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lockset: avoid starvation using a priority queue

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Mutex locks are released and acquired unfairly, causing arbitrary delays
in acquiring locks.  This prevents threads from releasing subvol FD's
which in turn blocks subvol deletes.

Fix by implementing a priority queue in LockSet to ensure fairness.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2017-10-01 15:53:11 -04:00
parent a3cd3ca07f
commit 8ea92202fc
1 changed files with 39 additions and 3 deletions
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42
include/crucible/lockset.h
View File

@ -1,6 +1,7 @@
#ifndef CRUCIBLE_LOCKSET_H
#define CRUCIBLE_LOCKSET_H
#include <crucible/cleanup.h>
#include <crucible/error.h>
#include <crucible/process.h>
@ -12,6 +13,8 @@
#include <map>
#include <memory>
#include <mutex>
#include <set>
#include <thread>
namespace crucible {
using namespace std;
@ -29,8 +32,11 @@ namespace crucible {
mutex m_mutex;
condition_variable m_condvar;
size_t m_max_size = numeric_limits<size_t>::max();
set<uint64_t> m_priorities;
uint64_t m_priority_counter;
bool full();
bool first_in_priority(uint64_t my_priority);
bool locked(const key_type &name);
class Lock {
@ -96,6 +102,26 @@ namespace crucible {
return m_set.size() >= m_max_size;
}
template <class T>
bool
LockSet<T>::first_in_priority(uint64_t my_priority)
{
#if 1
auto counter = m_max_size;
for (auto i : m_priorities) {
if (i == my_priority) {
return true;
}
if (++counter > m_max_size) {
return false;
}
}
THROW_ERROR(runtime_error, "my_priority " << my_priority << " not in m_priorities (size " << m_priorities.size() << ")");
#else
return *m_priorities.begin() == my_priority;
#endif
}
template <class T>
bool
LockSet<T>::locked(const key_type &name)
@ -105,9 +131,10 @@ namespace crucible {
template <class T>
void
LockSet<T>::max_size(size_t s)
LockSet<T>::max_size(size_t new_max_size)
{
m_max_size = s;
THROW_CHECK1(out_of_range, new_max_size, new_max_size > 0);
m_max_size = new_max_size;
}
template <class T>
@ -115,11 +142,18 @@ namespace crucible {
LockSet<T>::lock(const key_type &name)
{
unique_lock<mutex> lock(m_mutex);
while (full() || locked(name)) {
auto my_priority = m_priority_counter++;
Cleanup cleanup([&]() {
m_priorities.erase(my_priority);
});
m_priorities.insert(my_priority);
while (full() || locked(name) || !first_in_priority(my_priority)) {
m_condvar.wait(lock);
}
auto rv = m_set.insert(make_pair(name, gettid()));
THROW_CHECK0(runtime_error, rv.second);
// We removed our priority slot so other threads have to check again
m_condvar.notify_all();
}
template <class T>
@ -142,6 +176,8 @@ namespace crucible {
unique_lock<mutex> lock(m_mutex);
auto erase_count = m_set.erase(name);
m_condvar.notify_all();
lock.unlock();
this_thread::yield();
THROW_CHECK1(invalid_argument, erase_count, erase_count == 1);
}