On Sat, 29 Jan 2022 09:07:05 +0100, Jia-Ju Bai wrote: > > > > On 2022/1/29 12:27, Takashi Sakamoto wrote: > > Hi, > > > > On Sat, Jan 29, 2022 at 11:33:26AM +0800, Jia-Ju Bai wrote: > >> Hello, > >> > >> My static analysis tool reports a possible deadlock in the sound driver > >> in Linux 5.10: > >> > >> snd_card_disconnect_sync() > >> spin_lock_irq(&card->files_lock); --> Line 461 (Lock A) > >> wait_event_lock_irq(card->remove_sleep, ...); --> Line 462 (Wait X) > >> spin_unlock_irq(&card->files_lock); --> Line 465 (Unlock A) > >> > >> snd_hwdep_release() > >> mutex_lock(&hw->open_mutex); --> Line 152 (Lock B) > >> mutex_unlock(&hw->open_mutex); --> Line 157 (Unlock B) > >> snd_card_file_remove() > >> wake_up_all(&card->remove_sleep); --> Line 976 (Wake X) > >> > >> snd_hwdep_open() > >> mutex_lock(&hw->open_mutex); --> Line 95 (Lock B) > >> snd_card_file_add() > >> spin_lock(&card->files_lock); --> Line 932 (Lock A) > >> spin_unlock(&card->files_lock); --> Line 940 (Unlock A) > >> mutex_unlock(&hw->open_mutex); --> Line 139 (Unlock B) > >> > >> When snd_card_disconnect_sync() is executed, "Wait X" is performed by > >> holding "Lock A". If snd_hwdep_open() is executed at this time, it holds > >> "Lock B" and then waits for acquiring "Lock A". If snd_hwdep_release() > >> is executed at this time, it waits for acquiring "Lock B", and thus > >> "Wake X" cannot be performed to wake up "Wait X" in > >> snd_card_disconnect_sync(), causing a possible deadlock. > >> > >> I am not quite sure whether this possible problem is real and how to fix > >> it if it is real. > >> Any feedback would be appreciated, thanks :) > > I'm interested in your report about the deadlock, and seek the cause > > of issue. Then I realized that we should take care of the replacement of > > file_operation before acquiring spinlock in snd_card_disconnect_sync(). > > > > ``` > > snd_card_disconnect_sync() > > ->snd_card_disconnect() > > ->spin_lock() > > ->list_for_each_entry() > > mfile->file->f_op = snd_shutdown_f_ops > > ->spin_unlock() > > ->spin_lock_irq() > > ->wait_event_lock_irq() > > ->spin_unlock_irq() > > ``` > > > > The implementation of snd_shutdown_f_ops has no value for .open, therefore > > snd_hwdep_open() is not called anymore when waiting the event. The mutex > > (Lock B) is not acquired in process context of ALSA hwdep application. > > > > The original .release function can be called by snd_disconnect_release() > > via replaced snd_shutdown_f_ops. In the case, as you can see, the spinlock > > (Lock A) is not acquired. > > > > I think there are no race conditions against Lock A and B in process > > context of ALSA hwdep application after card disconnection. But it would > > be probable to overlook the other case. I would be glad to receive your > > check for the above procedure. > > Thanks a lot for the quick reply :) > Your explanation is reasonable, because snd_shutdown_f_ops indeed has > no value for .open. > > However, my static analysis tool finds another possible deadlock in > the mentioned code: > > snd_card_disconnect_sync() > spin_lock_irq(&card->files_lock); --> Line 461 (Lock A) > wait_event_lock_irq(card->remove_sleep, ...); --> Line 462 (Wait X) > spin_unlock_irq(&card->files_lock); --> Line 465 (Unlock A) > > snd_hwdep_release() > snd_card_file_remove() > spin_lock(&card->files_lock); --> Line 962 (Lock A) > wake_up_all(&card->remove_sleep); --> Line 976 (Wake X) > spin_unlock(&card->files_lock); --> Line 977 (Unlock A) > > When snd_card_disconnect_sync() is executed, "Wait X" is performed by > holding "Lock A". No, it's wait_event_lock_irq(), and this helper unlocks the given lock during waiting and re-locks it after schedule(). See the macro expansion in include/linux/wait.h. Takashi
