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Kernel: Disable interrupts while setting up a thread blocker

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There was a race window between instantiating a WaitQueueBlocker and
setting the thread state to Blocked. If a thread was preempted between
those steps, someone else might try to wake the wait queue and find an
unblocked thread in a wait queue, which is not sane.
This commit is contained in:
Andreas Kling 2019-12-01 12:45:51 +01:00
parent f067730f6b
commit 9ed272ce98
Notes: sideshowbarker 2024-07-19 11:00:39 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/9ed272ce985
2 changed files with 24 additions and 3 deletions
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15
Kernel/Arch/i386/CPU.h
View file

@ -289,6 +289,21 @@ private:
u32 m_flags;
};
inline bool cli_and_save_interrupt_flag()
{
u32 flags = cpu_flags();
cli();
return flags & 0x200;
}
inline void restore_interrupt_flag(bool flag)
{
if (flag)
sti();
else
cli();
}
class InterruptDisabler {
public:
InterruptDisabler()

12
Kernel/Thread.h
View file

@ -274,11 +274,17 @@ public:
ASSERT(state() == Thread::Running);
ASSERT(m_blocker == nullptr);
T t(AK::forward<Args>(args)...);
// NOTE: We disable interrupts here to avoid the situation where a WaitQueueBlocker
// adds the current thread to a WaitQueue, and then someone wakes up before
// we set the state to Blocked decides to wake the queue. They would find
// unblocked threads in a wait queue, which would not be good. We can't go
// into Blocked state earlier, since that would prevent this thread from
// getting scheduled.
auto saved_if = cli_and_save_interrupt_flag();
T t(forward<Args>(args)...);
m_blocker = &t;
// Enter blocked state.
set_state(Thread::Blocked);
restore_interrupt_flag(saved_if);
// Yield to the scheduler, and wait for us to resume unblocked.
if (beneficiary) {