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SubjectRe: [PATCH v5 06/10] Uprobes: Support SDT markers having reference count (semaphore)
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Hi Oleg,

On 07/03/2018 10:06 PM, Oleg Nesterov wrote:
> On 07/03, Ravi Bangoria wrote:
>>
>> Ok let me explain the difference.
>>
>> Current approach:
>>
>> ------------
>> register_for_each_vma() / uprobe_mmap()
>> install_breakpoint()
>> uprobe_write_opcode() {
>> if (instruction is not already patched) {
>> /* Gets called only _once_. */
>> increment the reference counter;
>> patch the instruction;
>> }
>> }
>
> Yes I see. And I am not sure this all is correct. And I still hope we can do
> something better, I'll write another email.
>
> For now, let's discuss your current approach.
>
>> Now, if I put it inside install_breakpoint():
>>
>> ------------
>> uprobe_register()
>> register_for_each_vma()
>> install_breakpoint() {
>> /* Called _for each consumer_ */
>
> How so? it is not called for each consumer. I think you misread this code.


Actually, I meant entire sequence

uprobe_register()
register_for_each_vma()
install_breakpoint()

gets called for each consumer. Not just install_breakpoint(). Sorry
for a bit of ambiguity.


>
>> increment the reference counter _once_;
>> uprobe_write_opcode()
>> ...
>> }
>
> So. I meant that you can move the _same_ logic into install_breakpoint() and
> remove_breakpoint(). And note that ref_ctr_updated in uprobe_write_opcode() is
> only needed because it can retry the fault.
>
> IOW, you can simply do update_ref_ctr(is_register => 1) at the start of
> install_breakpoint(), and update_ref_ctr(0) in remove_breakpoint(), there are
> no other callers of uprobe_write_opcode(). To clarify, it is indirectly called
> by set_swbp() and set_orig_insn(), but this doesn't matter.
>
> Or you can kill update_ref_ctr() and (roughly) do
>
> rc_vma = find_ref_ctr_vma(...);
> if (rc_vma)
> __update_ref_ctr(..., 1);
> else
> delayed_uprobe_add(...);
>
> at the start of install_breakpoint() and
>
> rc_vma = find_ref_ctr_vma(...);
> if (rc_vma)
> __update_ref_ctr(..., -1);
> delayed_uprobe_remove(...);
>
> in remove_breakpoint().
>
>
>> uprobe_mmap()
>> install_breakpoint() {
>> increment the reference counter _for each consumer_;
>
> Again, I do not understand where do you see the "for each consumer" thing.
>
>> uprobe_write_opcode()
>
> In short. There is a 1:1 relationship between uprobe_write_opcode(is_register => 1)
> and install_breakpoint(), and between uprobe_write_opcode(is_register => 0) and
> remove_breakpoint(). Whatever uprobe_write_opcode() can do if is_register == 1 can be
> done in install_breakpoint(), the same for is_register == 0 and remove_breakpont().
>
> What have I missed?


Yes, the verify_opcode() stuff as you have mentioned in another reply.

Thanks,
Ravi

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