Messages in this thread | | | From | Peter Rosin <> | Subject | RE: Domain faults when CONFIG_CPU_SW_DOMAIN_PAN is enabled | Date | Thu, 3 Dec 2015 21:37:51 +0000 |
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Russell King wrote: > On Thu, Dec 03, 2015 at 04:41:18PM +0000, Russell King - ARM Linux wrote: > > On Thu, Dec 03, 2015 at 04:12:06PM +0000, Peter Rosin wrote: > > > * uaccess_with_memcpy.c:__copy_to_user() has a mode in which it copies > > > "non-atomically" (if faulthandler_disabled() returns 0). If a fault > > > happens during __copy_to_user, what prevents some other thread from > > > clobbering DACR? > > > > See the second point above. Moreover, if we sleep in down_read(), > > then __switch_to() reads the current DACR value and saves it in the > > thread information, and will restore that value when resuming the > > thread - even if the thread has been migrated to a different CPU. > > I thought this was correct, but it isn't - that's what my original solution > did, but I think when Will reviewed it, we decided it wasn't necessary - > and it isn't necessary for every single case with the exception of this > one. This is exactly what's going wrong: the down_read() in these paths > calls into the scheduler, which switches away. When we come back, the > DACR value is reset by the other thread to 0x51. > > There's a few ways to solve this: > > 1. Make the thread switching code save and restore the DACR register as > it would do for domains. This imposes an overhead on every single > context switch whether or not we happen to be in this _single_ > troublesome code. (Patch attached - as there's several, I'm attaching > them.) > > 2. Add additional code to the uaccess-with-memcpy stuff to reset the > DACR value prior to using memcpy() or memset(). (Patch attached.)
I took both patches for a quick spin (a dozen boots and one hour uptime after that for each patch) and no incidents. I have not gathered data, but the crash on boot feels like it's quite a bit above 50% when there is a problem so this feels good (I used 5 clean reboots when I bisected and that worked).
Reported-by: Peter Rosin <peda@axentia.se> Tested-by: Peter Rosin <peda@axentia.se>
(and please don't forget to cc stable)
> 3. Make uaccess-with-memcpy depend on !CPU_SW_DOMAINS_PAN (suggested by > Will) > > 4. Delete the uaccess-with-memcpy code (also suggested by Will.) > > I think the best thing I can do is say... "Discuss amongst yourselves" :)
I have no personal preference on what should be done, I only had copy_to_user_memcpy enabled since that was what Atmel fed me.
Cheers, Peter
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