Messages in this thread |  | | Date | Mon, 17 Feb 2014 13:55:32 +0100 | From | Martin Schwidefsky <> | Subject | Re: [PATCH] sched/core: Create new task with twice disabled preemption |
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On Mon, 17 Feb 2014 10:40:06 +0000 Catalin Marinas <catalin.marinas@arm.com> wrote:
> On Mon, Feb 17, 2014 at 09:37:38AM +0000, Martin Schwidefsky wrote: > > 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 > > I don't see a problem with additional calls to > finish_arch_post_lock_switch() on arm and arm64 but I would have done > this in more than one step: > > 1. Introduce finish_switch_mm() > 2. Convert arm and arm64 to finish_switch_mm() (which means we no longer > check whether the interrupts are disabled in switch_mm() to defer the > switch > 3. Remove generic finish_arch_post_lock_switch() because its > functionality has been entirely replaced by finish_switch_mm() > > Anyway, we probably end up in the same place anyway.
Peter pointed me to finish_arch_post_lock switch as a replacement for finish_switch_mm. They are basically doing the same thing and I do not care too much how the function is called. finish_arch_post_lock_switch is ok from my point of view. If you want to change it be aware of the header file hell you are getting into. > But does this solve the problem of being preempted between switch_mm() > and finish_arch_post_lock_switch()? I guess we still need the same > guarantees that both switch_mm() and the hook happen on the same CPU.
By itself no, I do not think so. finish_arch_post_lock_switch is supposed to be called with no locks held, so preemption should be enabled as well.
> > https://git.kernel.org/cgit/linux/kernel/git/s390/linux.git/commit/?h=features&id=525d65f8f66ac29136ba6d2336f5a73b038701e2 > > That's a way to solve it for s390. I don't particularly like > transferring the mm switch pending TIF flag to the next task but I think > it does the job (just personal preference).
It gets the job done. The alternative is another per-cpu bitmap for each mm. I prefer the transfer of the TIF flag to the next task, we do that for the machine check flag TIF_MCCK_PENDING anyway. One more bit to transfer does not hurt.
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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