[PATCH 2/2] Add locking when reading BFD sections

[Tom Tromey <tom@tromey.com>]

This adds some per-BFD locking to gdb_bfd_map_section and
gdb_bfd_get_full_section_contents.

It turned out that the background DWARF reader could race with the
auto-load code, because the reader might try to mmap a section when
the main thread was trying to read in .debug_gdb_scripts.

The current BFD threading model is that only BFD globals will be
locked, so any multi-threaded use of a BFD has to be handled specially
by the application.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31626
---
 gdb/gdb_bfd.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/gdb/gdb_bfd.c b/gdb/gdb_bfd.c
index b142f985dcd99e6d29f4a6b295ebff50ca851293..fff71d0c3bdb900e180ec136429bf66c030fd1bc 100644
--- a/gdb/gdb_bfd.c
+++ b/gdb/gdb_bfd.c
@@ -144,6 +144,19 @@ struct gdb_bfd_data
 
   /* The registry.  */
   registry<bfd> registry_fields;
+
+#if CXX_STD_THREAD
+  /* Most of the locking needed for multi-threaded operation is
+     handled by BFD itself.  However, the current BFD model is that
+     locking is only needed for global operations -- but it turned out
+     that the background DWARF reader could race with the auto-load
+     code reading the .debug_gdb_scripts section from the same BFD.
+
+     This lock is the fix: wrappers for important BFD functions will
+     acquire this lock before performing operations that might modify
+     the state of this BFD.  */
+  std::mutex per_bfd_mutex;
+#endif
 };
 
 registry<bfd> *
@@ -777,6 +790,11 @@ gdb_bfd_map_section (asection *sectp, bfd_size_type *size)
 
   abfd = sectp->owner;
 
+#if CXX_STD_THREAD
+  gdb_bfd_data *gdata = (gdb_bfd_data *) bfd_usrdata (abfd);
+  std::lock_guard<std::mutex> guard (gdata->per_bfd_mutex);
+#endif
+
   descriptor = get_section_descriptor (sectp);
 
   /* If the data was already read for this BFD, just reuse it.  */
@@ -1108,6 +1126,11 @@ bool
 gdb_bfd_get_full_section_contents (bfd *abfd, asection *section,
 				   gdb::byte_vector *contents)
 {
+#if CXX_STD_THREAD
+  gdb_bfd_data *gdata = (gdb_bfd_data *) bfd_usrdata (abfd);
+  std::lock_guard<std::mutex> guard (gdata->per_bfd_mutex);
+#endif
+
   bfd_size_type section_size = bfd_section_size (section);
 
   contents->resize (section_size);

-- 
2.46.1