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Fix race in background index-cache writing

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Tom de Vries pointed out a bug in the index-cache background writer --
sometimes it will fail.  He also noted that it fails when the number
of worker threads is set to zero.  These turn out to be the same
problem -- the cache can't be written to until the per-BFD's
"index_table" member is set.

This patch avoids the race by rearranging the code slightly, to ensure
the cache cannot possibly be written before the member is set.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30261
This commit is contained in:
Tom Tromey
2023-03-24 15:53:22 -06:00
parent 03d83cd5f5
commit 6f214d0f39
3 changed files with 24 additions and 11 deletions
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24
gdb/dwarf2/cooked-index.c
View File

@@ -443,26 +443,32 @@ cooked_index_shard::wait (bool allow_quit) const
m_future.wait ();
}
cooked_index::cooked_index (vec_type &&vec, dwarf2_per_bfd *per_bfd)
cooked_index::cooked_index (vec_type &&vec)
: m_vector (std::move (vec))
{
for (auto &idx : m_vector)
idx->finalize ();
/* This must be set after all the finalization tasks have been
started, because it may call 'wait'. */
m_write_future
= gdb::thread_pool::g_thread_pool->post_task ([this, per_bfd] ()
{
maybe_write_index (per_bfd);
});
/* ACTIVE_VECTORS is not locked, and this assert ensures that this
will be caught if ever moved to the background. */
gdb_assert (is_main_thread ());
active_vectors.insert (this);
}
/* See cooked-index.h. */
void
cooked_index::start_writing_index (dwarf2_per_bfd *per_bfd)
{
/* This must be set after all the finalization tasks have been
started, because it may call 'wait'. */
m_write_future
= gdb::thread_pool::g_thread_pool->post_task ([this, per_bfd] ()
{
maybe_write_index (per_bfd);
});
}
cooked_index::~cooked_index ()
{
/* The 'finalize' method may be run in a different thread. If

5
gdb/dwarf2/cooked-index.h
View File

@@ -366,7 +366,7 @@ public:
object. */
using vec_type = std::vector<std::unique_ptr<cooked_index_shard>>;
cooked_index (vec_type &&vec, dwarf2_per_bfd *per_bfd);
explicit cooked_index (vec_type &&vec);
~cooked_index () override;
DISABLE_COPY_AND_ASSIGN (cooked_index);
@@ -429,6 +429,9 @@ public:
m_write_future.wait ();
}
/* Start writing to the index cache, if the user asked for this. */
void start_writing_index (dwarf2_per_bfd *per_bfd);
private:
/* Maybe write the index to the index cache. */

6
gdb/dwarf2/read.c
View File

@@ -5147,9 +5147,13 @@ dwarf2_build_psymtabs_hard (dwarf2_per_objfile *per_objfile)
indexes.push_back (index_storage.release ());
indexes.shrink_to_fit ();
cooked_index *vec = new cooked_index (std::move (indexes), per_bfd);
cooked_index *vec = new cooked_index (std::move (indexes));
per_bfd->index_table.reset (vec);
/* Cannot start writing the index entry until after the
'index_table' member has been set. */
vec->start_writing_index (per_bfd);
const cooked_index_entry *main_entry = vec->get_main ();
if (main_entry != nullptr)
{