Thanks a lot for your replay. > -----Original Message----- > From: Alan Stern [mailto:stern@xxxxxxxxxxxxxxxxxxx] > Sent: Thursday, July 18, 2013 11:09 PM > To: Ming Lei > Cc: Gioh Kim; linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > Mark Salter; namhyung.kim@xxxxxxx; Minchan Kim; Chanho Min; Jong-Sung Kim; > linux-arm-kernel > Subject: Re: [PATCH] [RFC] EHCI: add to memory barrier to updating hw_next > > On Thu, 18 Jul 2013, Ming Lei wrote: > > > > I guess that HC could have a use-after-free problem like following > situation. > > > > > > 1. A qtd which is not at the queue head should be removed in > qh_completions(). > > > 2. The last->hw_next become be pointing at the next qtd but the > hw_next value is delayed in write-buffer. > > > 3. The qtd is removed in the list. > > > 4. The qtd is freed into DMA pool and re-allocated for another urb. > > > 5. HC try to process last->hw_next and it is pointing re-allocated qtd. > > > > > > What do you think about it? Is it possible? > > > > I understand it might not be possible because: when 'stopped' is set, > > that said the HC might not advance the queue. But I don't understand > > why 'last->hw_next' is patched here under 'stopped' situation. > > It should not be possible. When "stopped" is set, the QH gets unlinked > and relinked before it can start up again. Relinking involves some memory > barriers, so the qTD will not be accessed again by the HC. > > last->hw_next gets patched because the qTD might belong to some URB in > the middle of the queue that is being unlinked. The URBs before it and > after it will still be active, so the queue link has to be updated. > You're right. I misunderstand those codes. Please forget about it. > > Even the 'stopped' case may be seldom triggered, do you know under > > which condition the stopped is triggered in your problem?(stall, short > > read or others) > > I was going to ask the same question. This particular piece of code gets > executed _only_ when an URB is unlinked. Not during any other kind of > error. I've got the problem when I listened to the mp3 file of USB HDD. I checked the urb data when the problem occurred, the last-status value of urb was EINPROGRESS and urb->unlinked was ECONNRESET. I think the 'stopped' case was occurred by the reset of USB port. The block device driver did reset USB port because there is no return from USB device. If I made block device driver could not reset USB port, the EHCI driver codes were not executed. Finally the halt of HC makes 'stopped' case. I think halt of the HC might be caused that store-buffer delays command for HC. When I applied the patch from https://lkml.org/lkml/2011/8/31/344 and added a mb() into hw_next updating to remove delay of store-buffer, My platform works well. Can the store-buffer delay halt HC? Is it possible? IMHO, if the qTD list is broken the HC think there is no qTD to send. So I added mb() at hw_next update code. > > Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
