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

[Jens Axboe <axboe@kernel.dk>]

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.

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);

-- 
Jens Axboe