov wrote:
> On 3/28/24 18:52, Jens Axboe wrote:
>> All our task_work handling is targeted at the state in the io_kiocb
>> itself, which is what it is being used for. However, MSG_RING rolls its
>> own task_work handling, ignoring how that is usually done.
>>
>> In preparation for switching MSG_RING to be able to use the normal
>> task_work handling, add io_req_task_work_add_remote() which allows the
>> caller to pass in the target io_ring_ctx and task.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>   io_uring/io_uring.c | 27 +++++++++++++++++++--------
>>   io_uring/io_uring.h |  2 ++
>>   2 files changed, 21 insertions(+), 8 deletions(-)
>>
>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>> index 9978dbe00027..609ff9ea5930 100644
>> --- a/io_uring/io_uring.c
>> +++ b/io_uring/io_uring.c
>> @@ -1241,9 +1241,10 @@ void tctx_task_work(struct callback_head *cb)
>>       WARN_ON_ONCE(ret);
>>   }
>>   -static inline void io_req_local_work_add(struct io_kiocb *req, unsigned tw_flags)
>> +static inline void io_req_local_work_add(struct io_kiocb *req,
>> +                     struct io_ring_ctx *ctx,
>> +                     unsigned tw_flags)
>>   {
>> -    struct io_ring_ctx *ctx = req->ctx;
>>       unsigned nr_wait, nr_tw, nr_tw_prev;
>>       unsigned long flags;
>>   @@ -1291,9 +1292,10 @@ static inline void io_req_local_work_add(struct io_kiocb *req, unsigned tw_flags
>>       wake_up_state(ctx->submitter_task, TASK_INTERRUPTIBLE);
>>   }
>>   -static void io_req_normal_work_add(struct io_kiocb *req)
>> +static void io_req_normal_work_add(struct io_kiocb *req,
>> +                   struct task_struct *task)
>>   {
>> -    struct io_uring_task *tctx = req->task->io_uring;
>> +    struct io_uring_task *tctx = task->io_uring;
>>       struct io_ring_ctx *ctx = req->ctx;
>>       unsigned long flags;
>>       bool was_empty;
>> @@ -1319,7 +1321,7 @@ static void io_req_normal_work_add(struct io_kiocb *req)
>>           return;
>>       }
>>   -    if (likely(!task_work_add(req->task, &tctx->task_work, ctx->notify_method)))
>> +    if (likely(!task_work_add(task, &tctx->task_work, ctx->notify_method)))
>>           return;
>>         io_fallback_tw(tctx, false);
>> @@ -1328,9 +1330,18 @@ static void io_req_normal_work_add(struct io_kiocb *req)
>>   void __io_req_task_work_add(struct io_kiocb *req, unsigned flags)
>>   {
>>       if (req->ctx->flags & IORING_SETUP_DEFER_TASKRUN)
>> -        io_req_local_work_add(req, flags);
>> +        io_req_local_work_add(req, req->ctx, flags);
>> +    else
>> +        io_req_normal_work_add(req, req->task);
>> +}
>> +
>> +void io_req_task_work_add_remote(struct io_kiocb *req, struct task_struct *task,
>> +                 struct io_ring_ctx *ctx, unsigned flags)
> 
> Urgh, even the declration screams that there is something wrong
> considering it _either_ targets @ctx or @task.
> 
> Just pass @ctx, so it either use ctx->submitter_task or
> req->task, hmm?

I actually since changed the above to use a common helper, so was
scratching my head a bit over your comment as it can't really work in
that setup without needing to check for whether ->submitter_task is set
or not. But I do agree this would be nicer, so I'll just return to using
the separate helpers for this and it should fall out nicely. The only
odd caller is the MSG_RING side, so makes sense to have it a bit more
separate rather than try and fold it in with the regular side of using
task_work.

> A side note, it's a dangerous game, I told it before. At least
> it would've been nice to abuse lockdep in a form of:
> 
> io_req_task_complete(req, tw, ctx) {
>     lockdep_assert(req->ctx == ctx);
>     ...
> }
> 
> but we don't have @ctx there, maybe we'll add it to tw later.

Agree, but a separate thing imho.

-- 
Jens Axboe