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Semaphores for ocaml probes #60

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Merged
merged 3 commits into from
Aug 5, 2021
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Semaphores for ocaml probes #60

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fb88229
Separate semaphore for ocaml probes to correctly support probe_is_ena…
gretay-js Nov 26, 2020
e7b5669
Emit semaphore symbol as weak and hidden and without compilation unit…
gretay-js Nov 26, 2020
d7189fb
Update version of ocaml probe notes
gretay-js Aug 4, 2021
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16 changes: 11 additions & 5 deletions backend/amd64/emit.mlp
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Original file line number Diff line number Diff line change
Expand Up @@ -559,7 +559,7 @@ let find_or_add_semaphore name =
match String.Map.find_opt name !probe_semaphores with
| Some label -> label
| None ->
let sym = Compilenv.make_symbol (Some ("semaphore_"^name)) in
let sym = "caml_probes_semaphore_"^name in
probe_semaphores := String.Map.add name sym !probe_semaphores;
sym

Expand Down Expand Up @@ -944,7 +944,11 @@ let emit_instr fallthrough i =
semaphores are of type unsigned short.
[1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
*)
I.mov (addressing (Ibased(semaphore_sym, 0)) WORD i 0) (res16 i 0);
(* OCaml has it's own semaphore, in addition to system tap,
to control ocaml probe handlers independently from stap probe handlers.
It is placed immediately after stap semaphore, and is the same
size - hence offset 2. *)
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I don't understand why this is offset 2 not 1. Is this value not in units of 8-byte words?

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The units are bytes, and the size of the new semaphore is set to be the same as the size of stap semaphores, so the code for reading them remains unchanged.

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ah, and the semaphores are 16-bit quantities. ok.

I.mov (addressing (Ibased(semaphore_sym, 2)) WORD i 0) (res16 i 0);
(* If the semaphore is 0, then the result is 0, otherwise 1. *)
I.cmp (int 0) (res16 i 0);
I.set (cond (Iunsigned Cne)) (res8 i 0);
Expand Down Expand Up @@ -1421,7 +1425,7 @@ let emit_probe_notes0 () =
D.qword (const 0)
end;
D.qword (ConstLabel semaphore_label);
D.bytes "ocaml\000";
D.bytes "ocaml.1\000";
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This name is a bit mysterious

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ah, suggestions for a better name are most welcome!
The prefix ".1" is the version number for ocaml probe notes. There is already a proposal for "ocaml.2" in PR #142.
The suffix "ocaml" is useful for distinguishing probes emitted by OCaml compiler from other stap "providers" of probes.

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Users will see these strings if they inspect probe notes using "readelf -n" or gdb's "info probes".

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This seems ok for now.

D.bytes (probe_name ^ "\000");
D.bytes (args ^ "\000");
def_label d;
Expand All @@ -1448,9 +1452,11 @@ let emit_probe_notes0 () =
end;
D.align 2;
String.Map.iter (fun _ label ->
D.global label;
D.weak label;
D.hidden label;
_label label;
D.word (const 0);
D.word (const 0); (* for systemtap probes *)
D.word (const 0); (* for ocaml probes *)
add_def_symbol label)
!probe_semaphores

Expand Down