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Kernel: Fix thread joining issues

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The thread joining logic hadn't been updated to account for the subtle
differences introduced by software context switching. This fixes several
race conditions related to thread destruction and joining, as well as
finalization which did not properly account for detached state and the
fact that threads can be joined after termination as long as they're not
detached.

Fixes #3596
This commit is contained in:
Tom 2020-09-25 21:44:43 -06:00 • committed by Andreas Kling
parent b245121f13
commit 1727b2d7cd
5 changed files with 196 additions and 82 deletions
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40
Kernel/Thread.cpp
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@ -110,6 +110,8 @@ Thread::~Thread()
auto thread_cnt_before = m_process->m_thread_count.fetch_sub(1, AK::MemoryOrder::memory_order_acq_rel);
ASSERT(thread_cnt_before != 0);
ASSERT(!m_joiner);
}
void Thread::unblock()
@ -192,6 +194,9 @@ void Thread::die_if_needed()
void Thread::yield_without_holding_big_lock()
{
bool did_unlock = unlock_process_if_locked();
// NOTE: Even though we call Scheduler::yield here, unless we happen
// to be outside of a critical section, the yield will be postponed
// until leaving it in relock_process.
Scheduler::yield();
relock_process(did_unlock);
}
@ -203,8 +208,20 @@ bool Thread::unlock_process_if_locked()
void Thread::relock_process(bool did_unlock)
{
if (did_unlock)
// Clearing the critical section may trigger the context switch
// flagged by calling Scheduler::donate_to or Scheduler::yield
// above. We have to do it this way because we intentionally
// leave the critical section here to be able to switch contexts.
u32 prev_flags;
u32 prev_crit = Processor::current().clear_critical(prev_flags, true);
if (did_unlock) {
// We've unblocked, relock the process if needed and carry on.
process().big_lock().lock();
}
// NOTE: We may be on a differenct CPU now!
Processor::current().restore_critical(prev_crit, prev_flags);
}
u64 Thread::sleep(u64 ticks)
@ -263,14 +280,9 @@ void Thread::finalize()
#endif
set_state(Thread::State::Dead);
if (m_joiner) {
ScopedSpinLock lock(m_joiner->m_lock);
ASSERT(m_joiner->m_joinee == this);
static_cast<JoinBlocker*>(m_joiner->m_blocker)->set_joinee_exit_value(m_exit_value);
static_cast<JoinBlocker*>(m_joiner->m_blocker)->set_interrupted_by_death();
m_joiner->m_joinee = nullptr;
// NOTE: We clear the joiner pointer here as well, to be tidy.
m_joiner = nullptr;
if (auto* joiner = m_joiner.exchange(nullptr, AK::memory_order_acq_rel)) {
// Notify joiner that we exited
static_cast<JoinBlocker*>(joiner->m_blocker)->joinee_exited(m_exit_value);
}
if (m_dump_backtrace_on_finalization)
@ -992,19 +1004,9 @@ Thread::BlockResult Thread::wait_on(WaitQueue& queue, const char* reason, timeva
Scheduler::yield();
}
// Clearing the critical section may trigger the context switch
// flagged by calling Scheduler::donate_to or Scheduler::yield
// above. We have to do it this way because we intentionally
// leave the critical section here to be able to switch contexts.
u32 prev_flags;
u32 prev_crit = Processor::current().clear_critical(prev_flags, true);
// We've unblocked, relock the process if needed and carry on.
relock_process(did_unlock);
// NOTE: We may be on a differenct CPU now!
Processor::current().restore_critical(prev_crit, prev_flags);
// This looks counter productive, but we may not actually leave
// the critical section we just restored. It depends on whether
// we were in one while being called.