Messages in this thread | | | Date | Thu, 4 Sep 2014 11:52:27 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v5 3/3] kprobes: arm: enable OPTPROBES for ARM 32 |
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On Thu, Sep 04, 2014 at 11:40:35AM +0100, Jon Medhurst (Tixy) wrote: > On Wed, 2014-09-03 at 11:30 +0100, Will Deacon wrote: > > On Wed, Sep 03, 2014 at 11:18:04AM +0100, Masami Hiramatsu wrote: > > > (2014/09/02 22:49), Jon Medhurst (Tixy) wrote: > > > > 1. On SMP systems it's very slow because of kprobe's use of stop_machine > > > > for applying and removing probes, this forces the system to idle and > > > > wait for the next scheduler tick for each probe change. > > > > > > Hmm, agreed. It seems that arm32 limitation of self-modifying code on SMP. > > > I'm not sure how we can handle it, but I guess; > > > - for some processors which have better coherent cache for SMP, we can > > > atomically replace the breakpoint code with original code. > > > > Except that it's not an architected breakpoint instruction, as I mentioned > > before. It's also not really a property of the cache. > > > > > - Even if we get an "undefined instruction" exception, its handler can > > > ask kprobes if the address is under modifying or not. And if it is, > > > we can just return from the exception to retry the execution. > > > > It's not as simple as that -- you could potentially see an interleaving of > > the two instructions. The architecture is even broader than that: > > > > Concurrent modification and execution of instructions can lead to the > > resulting instruction performing any behavior that can be achieved by > > executing any sequence of instructions that can be executed from the > > same Exception level, > > > > There are additional guarantees for some instructions (like the architected > > BKPT instruction). > > I should point out that the current implementation of kprobes doesn't > use stop_machine because it's trying to meet the above architecture > restrictions, and that arming kprobes (changing probed instruction to an > undefined instruction) isn't usually done under stop_machine, so other > CPUs could be executing the original instruction as it's being modified. > > So, should we be making patch_text unconditionally use stop machine and > remove all direct use of __patch_text? (E.g. by jump labels.)
You could take a look at what we do for arm64 (see aarch64_insn_hotpatch_safe) for inspiration.
Will
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