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

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

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);
}

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



















Regards,
Xiaoguang Wang