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

[Jens Axboe <axboe@kernel.dk>]

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
was idle. Probably mostly cosmetic, but we may as well just stick with
grabbing the sqd mutex. I'll send a v2.

-- 
Jens Axboe