Messages in this thread Patch in this message |  | | Date | Mon, 18 Nov 2013 09:11:43 +0100 | From | Martin Schwidefsky <> | Subject | Re: [PATCH 2/2] s390/mm,tlb: race of lazy TLB flush vs. recreation of TLB entries |
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On Fri, 15 Nov 2013 13:46:07 +0000 Catalin Marinas <catalin.marinas@arm.com> wrote:
> On 15 November 2013 13:29, Martin Schwidefsky <schwidefsky@de.ibm.com> wrote: > > On Fri, 15 Nov 2013 11:57:01 +0000 > > Catalin Marinas <catalin.marinas@arm.com> wrote: > > > >> On Fri, Nov 15, 2013 at 11:17:36AM +0000, Martin Schwidefsky wrote: > >> > On Fri, 15 Nov 2013 12:10:00 +0100 > >> > Martin Schwidefsky <schwidefsky@de.ibm.com> wrote: > >> > > >> > > On Fri, 15 Nov 2013 10:44:37 +0000 > >> > > Catalin Marinas <catalin.marinas@arm.com> wrote: > >> > > > 1. thread-A running with mm-A > >> > > > 2. context_switch() to thread-B1 causing a switch_mm(mm-B) > >> > > > 3. switch_mm(mm-B) sets thread-B1's TIF_TLB_WAIT but does _not_ call > >> > > > update_mm(mm-B). Hardware still using mm-A > >> > > > 4. scheduler unlocks and is about to call finish_mm_switch(mm-B) > >> > > > 5. interrupt and preemption before finish_mm_switch(mm-B) > >> > > > 6. context_switch() to thread-B2 causing a switch_mm(mm-B) (note here > >> > > > that thread-B1 and thread-B2 have the same mm-B) > >> > > > 7. switch_mm() as in this patch exits early because prev == next > >> > > > 8. finish_mm_switch(mm-B) is indeed called but TIF_TLB_WAIT is not set > >> > > > for thread-B2, therefore no call to update_mm(mm-B) > >> > > > > >> > > > So after point 8, you get thread-B2 running (and possibly returning to > >> > > > user space) with mm-A. Do you see a problem here? > >> > > > >> > > Oh, now I get it. Thanks for the patience, this is indeed a problem. > >> > > And I concur, a per-mm flag is the 'obvious' solution. > >> > > >> > Having said that and looking at the code I find this to be not as obvious > >> > any more. If you have multiple cpus using a per-mm flag can get you into > >> > trouble: > >> > > >> > 1. cpu #1 calls switch_mm and finds that irqs are disabled. > >> > mm->context.switch_pending is set > >> > 2. cpu #2 calls switch_mm for the same mm and finds that irqs are disabled. > >> > mm->context.switch_pending is set again > >> > 3. cpu #1 reaches finish_arch_post_lock_switch and finds switch_pending == 1 > >> > 4. cpu #1 zeroes mm->switch_pending and calls cpu_switch_mm > >> > 5. cpu #2 reaches finish_arch_post_lock_switch and finds switch_pending == 0 > >> > 6. cpu #2 continues with the old mm > >> > > >> > This is a race, no? > >> > >> Yes, but we only use this on ARMv5 and earlier and there is no SMP > >> support. > >> > >> On arm64 however, I need to fix that and you made a good point. In my > >> (not yet public) patch, the switch_pending is cleared after all the > >> IPIs have been acknowledged but it needs some more thinking. A solution > >> could be to always do the cpu_switch_mm() in finish_mm_switch() without > >> any checks but this requires that any switch_mm() call from the kernel > >> needs to be paired with finish_mm_switch(). So your first patch comes in > >> handy (but I still need to figure out a quick arm64 fix for cc stable). > > > > I am currently thinking about the following solution for s390: keep the > > TIF_TLB_FLUSH bit per task but do a preempt_disable() in switch_mm() > > if the switch is incomplete. This pairs with a preempt_enable() in > > finish_switch_mm() after the update_mm has been done. > > That's the first thing I tried when I noticed the problem but I got > weird kernel warnings with preempt_enable/disabling spanning across > the scheduler unlocking. So doesn't seem safe. > > It may work if instead a simple flag you use atomic_inc/dec for the mm flag.
