hi,
On 3/16/20 6:14 AM, Xiaoguang Wang wrote:
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 :)
For the "oh crap I can't allocate data" stack path, I think the below
should fix it. Might not be a bad idea to re-think the live updates in
general, though.
I'm not a native english speaker and afraid that I may misread your replies :)
So I'd like to confirm that do you mind that I implement a percpu_ref per
registered file to track every registered file's status?
Regards,
Xiaoguang Wang
diff --git a/fs/io_uring.c b/fs/io_uring.c
index b1fbc4424aa6..3f0c8291a17c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5612,10 +5612,12 @@ static void io_ring_file_ref_flush(struct fixed_file_data *data)
while ((node = llist_del_all(&data->put_llist)) != NULL) {
llist_for_each_entry_safe(pfile, tmp, node, llist) {
io_ring_file_put(data->ctx, pfile->file);
- if (pfile->done)
+ if (pfile->done) {
+ percpu_ref_get(&data->refs);
complete(pfile->done);
- else
+ } else {
kfree(pfile);
+ }
}
}
}
@@ -5830,6 +5836,7 @@ static bool io_queue_file_removal(struct fixed_file_data *data,
llist_add(&pfile->llist, &data->put_llist);
if (pfile == &pfile_stack) {
+ percpu_ref_put(&data->refs);
percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch);
wait_for_completion(&done);
flush_work(&data->ref_work);
