+CC linux-arch, Peter for any preemption insights ! On 03/14/2018 09:36 AM, Alexey Brodkin wrote: > Hi Vineet, > > While debugging a segfault of user-space app on system without atomic ops > (I mean LLOCK/SCOND) I understood the root-cause is in implementation > of kernel's __NR_arc_usr_cmpxchg syscall which is supposed to emulate mentioned > atomic ops for user-space. > > So here's a problem. > > 1. User-space app [via libc] triggers __NR_arc_usr_cmpxchg syscall, > we enter arc_usr_cmpxchg()which basically does: > ---------------------------->8------------------------------- > preempt_disable(); > __get_user(uval, uaddr); > __put_user(new, uaddr); > preempt_enable(); > ---------------------------->8------------------------------- > > 2. Most of the time everything is fine because __get_user()/__put_user() > for ARC is just LD/ST. > > 3. Rarely user's variable is situated in not yet allocated page. > Here I mean copy-on-write case, when a page has read-only flag in TLB. > In that case __get_user() succeeds but __put_user() causes Privilege > Violation exception and we enter do_page_fault() where new page allocation > with proper access bits is supposed to happen... but that never happens > because with preempt_disable() we set in_atomic() which set > faulthandler_disabled() and so we exit early from page fault handler > effectively with nothing done, i.e. user's variable is left unchanged > which in its turn causes very strange problems later down the line because > we don't notify user-space app about failed data modification. Interesting problem ! But what is special here, I would think syscalls in general could hit this. > > The simplest fix is to not mess with preemption: > ---------------------------->8------------------------------- > diff --git a/arch/arc/kernel/process.c b/arch/arc/kernel/process.c > index 5ac3b547453f..d1713d8d3981 100644 > --- a/arch/arc/kernel/process.c > +++ b/arch/arc/kernel/process.c > @@ -63,8 +63,6 @@ SYSCALL_DEFINE3(arc_usr_cmpxchg, int *, uaddr, int, expected, int, new) > if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int))) > return -EFAULT; > > - preempt_disable(); > - > if (__get_user(uval, uaddr)) > goto done; ... ... > done: > - preempt_enable(); > - > return uval; > } > ---------------------------->8------------------------------- > > But I'm not really sure how safe is that. Well it is broken wrt the semantics the syscall is supposed to provide. Preemption disabling is what prevents a concurrent thread from coming in and modifying the same location (Imagine a variable which is being cmpxchg concurrently by 2 threads). One approach is to do it the MIPS way, emulate the llsc flag - set it under preemption disabled section and clear it in switch_to see arch/mips/kernel/syscall.c
