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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <78180591-3f3e-f4e8-7618-de7250b479f0@linux.alibaba.com> Content-Type: text/plain; charset=gbk Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 3/17/20 6:13 AM, Xiaoguang Wang wrote: > 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? That'd be great, as long as we're ensuring that memory bloat doesn't become a problem. But never doubt that you can send patches to improve things - even if they sometimes don't get applied, they may help spark discussion that will end up leading to a great fix. -- Jens Axboe