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

> 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?

Oleg.

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