I have not seen the kernel warnings because the detour over finish_switch_mm is used only rarely. After forcing the detour I got the fallout, doing the preempt_disable in switch_mm and preempt_enable in finish_switch_mm does not work. But what does work is to copy the TIF_TLB_FLUSH bit in the __switch_to function just like the TIF_MCCK_PENDING. That way the TIF_TLB_FLUSH can not get "hidden" by a preemptive schedule.
The patch to use finish_arch_post_lock_switch instead of finish_switch_mm would look like this: -- From c4d83a9ff8c6d5ca821bab24b5bd77b782a69819 Mon Sep 17 00:00:00 2001 From: Martin Schwidefsky <schwidefsky@de.ibm.com> Date: Fri, 26 Oct 2012 17:17:44 +0200 Subject: [PATCH] sched/mm: call finish_arch_post_lock_switch in idle_task_exit and use_mm
The finish_arch_post_lock_switch is called at the end of the task switch after all locks have been released. In concept it is paired with the switch_mm function, but the current code only does the call in finish_task_switch. Add the call to idle_task_exit and use_mm. One use case for the additional calls is s390 which will use finish_arch_post_lock_switch to wait for the completion of TLB flush operations.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com> --- include/linux/mmu_context.h | 6 ++++++ kernel/sched/core.c | 6 ++++-- kernel/sched/sched.h | 3 --- mm/mmu_context.c | 3 +-- 4 files changed, 11 insertions(+), 7 deletions(-)
diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h index 70fffeb..38f5550 100644 --- a/include/linux/mmu_context.h +++ b/include/linux/mmu_context.h @@ -1,9 +1,15 @@ #ifndef _LINUX_MMU_CONTEXT_H #define _LINUX_MMU_CONTEXT_H +#include <asm/mmu_context.h> + struct mm_struct; void use_mm(struct mm_struct *mm); void unuse_mm(struct mm_struct *mm); +#ifndef finish_arch_post_lock_switch +# define finish_arch_post_lock_switch() do { } while (0) +#endif + #endif diff --git a/kernel/sched/core.c b/kernel/sched/core.c index c180860..ffa234c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -32,7 +32,7 @@ #include <linux/init.h> #include <linux/uaccess.h> #include <linux/highmem.h> -#include <asm/mmu_context.h> +#include <linux/mmu_context.h> #include <linux/interrupt.h> #include <linux/capability.h> #include <linux/completion.h> @@ -4154,8 +4154,10 @@ void idle_task_exit(void) BUG_ON(cpu_online(smp_processor_id())); - if (mm != &init_mm) + if (mm != &init_mm) { switch_mm(mm, &init_mm, current); + finish_arch_post_lock_switch(); + } mmdrop(mm); } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 88c85b2..ad48db3 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -850,9 +850,6 @@ static inline int task_running(struct rq *rq, struct task_struct *p) #ifndef finish_arch_switch # define finish_arch_switch(prev) do { } while (0) #endif -#ifndef finish_arch_post_lock_switch -# define finish_arch_post_lock_switch() do { } while (0) -#endif #ifndef __ARCH_WANT_UNLOCKED_CTXSW static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next) diff --git a/mm/mmu_context.c b/mm/mmu_context.c index 8a8cd02..56ecbbd 100644 --- a/mm/mmu_context.c +++ b/mm/mmu_context.c @@ -8,8 +8,6 @@ #include <linux/export.h> #include <linux/sched.h> -#include <asm/mmu_context.h> - /* * use_mm * Makes the calling kernel thread take on the specified @@ -31,6 +29,7 @@ void use_mm(struct mm_struct *mm) tsk->mm = mm; switch_mm(active_mm, mm, tsk); task_unlock(tsk); + finish_arch_post_lock_switch(); if (active_mm != mm) mmdrop(active_mm); -- 1.8.3.4 -- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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