Kernel+LibC: Remove sys$donate()

This was an old SerenityOS-specific syscall for donating the remainder
of the calling thread's time-slice to another thread within the same
process.

Now that Threading::Lock uses a pthread_mutex_t internally, we no
longer need this syscall, which allows us to get rid of a surprising
amount of unnecessary scheduler logic. :^)

Kernel/API/Syscall.h

@@ -113,7 +113,6 @@ namespace Kernel {
     S(setsockopt)                 \
     S(create_thread)              \
     S(gettid)                     \
-    S(donate)                     \
     S(rename)                     \
     S(ftruncate)                  \
     S(exit_thread)                \

Kernel/Process.h

@@ -276,7 +276,6 @@ public:
     KResultOr<FlatPtr> sys$dbgputstr(Userspace<const u8*>, size_t);
     KResultOr<FlatPtr> sys$dump_backtrace();
     KResultOr<FlatPtr> sys$gettid();
-    KResultOr<FlatPtr> sys$donate(pid_t tid);
     KResultOr<FlatPtr> sys$setsid();
     KResultOr<FlatPtr> sys$getsid(pid_t);
     KResultOr<FlatPtr> sys$setpgid(pid_t pid, pid_t pgid);

Kernel/Scheduler.cpp

@@ -31,8 +31,6 @@ class SchedulerPerProcessorData {
 public:
     SchedulerPerProcessorData() = default;
 
-    WeakPtr<Thread> m_pending_beneficiary;
-    const char* m_pending_donate_reason { nullptr };
     bool m_in_scheduler { true };
 };
 
@@ -206,26 +204,6 @@ bool Scheduler::pick_next()
         dump_thread_list();
     }
 
-    auto pending_beneficiary = scheduler_data.m_pending_beneficiary.strong_ref();
-    if (pending_beneficiary && dequeue_runnable_thread(*pending_beneficiary, true)) {
-        // The thread we're supposed to donate to still exists and we can
-        const char* reason = scheduler_data.m_pending_donate_reason;
-        scheduler_data.m_pending_beneficiary = nullptr;
-        scheduler_data.m_pending_donate_reason = nullptr;
-
-        // We need to leave our first critical section before switching context,
-        // but since we're still holding the scheduler lock we're still in a critical section
-        critical.leave();
-
-        dbgln_if(SCHEDULER_DEBUG, "Processing pending donate to {} reason={}", *pending_beneficiary, reason);
-        return donate_to_and_switch(pending_beneficiary.ptr(), reason);
-    }
-
-    // Either we're not donating or the beneficiary disappeared.
-    // Either way clear any pending information
-    scheduler_data.m_pending_beneficiary = nullptr;
-    scheduler_data.m_pending_donate_reason = nullptr;
-
     auto& thread_to_schedule = pull_next_runnable_thread();
     if constexpr (SCHEDULER_DEBUG) {
 #if ARCH(I386)
@@ -250,11 +228,6 @@ bool Scheduler::yield()
 {
     InterruptDisabler disabler;
     auto& proc = Processor::current();
-    auto& scheduler_data = proc.get_scheduler_data();
-
-    // Clear any pending beneficiary
-    scheduler_data.m_pending_beneficiary = nullptr;
-    scheduler_data.m_pending_donate_reason = nullptr;
 
     auto current_thread = Thread::current();
     dbgln_if(SCHEDULER_DEBUG, "Scheduler[{}]: yielding thread {} in_irq={}", proc.get_id(), *current_thread, proc.in_irq());
@@ -275,66 +248,6 @@ bool Scheduler::yield()
     return true;
 }
 
