On Thu, Feb 13, 2014 at 09:32:22PM +0400, Kirill Tkhai wrote: > On 13.02.2014 20:00, Peter Zijlstra wrote: > > On Thu, Feb 13, 2014 at 07:51:56PM +0400, Kirill Tkhai wrote: > >> For archs without __ARCH_WANT_UNLOCKED_CTXSW set this means > >> that all newly created tasks execute finish_arch_post_lock_switch() > >> and post_schedule() with preemption enabled. > > > > That's IA64 and MIPS; do they have a 'good' reason to use this? > > It seems my description misleads reader, I'm sorry if so. > > I mean all architectures *except* IA64 and MIPS. All, which > has no __ARCH_WANT_UNLOCKED_CTXSW defined. > > IA64 and MIPS already have preempt_enable() in schedule_tail(): > > #ifdef __ARCH_WANT_UNLOCKED_CTXSW > /* In this case, finish_task_switch does not reenable preemption */ > preempt_enable(); > #endif > > Their initial preemption is not decremented in finish_lock_switch(). > > So, we speak about x86, ARM64 etc. > > Look at ARM64's finish_arch_post_lock_switch(). It looks a task > must to not be preempted between switch_mm() and this function. > But in case of new task this is possible.
We had a thread about this at the end of last year: https://lkml.org/lkml/2013/11/15/82 There is indeed a problem on arm64, something like this (and I think s390 also needs a fix): 1. switch_mm() via check_and_switch_context() defers the actual mm switch by setting TIF_SWITCH_MM 2. the context switch is considered 'done' by the kernel before finish_arch_post_lock_switch() and therefore we can be preempted to a new thread before finish_arch_post_lock_switch() 3. The new thread has the same mm as the preempted thread but we actually missed the mm switching in finish_arch_post_lock_switch() because TIF_SWITCH_MM is per thread rather than mm > This is the problem I tried to solve. I don't know arm64, and I can't > say how it is serious. Have you managed to reproduce this? I don't say it doesn't exist, but I want to make sure that any patch actually fixes it. So we have more solutions, one of the first two suitable for stable: 1. Propagate the TIF_SWITCH_MM to the next thread (suggested by Martin) 2. Get rid of TIF_SWITCH_MM and use mm_cpumask for tracking (I already have the patch, it just needs a lot more testing) 3. Re-write the ASID allocation algorithm to no longer require IPIs and therefore drop finish_arch_post_lock_switch() (this can be done, so pretty intrusive for stable) 4. Replace finish_arch_post_lock_switch() with finish_mm_switch() as per Martin's patch and I think this would guarantee a call always, we can move the mm switching from switch_mm() to finish_mm_switch() and no need for flags to mark deferred mm switching For arm64, we'll most likely go with 2 for stable and move to 3 shortly after, no need for other deferred mm switching. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
