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From: James Hogan <james.hogan@imgtec.com>
To: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: <yhb@ruijie.com.cn>, <ralf@linux-mips.org>, <linux-mips@linux-mips.org>
Subject: Re: MIPS: We need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0.
Date: Mon, 11 Jul 2016 19:07:55 +0100	[thread overview]
Message-ID: <20160711180755.GA29839@jhogan-linux.le.imgtec.org> (raw)
In-Reply-To: <5783DF18.1080408@imgtec.com>

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Hi Leonid,

On Mon, Jul 11, 2016 at 11:02:00AM -0700, Leonid Yegoshin wrote:
> On 07/10/2016 06:04 AM, yhb@ruijie.com.cn wrote:
> > Subject: [PATCH] MIPS: We need to clear MMU contexts of all other processes
> >   when asid_cache(cpu) wraps to 0.
> >
> > Suppose that asid_cache(cpu) wraps to 0 every n days.
> > case 1:
> > (1)Process 1 got ASID 0x101.
> > (2)Process 1 slept for n days.
> > (3)asid_cache(cpu) wrapped to 0x101, and process 2 got ASID 0x101.
> > (4)Process 1 is woken,and ASID of process 1 is same as ASID of process 2.
> >
> > case 2:
> > (1)Process 1 got ASID 0x101 on CPU 1.
> > (2)Process 1 migrated to CPU 2.
> > (3)Process 1 migrated to CPU 1 after n days.
> > (4)asid_cache on CPU 1 wrapped to 0x101, and process 2 got ASID 0x101.
> > (5)Process 1 is scheduled, and ASID of process 1 is same as ASID of process 2.
> >
> > So we need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0.
> >
> > Signed-off-by: yhb <yhb@ruijie.com.cn>
> >
> I think a more clear description should be given here - there is no 
> indication that wrap happens over 32bit integer.
> 
> And taking into account "n days" frequency - can we just kill all local 
> ASIDs in all processes (additionally to local_flush_tlb_all) and enforce 
> reassignment if wrap happens? It should be a very rare event, you are 
> first to hit this.
> 
> It seems to be some localized stuff in get_new_mmu_context() instead of 
> widespread patching.

That is what this patch does, but to do so it appears you need to lock
the other tasks one by one, and that must be doable from a context
switch, i.e. hardirq context, and that requires the task lock to be of
the _irqsave variant, hence the widespread changes and the relatively
tiny MIPS change hidden in the middle.

Cheers
James

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WARNING: multiple messages have this Message-ID (diff)
From: James Hogan <james.hogan@imgtec.com>
To: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: yhb@ruijie.com.cn, ralf@linux-mips.org, linux-mips@linux-mips.org
Subject: Re: MIPS: We need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0.
Date: Mon, 11 Jul 2016 19:07:55 +0100	[thread overview]
Message-ID: <20160711180755.GA29839@jhogan-linux.le.imgtec.org> (raw)
Message-ID: <20160711180755.6WkxQ4tawILq-ZOCJPAI6WiKG2Qfkt0PkE12osRPfuY@z> (raw)
In-Reply-To: <5783DF18.1080408@imgtec.com>

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Hi Leonid,

On Mon, Jul 11, 2016 at 11:02:00AM -0700, Leonid Yegoshin wrote:
> On 07/10/2016 06:04 AM, yhb@ruijie.com.cn wrote:
> > Subject: [PATCH] MIPS: We need to clear MMU contexts of all other processes
> >   when asid_cache(cpu) wraps to 0.
> >
> > Suppose that asid_cache(cpu) wraps to 0 every n days.
> > case 1:
> > (1)Process 1 got ASID 0x101.
> > (2)Process 1 slept for n days.
> > (3)asid_cache(cpu) wrapped to 0x101, and process 2 got ASID 0x101.
> > (4)Process 1 is woken,and ASID of process 1 is same as ASID of process 2.
> >
> > case 2:
> > (1)Process 1 got ASID 0x101 on CPU 1.
> > (2)Process 1 migrated to CPU 2.
> > (3)Process 1 migrated to CPU 1 after n days.
> > (4)asid_cache on CPU 1 wrapped to 0x101, and process 2 got ASID 0x101.
> > (5)Process 1 is scheduled, and ASID of process 1 is same as ASID of process 2.
> >
> > So we need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0.
> >
> > Signed-off-by: yhb <yhb@ruijie.com.cn>
> >
> I think a more clear description should be given here - there is no 
> indication that wrap happens over 32bit integer.
> 
> And taking into account "n days" frequency - can we just kill all local 
> ASIDs in all processes (additionally to local_flush_tlb_all) and enforce 
> reassignment if wrap happens? It should be a very rare event, you are 
> first to hit this.
> 
> It seems to be some localized stuff in get_new_mmu_context() instead of 
> widespread patching.

That is what this patch does, but to do so it appears you need to lock
the other tasks one by one, and that must be doable from a context
switch, i.e. hardirq context, and that requires the task lock to be of
the _irqsave variant, hence the widespread changes and the relatively
tiny MIPS change hidden in the middle.

Cheers
James

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  parent reply	other threads:[~2016-07-11 18:08 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-10 13:04 MIPS: We need to clear MMU contexts of all other processes when asid_cache(cpu) wraps to 0 yhb
2016-07-10 13:04 ` yhb
2016-07-11  9:30 ` James Hogan
2016-07-11  9:30   ` James Hogan
2016-07-11 18:02 ` Leonid Yegoshin
2016-07-11 18:02   ` Leonid Yegoshin
2016-07-11 18:05   ` [PATCH] " Leonid Yegoshin
2016-07-11 18:05     ` Leonid Yegoshin
2016-07-11 18:07   ` James Hogan [this message]
2016-07-11 18:07     ` James Hogan
2016-07-11 18:19     ` [PATCH] " Leonid Yegoshin
2016-07-11 18:19       ` Leonid Yegoshin
2016-07-11 19:21       ` James Hogan
2016-07-11 19:21         ` James Hogan
2016-07-11 19:39         ` Leonid Yegoshin
2016-07-11 19:39           ` Leonid Yegoshin
2016-07-11 20:18           ` James Hogan
2016-07-11 20:18             ` James Hogan

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