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Date: Tue, 06 Oct 2009 23:10:28 -0400 From: Masami Hiramatsu <mhiramat@...hat.com> To: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> CC: Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>, Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org, tglx@...utronix.de, ak@...e.de, roland@...hat.com, rth@...hat.com Subject: Re: [PATCH 1/4] jump label - make init_kernel_text() global Mathieu Desnoyers wrote: > * Steven Rostedt (rostedt@...dmis.org) wrote: >> On Sat, 2009-10-03 at 08:39 -0400, Mathieu Desnoyers wrote: >> >>> I might be missing a bit of context here, I just want to make sure we >>> are on the same page: patching a jmp instruction is safe on UP, safe >>> with stop_machine(), is very likely safe with the breakpoint-ipi >> >> Hi Mathieu, >> >> I've been reading through these threads (both this one and the immediate >> one) and I'm still a bit confused. I really want to understand this in a >> simple way, thus make sure everyone else understands it too. >> >> >From what Arjan said here: >> >> http://lkml.org/lkml/2009/9/25/98 >> >> The issue is going back from the int3 to the old value. How does the >> breakpoint-ipi work? >> >> Supposedly, we can add an int3 to the code without any worry. If another >> CPU at that same time hits that code path, it will either run the old >> code, or take the interrupt. The breakpoint interrupt handler, will >> handle that code path, and the execution continues. >> >> Now what is the issues with removing the int3 and placing back the old >> (or new) value. Is there an issue if another CPU is about to execute >> that code path as we remove the int3? If so, how does sending an IPI >> help the matter without adding more races? >> >> Is there only an issue if we change the old value with something else, >> and you just need to send the IPI after you modify the old code and >> before removing the int3? >> >> I may just be totally confused, which I usually am. But when I'm not >> confused, I feel that the code is practical ;-) >> > > Hi Steven, > > OK, I'll make the explanation as straightforward as possible. I'll use a > race example to illustrate what we try to avoid by using the > breakpoint+ipi scheme. After that, I present the same scenario with the > breakpoint+ipi in place. > > Each step shows what is executed, and what is the memory values seen by > the CPU. CPU A is doing the code patching, CPU B executing the code. > I intentionally left out some sfence required on CPU A for simplicity.) > > Initially, let's say we have: > (1) (2) > 0xeb 0xe5 (jmp to offset 0xe5) > > And we want to change this to: > (1) (2) > 0xeb 0xf0 (jmp to offset 0xf0) > > (scenario "buggy") > > CPU A | CPU B (this is about as far as my ascii-art skills go) > ------------------------- ;) > 0xeb 0xe5 0xeb 0xe5 > 0: CPU B instruction pointer is earlier than (1) > CPU B pipeline speculatively predicts branches, > prefetches data, calculates speculated values. > 1: CPU B loads 0xeb > 2: CPU B loads 0xe5 > 3: > Write to (2) > 0xeb 0xf0 0xeb 0xf0 > 4: CPU B instruction pointer gets to (1), needs to validate > all the pipeline speculation. > But ! The CPU does not expect code to change underneath. > General protection fault (or any other fault.. random..) > > > Now with the breakpoint+ipi/mb() scheme: > (scenario A: CPU B does not hit the breakpoint) > > CPU A | CPU B > ------------------------- > 0xeb 0xe5 0xeb 0xe5 > 0: CPU B instruction pointer is earlier than (1) > CPU B pipeline speculatively predicts branches, > prefetches data, calculates speculated values. > 1: CPU B loads 0xeb > 2: CPU B loads 0xe5 > 3: > Write to (1) > 0xcc 0xe5 0xcc 0xe5 # breakpoint inserted > 4: send IPI > 5: mfence # serializing instruction. Flushes CPU B's > # pipeline > 6: > Write to (2) > 0xcc 0xf0 0xcc 0xf0 > 7: > Write to (1) > 0xeb 0xf0 0xeb 0xf0 > 8: CPU B instruction pointer gets to (1), needs to validate > all the pipeline speculation. Because we flushed any > speculation prior to the mfence, we're ok. > > > Now, I'll show why just using the breakpoint, without IPI, is > problematic: > > CPU A | CPU B > ------------------------- > 0xeb 0xe5 0xeb 0xe5 > 0: CPU B instruction pointer is earlier than (1) > CPU B pipeline speculatively predicts branches, > prefetches data, calculates speculated values. > 1: CPU B loads 0xeb > 2: CPU B loads 0xe5 > 3: > Write to (1) > 0xcc 0xe5 0xcc 0xf0 # breakpoint inserted > 4: > Write to (2) > 0xcc 0xf0 0xeb 0xf0 # Silly CPU B. Did not see nor use the breakpoint. > # Same problem as scenario "buggy". > 5: > Write to (1) > 0xeb 0xf0 0xeb 0xf0 > 4: CPU B instruction pointer gets to (1), needs to validate > all the pipeline speculation. > But ! The CPU does not expect code to change underneath. > General protection fault (or any other fault.. random..) > > So, basically, we ensure that the only transitions CPU B will see are > either: > > 0xeb 0xe5 -> 0xcc 0xe5 : OK, adding breakpoint > 0xcc 0xe5 -> 0xcc 0xf0 : OK, not using the operand anyway, it's a > breakpoint! > 0xcc 0xf0 -> 0xeb 0xf0 : OK, removing breakpoint > > *but*, the transition we guarantee that CPU B will *never* see without > having a mfence executed between the old and the new version is: > > 0xeb 0xe5 -> 0xeb 0xf0 <----- buggy. > > Hope the explanation helps, Thanks for explanation. One thing I'd like to know is why you are using mfence instead of cpuid (a.k.a. sync_core()). I assume that old processor doesn't have mfence and is that OK? Thank you, -- Masami Hiramatsu Software Engineer Hitachi Computer Products (America), Inc. Software Solutions Division e-mail: mhiramat@...hat.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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