[binutils-gdb] Avoid race when writing to index cache

[Tom Tromey <tromey@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=ed29a346be439466ff2a5ce33e715e02c49fbdac

commit ed29a346be439466ff2a5ce33e715e02c49fbdac
Author: Tom Tromey <tom@tromey.com>
Date:   Sun Jan 28 09:14:04 2024 -0700

    Avoid race when writing to index cache
    
    The background DWARF reader changes introduced a race when writing to
    the index cache.  The problem here is that constructing the
    index_cache_store_context object should only happen on the main
    thread, to ensure that the various value captures do not race.
    
    This patch adds an assert to the construct to that effect, and then
    arranges for this object to be constructed by the cooked_index_worker
    constructor -- which is only invoked on the main thread.
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31262

Diff:
---
 gdb/dwarf2/cooked-index.c | 27 ++++++++++++---------------
 gdb/dwarf2/cooked-index.h | 15 ++++++++++-----
 gdb/dwarf2/index-cache.c  |  4 ++++
 3 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/gdb/dwarf2/cooked-index.c b/gdb/dwarf2/cooked-index.c
index 5cbee83228b..2a1ca6fd225 100644
--- a/gdb/dwarf2/cooked-index.c
+++ b/gdb/dwarf2/cooked-index.c
@@ -578,6 +578,15 @@ cooked_index_worker::set (cooked_state desired_state)
 #endif /* CXX_STD_THREAD */
 }
 
+/* See cooked-index.h.  */
+
+void
+cooked_index_worker::write_to_cache (const cooked_index *idx) const
+{
+  if (idx != nullptr)
+    m_cache_store.store ();
+}
+
 cooked_index::cooked_index (dwarf2_per_objfile *per_objfile,
 			    std::unique_ptr<cooked_index_worker> &&worker)
   : m_state (std::move (worker)),
@@ -621,17 +630,16 @@ cooked_index::set_contents (vec_type &&vec)
 
   m_state->set (cooked_state::MAIN_AVAILABLE);
 
-  index_cache_store_context ctx (global_index_cache, m_per_bfd);
-
   /* This is run after finalization is done -- but not before.  If
      this task were submitted earlier, it would have to wait for
      finalization.  However, that would take a slot in the global
      thread pool, and if enough such tasks were submitted at once, it
      would cause a livelock.  */
-  gdb::task_group finalizers ([this, ctx = std::move (ctx)] ()
+  gdb::task_group finalizers ([this] ()
   {
     m_state->set (cooked_state::FINALIZED);
-    maybe_write_index (ctx);
+    m_state->write_to_cache (index_for_writing ());
+    m_state->set (cooked_state::CACHE_DONE);
   });
 
   for (auto &idx : m_vector)
@@ -836,17 +844,6 @@ cooked_index::dump (gdbarch *arch)
     }
 }
 
-void
-cooked_index::maybe_write_index (const index_cache_store_context &ctx)
-{
-  if (index_for_writing () != nullptr)
-    {
-      /* (maybe) store an index in the cache.  */
-      ctx.store ();
-    }
-  m_state->set (cooked_state::CACHE_DONE);
-}
-
 /* Wait for all the index cache entries to be written before gdb
    exits.  */
 static void
diff --git a/gdb/dwarf2/cooked-index.h b/gdb/dwarf2/cooked-index.h
index 17659df0531..91419b79edc 100644
--- a/gdb/dwarf2/cooked-index.h
+++ b/gdb/dwarf2/cooked-index.h
@@ -467,7 +467,8 @@ class cooked_index_worker
 public:
 
   explicit cooked_index_worker (dwarf2_per_objfile *per_objfile)
-    : m_per_objfile (per_objfile)
+    : m_per_objfile (per_objfile),
+      m_cache_store (global_index_cache, per_objfile->per_bfd)
   { }
   virtual ~cooked_index_worker ()
   { }
@@ -486,10 +487,15 @@ public:
 
 protected:
 
-  /* Let cooked_index call the 'set' method.  */
+  /* Let cooked_index call the 'set' and 'write_to_cache' methods.  */
   friend class cooked_index;
+
+  /* Set the current state.  */
   void set (cooked_state desired_state);
 
+  /* Write to the index cache.  */
+  void write_to_cache (const cooked_index *idx) const;
+
   /* Helper function that does the work of reading.  This must be able
      to be run in a worker thread without problems.  */
   virtual void do_reading () = 0;
@@ -534,6 +540,8 @@ protected:
      scanning is stopped and this exception will later be reported by
      the 'wait' method.  */
   std::optional<gdb_exception> m_failed;
+  /* An object used to write to the index cache.  */
+  index_cache_store_context m_cache_store;
 };
 
 /* The main index of DIEs.
@@ -671,9 +679,6 @@ public:
 
 private:
 
-  /* Maybe write the index to the index cache.  */
-  void maybe_write_index (const index_cache_store_context &);
-
   /* The vector of cooked_index objects.  This is stored because the
      entries are stored on the obstacks in those objects.  */
   vec_type m_vector;
diff --git a/gdb/dwarf2/index-cache.c b/gdb/dwarf2/index-cache.c
index 780d2c4f200..d9047ef46bb 100644
--- a/gdb/dwarf2/index-cache.c
+++ b/gdb/dwarf2/index-cache.c
@@ -33,6 +33,7 @@
 #include "gdbsupport/selftest.h"
 #include <string>
 #include <stdlib.h>
+#include "run-on-main-thread.h"
 
 /* When set to true, show debug messages about the index cache.  */
 static bool debug_index_cache = false;
@@ -94,6 +95,9 @@ index_cache_store_context::index_cache_store_context (const index_cache &ic,
      m_dir (ic.m_dir),
      m_per_bfd (per_bfd)
 {
+  /* Capturing globals may only be done on the main thread.  */
+  gdb_assert (is_main_thread ());
+
   if (!m_enabled)
     return;