Re: bug report about patch "io_uring: avoid ring quiesce for fixed file set unregister and update"

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> hi,
> 
> While diving into iouring file register/unregister/update codes, seems that
> there is one bug in __io_sqe_files_update():
>      if (ref_switch)
>          percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch);
> 
> The initial fixed_file_data's refs is 1, assume there are no requests
> to get/put this refs, and we firstly register 10 files and later update
> these 10 files, and no memory allocations fails, then above two line of
> codes in __io_sqe_files_update() will be called, before entering
> percpu_ref_switch_to_atomic(), the count of refs is still one, and
> |--> percpu_ref_switch_to_atomic
> |----> __percpu_ref_switch_mode
> |------> __percpu_ref_switch_to_atomic
> |-------- > percpu_ref_get(ref), # now the count of refs will be 2.
> 
> a while later
> |--> percpu_ref_switch_to_atomic_rcu
> |----> percpu_ref_call_confirm_rcu
> |------ > confirm_switch(), # calls io_atomic_switch, note that the count of refs is 2.
> |------ > percpu_ref_put # drop one ref
> 
> static void io_atomic_switch(struct percpu_ref *ref)
> {
>      struct fixed_file_data *data;
> 
>      /*
>       * Juggle reference to ensure we hit zero, if needed, so we can
>       * switch back to percpu mode
>       */
>      data = container_of(ref, struct fixed_file_data, refs);
>      percpu_ref_put(&data->refs);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> After this operation, the count of refs is 1 now, still not zero, so
> io_file_data_ref_zero won't be called, then io_ring_file_ref_flush()
> won't be called, this fixed_file_data's refs will always be in atomic mode,
> which is bad.
> 
>      percpu_ref_get(&data->refs);
> }
Even don't consider below pfile memory allocation failure, because before entering
io_atomic_switch(), the count of refs is 2, the percpu_ref_put calls in io_atomic_switch
makes the count of refs to be 1, so io_file_data_ref_zero  still won't be called, and
refs will always be in atomic mode? Normal reqs just add/dec 1 to the refs, so the cout
of refs will never have a chance to reach zero.

What I worry is that when we finish __io_sqe_files_update, whether fixed_file_data's refs
will always be in atomic mode.

Regards,
Xiaoguang Wang
> 
> To confirm this bug, I did a hack to kernel:
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -5812,7 +5812,10 @@ static bool io_queue_file_removal(struct fixed_file_data *data,
>           * If we fail allocating the struct we need for doing async reomval
>           * of this file, just punt to sync and wait for it.
>           */
> +       /*
>          pfile = kzalloc(sizeof(*pfile), GFP_KERNEL);
> +       */
> +       pfile = NULL;
>          if (!pfile) {
>                  pfile = &pfile_stack;
>                  pfile->done = &done;
> To simulate memory allocation failures, then run liburing/test/file-update,
> 
> [lege@localhost test]$ sudo cat /proc/2091/stack
> [sudo] password for lege:
> [<0>] __io_sqe_files_update.isra.85+0x175/0x330
> [<0>] __io_uring_register+0x178/0xe20
> [<0>] __x64_sys_io_uring_register+0xa0/0x160
> [<0>] do_syscall_64+0x55/0x1b0
> [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> (gdb) list * __io_sqe_files_update+0x175
> 0xffffffff812ec255 is in __io_sqe_files_update (fs/io_uring.c:5830).
> 5825            llist_add(&pfile->llist, &data->put_llist);
> 5826
> 5827            if (pfile == &pfile_stack) {
> 5828                    percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch);
> 5829                    wait_for_completion(&done);
> 5830                    flush_work(&data->ref_work);
> 5831                    return false;
> 
> file-update will always hang in wait_for_completion(&done), it's because
> io_ring_file_ref_flush never has a chance to run.
> 
> I think how to fix this issue a while, doesn't find a elegant method yet.
> And applications may issue requests continuously, then fixed_file_data's refs
> may never have a chance to reach zero, refs will always be in atomic mode.
> Or the simplest method is to use percpu_ref per registered file :)
> 
> Regards,
> Xiaoguang Wang
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> Regards,
> Xiaoguang Wang