[committed][gdb/symtab] Make per_cu->unit_type atomic

Tom de Vries via Gdb-patches
2022-07-14T06:20:05Z

Hi,

With gdb with -fsanitize=thread and test-case gdb.ada/array_bounds.exp, I run
into a data race between:
...
Read of size 1 at 0x7b2000025f0f by main thread:
#0 packed<dwarf_unit_type, 1ul>::operator dwarf_unit_type() const packed.h:54
#1 dwarf2_per_cu_data::set_unit_type(dwarf_unit_type) read.h:339
...
and:
...
Previous write of size 1 at 0x7b2000025f0f by thread T3:
#0 dwarf2_per_cu_data::set_unit_type(dwarf_unit_type) read.h:341
...

Fix this by making per_cu->unit_type atomic.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29286

Committed to trunk.

Thanks,
- Tom

[gdb/symtab] Make per_cu->unit_type atomic

---
gdb/dwarf2/read.h | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index fcf0cd70530..acbe298b310 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -178,7 +178,7 @@ struct dwarf2_per_cu_data

private:
/* The unit type of this CU. */
- packed<dwarf_unit_type, 1> m_unit_type = (dwarf_unit_type) 0;
+ std::atomic<packed<dwarf_unit_type, 1>> m_unit_type {(dwarf_unit_type)0};

/* The language of this CU. */
packed<language, LANGUAGE_BYTES> m_lang = language_unknown;
@@ -329,19 +329,24 @@ struct dwarf2_per_cu_data

dwarf_unit_type unit_type (bool strict_p = true) const
{
+ dwarf_unit_type ut = m_unit_type.load ();
if (strict_p)
- gdb_assert (m_unit_type != 0);
- return m_unit_type;
+ gdb_assert (ut != 0);
+ return ut;
}

void set_unit_type (dwarf_unit_type unit_type)
{
- if (m_unit_type == 0)
- /* Set if not set already. */
- m_unit_type = unit_type;
- else
- /* If already set, verify that it's the same value. */
- gdb_assert (m_unit_type == unit_type);
+ /* Set if not set already. */
+ packed<dwarf_unit_type, 1> nope = (dwarf_unit_type)0;
+ if (m_unit_type.compare_exchange_strong (nope, unit_type))
+ return;
+
+ /* If already set, verify that it's the same value. */
+ nope = unit_type;
+ if (m_unit_type.compare_exchange_strong (nope, unit_type))
+ return;
+ gdb_assert_not_reached ();
}

enum language lang (bool strict_p = true) const