Re: [PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/25/23 14:44, Jens Axboe wrote:
> On 10/25/23 6:09 AM, Pavel Begunkov wrote:
>> On 10/23/23 16:27, Jens Axboe wrote:
>>> On 10/23/23 9:17 AM, Gabriel Krisman Bertazi wrote:
>>>> Jens Axboe <axboe@kernel.dk> writes:
>>>>
>>>>> We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
>>>>> dereference. Park the SQPOLL thread while getting the task cpu and pid for
>>>>> fdinfo, this ensures we have a stable view of it.
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>
>>>>> ---
>>>>>
>>>>> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
>>>>> index c53678875416..cd2a0c6b97c4 100644
>>>>> --- a/io_uring/fdinfo.c
>>>>> +++ b/io_uring/fdinfo.c
>>>>> @@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
>>>>>    __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>    {
>>>>>        struct io_ring_ctx *ctx = f->private_data;
>>>>> -    struct io_sq_data *sq = NULL;
>>>>>        struct io_overflow_cqe *ocqe;
>>>>>        struct io_rings *r = ctx->rings;
>>>>>        unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
>>>>> @@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>        unsigned int cq_shift = 0;
>>>>>        unsigned int sq_shift = 0;
>>>>>        unsigned int sq_entries, cq_entries;
>>>>> +    int sq_pid = -1, sq_cpu = -1;
>>>>>        bool has_lock;
>>>>>        unsigned int i;
>>>>>    @@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>        has_lock = mutex_trylock(&ctx->uring_lock);
>>>>>          if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>>>>> -        sq = ctx->sq_data;
>>>>> -        if (!sq->thread)
>>>>> -            sq = NULL;
>>>>> +        struct io_sq_data *sq = ctx->sq_data;
>>>>> +
>>>>> +        io_sq_thread_park(sq);
>>>>> +        if (sq->thread) {
>>>>> +            sq_pid = task_pid_nr(sq->thread);
>>>>> +            sq_cpu = task_cpu(sq->thread);
>>>>> +        }
>>>>> +        io_sq_thread_unpark(sq);
>>>>
>>>> Jens,
>>>>
>>>> io_sq_thread_park will try to wake the sqpoll, which is, at least,
>>>> unnecessary. But I'm thinking we don't want to expose the ability to
>>>> schedule the sqpoll from procfs, which can be done by any unrelated
>>>> process.
>>>>
>>>> To solve the bug, it should be enough to synchronize directly on
>>>> sqd->lock, preventing sq->thread from going away inside the if leg.
>>>> Granted, it is might take longer if the sqpoll is busy, but reading
>>>> fdinfo is not supposed to be fast.  Alternatively, don't call
>>>> wake_process in this case?
>>>
>>> I did think about that but just went with the exported API. But you are
>>> right, it's a bit annoying that it'd also wake the thread, in case it
>>
>> Waking it up is not a problem but without parking sq thread won't drop
>> the lock until it's time to sleep, which might be pretty long leaving
>> the /proc read stuck on the lock uninterruptibly.
>>
>> Aside from parking vs lock, there is a lock inversion now:
>>
>> proc read                   | SQPOLL
>>                              |
>> try_lock(ring) // success   |
>>                              | woken up
>>                              | lock(sqd); // success
>> lock(sqd); // stuck         |
>>                              | try to submit requests
>>                              | -- lock(ring); // stuck
> 
> Yeah good point, forgot we nest these opposite of what you'd expect.
> Honestly I think the fix here is just to turn it into a trylock. Yes
> that'll miss some cases where we could've gotten the pid/cpu, but
> doesn't seem worth caring about.
> 
> IOW, fold in this incremental.

Should work, otherwise you probably can just park first.

Long term it'd be nice to make sqpoll to not hold sqd->lock during
submission + polling as it currently does. Park callers sleep on the
lock, but if we replace it with some struct completion the rest
should be easy enough.


> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
> index af1bdcc0703e..f04a43044d91 100644
> --- a/io_uring/fdinfo.c
> +++ b/io_uring/fdinfo.c
> @@ -145,12 +145,13 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>   	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>   		struct io_sq_data *sq = ctx->sq_data;
>   
> -		mutex_lock(&sq->lock);
> -		if (sq->thread) {
> -			sq_pid = task_pid_nr(sq->thread);
> -			sq_cpu = task_cpu(sq->thread);
> +		if (mutex_trylock(&sq->lock)) {
> +			if (sq->thread) {
> +				sq_pid = task_pid_nr(sq->thread);
> +				sq_cpu = task_cpu(sq->thread);
> +			}
> +			mutex_unlock(&sq->lock);
>   		}
> -		mutex_unlock(&sq->lock);
>   	}
>   
>   	seq_printf(m, "SqThread:\t%d\n", sq_pid);
> 

-- 
Pavel Begunkov