Thinh Nguyen wrote: > 정재훈 wrote: >>> -----Original Message----- >>> From: Thinh Nguyen [mailto:Thinh.Nguyen@xxxxxxxxxxxx] >>> Sent: Friday, March 11, 2022 10:57 AM >>> To: 정재훈; Thinh Nguyen; 'Felipe Balbi'; 'Greg Kroah-Hartman' >>> Cc: 'open list:USB XHCI DRIVER'; 'open list'; 'Seungchull Suh'; 'Daehwan >>> Jung'; cpgs@xxxxxxxxxxx; cpgsproxy5@xxxxxxxxxxx >>> Subject: Re: [PATCH] usb: dwc3: Add dwc3 lock for blocking interrupt >>> storming >>> >>> 정재훈 wrote: >>>> Hi. >>>> >>>>> -----Original Message----- >>>>> From: Thinh Nguyen [mailto:Thinh.Nguyen@xxxxxxxxxxxx] >>>>> Sent: Thursday, March 10, 2022 11:14 AM >>>>> To: JaeHun Jung; Felipe Balbi; Greg Kroah-Hartman >>>>> Cc: open list:USB XHCI DRIVER; open list; Seungchull Suh; Daehwan >>>>> Jung >>>>> Subject: Re: [PATCH] usb: dwc3: Add dwc3 lock for blocking interrupt >>>>> storming >>>>> >>>>> Hi, >>>>> >>>>> JaeHun Jung wrote: >>>>>> Interrupt Storming occurred with a very low probability of occurrence. >>>>>> The occurrence of the problem is estimated to be caused by a race >>>>>> condition between the top half and bottom half of the interrupt >>>>>> service >>>>> routine. >>>>>> It was confirmed that variables have values that cannot be held when >>>>>> ISR occurs through normal H / W irq. >>>>>> ==================================================================== >>>>>> = (struct dwc3_event_buffer *) ev_buf = 0xFFFFFF88DE6A0380 ( >>>>>> (void *) buf = 0xFFFFFFC01594E000, >>>>>> (void *) cache = 0xFFFFFF88DDC14080, >>>>>> (unsigned int) length = 4096, >>>>>> (unsigned int) lpos = 0, >>>>>> (unsigned int) count = 0, << >>>>>> (unsigned int) flags = 1, << >>>>>> ==================================================================== >>>>>> = "evt->count=0" and "evt->flags=DWC3_EVENT_PENDING" cannot be set >>>>>> at the same time. >>>>>> >>>>>> We estimate that a race condition occurred between dwc3_interrupt() >>>>>> and dwc3_process_event_buf() called by >>>>>> dwc3_gadget_process_pending_events(). >>>>>> So I try to block the race condition through spin_lock. >>>>> >>>>> This looks like it needs a memory barrier. Would this work for you? >>>> Maybe it could be. But "evt->count = 0;" is updated on >>> dwc3_process_event_buf(). >>>> So, I think spin_lock is more clear routine for this issue. >>>> >>> >>> Not really. If problem is due to the evt->flags not updated in time, then >>> the solution should be using the memory barrier. The spin_lock would >>> obfuscate the issue. And we should avoid using spin_lock in the top-half. >> >> This issue was occurred by watchdog. The interrupt occurred in units of 4 to 5us and cannot be released until the bottom is executed. >> If it is a problem with the memory barrier, the value should be updated after a few clocks and the TOP should run normally. Isn't it? > > Can you guarantee that a value is stored after X amount of time, every time? > >> And Could you explain me why we should avoid using spin_lock in the top-half. >> > > The top-half and bottom-half are serialized. While the bottom-half > handler is running, the interrupt should be masked. If the top-half got > called in the middle of the bottom-half handler, something else is > wrong. There should not be a race that requires a spin_lock for this > particular critical section. > > The problem you're seeing is pointing toward a memory barrier issue. > > Also you noted that there's an "interrupt storm", which doesn't indicate > to me that it's due to PCIe legacy interrupt de-assertion delay response > either. > > Can you test it out and we can take a look further? > We want to avoid spin_lock because the top-half shouldn't stall for too long, affecting performance. This can happen if some async call from the upperlayer driver's holding the lock. Thinh
