Messages in this thread |  | | Date | Mon, 17 Feb 2014 10:37:38 +0100 | From | Martin Schwidefsky <> | Subject | Re: [PATCH] sched/core: Create new task with twice disabled preemption |
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On Fri, 14 Feb 2014 10:52:55 +0000 Catalin Marinas <catalin.marinas@arm.com> wrote:
> 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)
This is what I put in place for s390 but with the name TIF_TLB_WAIT instead of TIF_SWITCH_MM. I took the liberty to add the code to the features branch of the linux-s390 tree including the common code change that is necessary:
https://git.kernel.org/cgit/linux/kernel/git/s390/linux.git/commit/?h=features&id=09ddfb4d5602095aad04eada8bc8df59e873a6ef https://git.kernel.org/cgit/linux/kernel/git/s390/linux.git/commit/?h=features&id=525d65f8f66ac29136ba6d2336f5a73b038701e2
These patches will be included in a please-pull request with the next merge window.
> 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. >
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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