linux-kernel - Re: [PATCH v4 2/2] powerpc: Uprobes port to powerpc

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Message-ID: <20120823160210.GF25338@linux.vnet.ibm.com>
Date:	Thu, 23 Aug 2012 21:32:10 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <michael@...erman.id.au>, ananth@...ibm.com,
	ppcdev <linuxppc-dev@...ts.ozlabs.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>, peterz@...radead.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v4 2/2] powerpc: Uprobes port to powerpc

* Oleg Nesterov <oleg@...hat.com> [2012-08-23 11:02:09]:

> On 08/23, Benjamin Herrenschmidt wrote:
> >
> > On Thu, 2012-08-23 at 11:02 +0530, Srikar Dronamraju wrote:
> > > >
> > >
> > > insn is updated/accessed in the arch independent code. Size of
> > > uprobe_opcode_t could be different for different archs.
> > > uprobe_opcode_t
> > > represents the size of the smallest breakpoint instruction for an
> > > arch.
> > >
> > > Hence u8 works out the best. I know we could still use uprobe_opcode_t
> > > and achieve the same. In which case, we would have to interpret
> > > MAX_UINSN_BYTES differently. Do you see any advantages of using
> > > uprobe_opcode_t instead of u8 across archs?
> >
> > But don't you actively rely on the fact that on powerpc, unlike x86, you
> > -can- atomically replace an instruction with a single 32-bit store ?
> 
> I must have missed something...
> 
> But powerpc does not replace an instruction, the arch independent code
> does this and it assumes that uprobe->arch.insn is u8[MAX_UINSN_BYTES].
> 
> Perhaps you meant that on powerpc it is "safe" to replace the insn
> even if this can race with some CPU executing this code? But uprobes
> has to replace the original page anyway, we should not write to
> ->vm_file.

I think Ben is referring to the fact that because we use an array we
endup using memcpy to copy the original instruction from the ->vm_file.

> 
> I agree that memcpy() in arch_uprobe_analyze_insn() and
> arch_uprobe_skip_sstep() looks a bit strange. May be powerpc can do
> 
> 	struct arch_uprobe {
> 		union {
> 			u8		insn[MAX_UINSN_BYTES];
> 			u32		ainsn;
> 		};
> 	};
> 
> and use auprobe->ainsn directly, I dunno.

I think this should work.

Ben  would this suffice?

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