Re: task hung in ext4_fallocate #2

[Jens Axboe <axboe@kernel.dk>]

On 10/24/23 6:06 PM, Dave Chinner wrote:
> On Tue, Oct 24, 2023 at 12:35:26PM -0600, Jens Axboe wrote:
>> On 10/24/23 8:30 AM, Jens Axboe wrote:
>>> I don't think this is related to the io-wq workers doing non-blocking
>>> IO.
> 
> The io-wq worker that has deadlocked _must_ be doing blocking IO. If
> it was doing non-blocking IO (i.e. IOCB_NOWAIT) then it would have
> done a trylock and returned -EAGAIN to the worker for it to try
> again later. I'm not sure that would avoid the issue, however - it
> seems to me like it might just turn it into a livelock rather than a
> deadlock....

Sorry typo, yes they are doing blocking IO, that's all they ever do. My
point is that it's not related to the issue.

>>> The callback is eventually executed by the task that originally
>>> submitted the IO, which is the owner and not the async workers. But...
>>> If that original task is blocked in eg fallocate, then I can see how
>>> that would potentially be an issue.
>>>
>>> I'll take a closer look.
>>
>> I think the best way to fix this is likely to have inode_dio_wait() be
>> interruptible, and return -ERESTARTSYS if it should be restarted. Now
>> the below is obviously not a full patch, but I suspect it'll make ext4
>> and xfs tick, because they should both be affected.
> 
> How does that solve the problem? Nothing will issue a signal to the
> process that is waiting in inode_dio_wait() except userspace, so I
> can't see how this does anything to solve the problem at hand...

Except task_work, which when it completes, will increment the i_dio
count again. This is the whole point of the half assed patch I sent out.

> I'm also very leary of adding new error handling complexity to paths
> like truncate, extent cloning, fallocate, etc which expect to block
> on locks until they can perform the operation safely.

I actually looked at all of them, ext4 and xfs specifically. It really
doesn't seem to bad.

> On further thinking, this could be a self deadlock with
> just async direct IO submission - submit an async DIO with
> IOCB_CALLER_COMP, then run an unaligned async DIO that attempts to
> drain in-flight DIO before continuing. Then the thread waits in
> inode_dio_wait() because it can't run the completion that will drop
> the i_dio_count to zero.

No, because those will be non-blocking. Any blocking IO will go via
io-wq, and that won't then hit the deadlock. If you're doing
inode_dio_wait() from the task itself for a non-blocking issue, then
that would surely be an issue. But we should not be doing that, and we
are checking for it.

> Hence it appears to me that we've missed some critical constraints
> around nesting IO submission and completion when using
> IOCB_CALLER_COMP. Further, it really isn't clear to me how deep the
> scope of this problem is yet, let alone what the solution might be.

I think you're missing exactly what the deadlock is.

> With all this in mind, and how late this is in the 6.6 cycle, can we
> just revert the IOCB_CALLER_COMP changes for now?

Yeah I'm going to do a revert of the io_uring side, which effectively
disables it. Then a revised series can be done, and when done, we could
bring it back.

-- 
Jens Axboe