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Code Packages Activity

namedptr: concurrency and const cleanup

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Fix the locking order for the case where an exception is thrown
in shared_ptr's allocator.

More const.

Drop the explicit closure return type since the compiler can deduce it.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell
2021-11-15 10:37:25 -05:00
parent 5e379b4c48
commit 73f94750ec
1 changed files with 19 additions and 18 deletions
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37
include/crucible/namedptr.h
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@@ -82,7 +82,7 @@ namespace crucible {
// "our" map entry if it exists and is expired. The other
// thread would have done the same for us if the race had
// a different winner.
auto found = m_map_rep->m_map.find(m_ret_key);
const auto found = m_map_rep->m_map.find(m_ret_key);
if (found != m_map_rep->m_map.end() && found->second.expired()) {
m_map_rep->m_map.erase(found);
}
@@ -93,10 +93,10 @@ namespace crucible {
NamedPtr<Return, Arguments...>::lookup_item(const Key &k)
{
// Must be called with lock held
auto found = m_map_rep->m_map.find(k);
const auto found = m_map_rep->m_map.find(k);
if (found != m_map_rep->m_map.end()) {
// Get the strong pointer back
auto rv = found->second.lock();
const auto rv = found->second.lock();
if (rv) {
// Have strong pointer. Return value that shares map entry.
return shared_ptr<Return>(rv, rv->m_ret_ptr.get());
@@ -116,34 +116,36 @@ namespace crucible {
Key k(args...);
// Is it already in the map?
unique_lock<mutex> lock(m_map_rep->m_mutex);
unique_lock<mutex> lock_lookup(m_map_rep->m_mutex);
auto rv = lookup_item(k);
if (rv) {
return rv;
}
// Release map lock and acquire key lock
lock.unlock();
auto key_lock = m_lockset.make_lock(k);
lock_lookup.unlock();
const auto key_lock = m_lockset.make_lock(k);
// Did item appear in map while we were waiting for key?
lock.lock();
lock_lookup.lock();
rv = lookup_item(k);
if (rv) {
return rv;
}
// We now hold key and index locks, but item not in map (or expired).
// Release map lock
lock.unlock();
// Release map lock so other threads can use the map
lock_lookup.unlock();
// Call the function and create a new Value outside of the map
const auto new_value_ptr = make_shared<Value>(fn(args...), k, m_map_rep);
// Call the function and create a new Value
auto new_value_ptr = make_shared<Value>(fn(args...), k, m_map_rep);
// Function must return a non-null pointer
THROW_CHECK0(runtime_error, new_value_ptr->m_ret_ptr);
// Reacquire index lock for map insertion
lock.lock();
// Reacquire index lock for map insertion. We still hold the key lock.
// Use a different lock object to make exceptions unlock in the right order
unique_lock<mutex> lock_insert(m_map_rep->m_mutex);
// Insert return value in map or overwrite existing
// empty or expired weak_ptr value.
@@ -158,14 +160,13 @@ namespace crucible {
// to find and fix.
assert(new_item_ref.expired());
// Update the empty map slot
// Update the map slot we are sure is empty
new_item_ref = new_value_ptr;
// Drop lock so we don't deadlock in constructor exceptions
lock.unlock();
// Return shared_ptr to Return using strong pointer's reference counter
return shared_ptr<Return>(new_value_ptr, new_value_ptr->m_ret_ptr.get());
// Release map lock, then key lock
}
template <class Return, class... Arguments>
@@ -188,7 +189,7 @@ namespace crucible {
NamedPtr<Return, Arguments...>::insert(const Ptr &r, Arguments... args)
{
THROW_CHECK0(invalid_argument, r);
return insert_item([&](Arguments...) -> Ptr { return r; }, args...);
return insert_item([&](Arguments...) { return r; }, args...);
}
}