linux-kernel - Re: [PATCH 1/5] ftrace: Synchronize variable setting with breakpoints

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Date:	Thu, 31 May 2012 13:06:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/5] ftrace: Synchronize variable setting with
 breakpoints

On Wed, 2012-05-30 at 21:28 -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> When the function tracer starts modifying the code via breakpoints
> it sets a variable (modifying_ftrace_code) to inform the breakpoint
> handler to call the ftrace int3 code.
> 
> But there's no synchronization between setting this code and the
> handler, thus it is possible for the handler to be called on another
> CPU before it sees the variable. This will cause a kernel crash as
> the int3 handler will not know what to do with it.
> 
> I originally added smp_mb()'s to force the visibility of the variable
> but H. Peter Anvin suggested that I just make it atomic.

Uhm,. maybe. atomic_{inc,dec}() implies a full memory barrier on x86,
but atomic_read() never has the smp_rmb() required.

Now smp_rmb() is mostly a nop on x86, except for CONFIG_X86_PPRO_FENCE.

So this should mostly work, but yuck.


Also, why does this stuff live in ftrace? I always thought you were
going to replace text_poke() so everybody that uses cross-modifying code
could profit?


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