linux-kernel - Re: [PATCH 1/3] jump label: add enabled/disabled state to jump label key entries

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Message-ID: <20101124154200.GD2815@redhat.com>
Date:	Wed, 24 Nov 2010 10:42:01 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	rostedt@...dmis.org, mingo@...e.hu, mathieu.desnoyers@...ymtl.ca,
	hpa@...or.com, tglx@...utronix.de, andi@...stfloor.org,
	roland@...hat.com, rth@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, avi@...hat.com, davem@...emloft.net,
	sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] jump label: add enabled/disabled state to jump
	label key entries

On Wed, Nov 24, 2010 at 04:24:05PM +0100, Peter Zijlstra wrote:
> On Wed, 2010-11-24 at 10:19 -0500, Jason Baron wrote:
> > On Wed, Nov 24, 2010 at 04:11:18PM +0100, Peter Zijlstra wrote:
> > > On Wed, 2010-11-24 at 09:54 -0500, Jason Baron wrote:
> > > > On Wed, Nov 24, 2010 at 09:20:09AM +0100, Peter Zijlstra wrote:
> > > > > On Tue, 2010-11-23 at 16:27 -0500, Jason Baron wrote:
> > > > > >         struct hlist_head modules;
> > > > > >         unsigned long key;
> > > > > > +       u32 nr_entries : 31,
> > > > > > +              enabled :  1;
> > > > > >  }; 
> > > > > 
> > > > > I still don't see why you do this, why not simply mandate that the key
> > > > > is of type atomic_t* and use *key as enabled state?
> > > > > 
> > > > 
> > > > Because I want to use *key as a pointer directly to 'struct jump_label_entry'.
> > > > In this way jump_label_enable(), jump_label_disable(), become O(1) operations.
> > > > That way we don't need any hashing.
> > > 
> > > But but but, you're doing a friggin stop_machine to poke text, that's
> > > way more expensive than anything else.
> > > 
> > 
> > Yes, but other arches do not require stop_machine(). Also, there is work
> > for x86 to make the code patching happen without stop_machine().
> 
> Even without stop machine you're sending IPIs to all CPUs, that's not
> free either.
> 
> And I think the only arch where you can do text pokes without cross-cpu
> synchronization is one that doesn't have SMP support.
> 
> 

is this really true?

The powerpc implementation uses patch_instruction():


arch/powerpc/lib/code-patching.c:

void patch_instruction(unsigned int *addr, unsigned int instr)
{
        *addr = instr;
        asm ("dcbst 0, %0; sync; icbi 0,%0; sync; isync" : : "r"
(addr));
}


And sparc does uses flushi():


include/asm/system_64.h:

#define flushi(addr)    __asm__ __volatile__ ("flush %0" : : "r" (addr)
: "memory")

thanks,

-Jason
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