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signal: fix starvation by fast signal contexts

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In the past, a signal context, that was chosen for handling by
'Signal_receiver::pending_signal and always triggered again before
the next call of 'pending_signal', caused all other contexts behind
in the list to starve. This was the case because 'pending_signal'
always took the first pending context in its context list.

We avoid this problem now by handling pending signals in a round-robin
fashion instead.

Ref #2532
This commit is contained in:
Martin Stein
2017-09-17 12:30:20 +02:00
committed by Christian Helmuth
parent cd21074201
commit b811ef4331
4 changed files with 75 additions and 16 deletions
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2
repos/base/src/lib/base/signal.cc
View File

@ -224,7 +224,7 @@ Signal_context_capability Signal_receiver::manage(Signal_context *context)
Lock::Guard list_lock_guard(_contexts_lock);
/* insert context into context list */
_contexts.insert(&context->_receiver_le);
_contexts.insert(context);
/* register context at process-wide registry */
signal_context_registry()->insert(&context->_registry_le);

56
repos/base/src/lib/base/signal_common.cc
View File

@ -163,9 +163,7 @@ Signal Signal_receiver::pending_signal()
Lock::Guard list_lock_guard(_contexts_lock);
/* look up the contexts for the pending signal */
for (List_element<Signal_context> *le = _contexts.first(); le; le = le->next()) {
Signal_context *context = le->object();
for (Signal_context *context = _contexts.head(); context; context = context->_next) {
Lock::Guard lock_guard(context->_lock);
@ -173,6 +171,7 @@ Signal Signal_receiver::pending_signal()
if (!context->_pending)
continue;
_contexts.head(context);
context->_pending = false;
Signal::Data result = context->_curr_signal;
@ -206,8 +205,8 @@ Signal_receiver::~Signal_receiver()
Lock::Guard list_lock_guard(_contexts_lock);
/* disassociate contexts from the receiver */
for (List_element<Signal_context> *le; (le = _contexts.first()); )
_unsynchronized_dissolve(le->object());
while (_contexts.head())
_unsynchronized_dissolve(_contexts.head());
_platform_destructor();
}
@ -225,7 +224,7 @@ void Signal_receiver::_unsynchronized_dissolve(Signal_context * const context)
context->_cap = Signal_context_capability();
/* remove context from context list */
_contexts.remove(&context->_receiver_le);
_contexts.remove(context);
_platform_finish_dissolve(context);
}
@ -250,9 +249,7 @@ bool Signal_receiver::pending()
Lock::Guard list_lock_guard(_contexts_lock);
/* look up the contexts for the pending signal */
for (List_element<Signal_context> *le = _contexts.first(); le; le = le->next()) {
Signal_context *context = le->object();
for (Signal_context *context = _contexts.head(); context; context = context->_next) {
Lock::Guard lock_guard(context->_lock);
@ -261,3 +258,44 @@ bool Signal_receiver::pending()
}
return false;
}
void Signal_receiver::Context_list::insert(Signal_context *re)
{
if (_head) {
re->_prev = _head->_prev;
re->_next = nullptr;
_head->_prev->_next = re;
_head->_prev = re;
} else {
re->_prev = re;
re->_next = nullptr;
_head = re;
}
}
void Signal_receiver::Context_list::remove(Signal_context const *re)
{
if (re->_prev == re) {
_head = nullptr;
} else {
if (_head == re) {
_head = re->_next;
}
re->_prev->_next = re->_next;
if (re->_next) {
re->_next->_prev = re->_prev;
} else {
_head->_prev = re->_prev;
}
}
}
void Signal_receiver::Context_list::head(Signal_context *re)
{
_head->_prev->_next = _head;
_head = re;
_head->_prev->_next = nullptr;
}