-bool Scheduler::donate_to_and_switch(Thread* beneficiary, [[maybe_unused]] const char* reason)
-{
-    VERIFY(g_scheduler_lock.own_lock());
-
-    auto& proc = Processor::current();
-    VERIFY(proc.in_critical() == 1);
-
-    unsigned ticks_left = Thread::current()->ticks_left();
-    if (!beneficiary || beneficiary->state() != Thread::Runnable || ticks_left <= 1)
-        return Scheduler::yield();
-
-    unsigned ticks_to_donate = min(ticks_left - 1, time_slice_for(*beneficiary));
-    dbgln_if(SCHEDULER_DEBUG, "Scheduler[{}]: Donating {} ticks to {}, reason={}", proc.get_id(), ticks_to_donate, *beneficiary, reason);
-    beneficiary->set_ticks_left(ticks_to_donate);
-
-    return Scheduler::context_switch(beneficiary);
-}
-
-bool Scheduler::donate_to(RefPtr<Thread>& beneficiary, const char* reason)
-{
-    VERIFY(beneficiary);
-
-    if (beneficiary == Thread::current())
-        return Scheduler::yield();
-
-    // Set the m_in_scheduler flag before acquiring the spinlock. This
-    // prevents a recursive call into Scheduler::invoke_async upon
-    // leaving the scheduler lock.
-    ScopedCritical critical;
-    auto& proc = Processor::current();
-    auto& scheduler_data = proc.get_scheduler_data();
-    scheduler_data.m_in_scheduler = true;
-    ScopeGuard guard(
-        []() {
-            // We may be on a different processor after we got switched
-            // back to this thread!
-            auto& scheduler_data = Processor::current().get_scheduler_data();
-            VERIFY(scheduler_data.m_in_scheduler);
-            scheduler_data.m_in_scheduler = false;
-        });
-
-    VERIFY(!proc.in_irq());
-
-    if (proc.in_critical() > 1) {
-        scheduler_data.m_pending_beneficiary = beneficiary; // Save the beneficiary
-        scheduler_data.m_pending_donate_reason = reason;
-        proc.invoke_scheduler_async();
-        return false;
-    }
-
-    ScopedSpinLock lock(g_scheduler_lock);
-
-    // "Leave" the critical section before switching context. Since we
-    // still hold the scheduler lock, we're not actually leaving it.
-    // Processor::switch_context expects Processor::in_critical() to be 1
-    critical.leave();
-    donate_to_and_switch(beneficiary, reason);
-    return false;
-}
-
 bool Scheduler::context_switch(Thread* thread)
 {
     if (s_mm_lock.own_lock()) {

Kernel/Scheduler.h

@@ -36,8 +36,6 @@ public:
     static bool pick_next();
     static bool yield();
     static void yield_from_critical();
-    static bool donate_to_and_switch(Thread*, const char* reason);
-    static bool donate_to(RefPtr<Thread>&, const char* reason);
     static bool context_switch(Thread*);
     static void enter_current(Thread& prev_thread, bool is_first);
     static void leave_on_first_switch(u32 flags);

Kernel/Syscalls/sched.cpp

@@ -15,20 +15,6 @@ KResultOr<FlatPtr> Process::sys$yield()
     return 0;
 }
 
-KResultOr<FlatPtr> Process::sys$donate(pid_t tid)
-{
-    REQUIRE_PROMISE(stdio);
-    if (tid < 0)
-        return EINVAL;
-
-    ScopedCritical critical;
-    auto thread = Thread::from_tid(tid);
-    if (!thread || thread->pid() != pid())
-        return ESRCH;
-    Thread::current()->donate_without_holding_big_lock(thread, "sys$donate");
-    return 0;
-}
-
 KResultOr<FlatPtr> Process::sys$sched_setparam(int pid, Userspace<const struct sched_param*> user_param)
 {
     REQUIRE_PROMISE(proc);

Kernel/Thread.cpp

@@ -334,18 +334,6 @@ void Thread::yield_without_holding_big_lock()
     relock_process(previous_locked, lock_count_to_restore);
 }
 
-void Thread::donate_without_holding_big_lock(RefPtr<Thread>& thread, const char* reason)
-{
-    VERIFY(!g_scheduler_lock.own_lock());
-    u32 lock_count_to_restore = 0;
-    auto previous_locked = unlock_process_if_locked(lock_count_to_restore);
-    // 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::donate_to(thread, reason);
-    relock_process(previous_locked, lock_count_to_restore);
-}
-
 LockMode Thread::unlock_process_if_locked(u32& lock_count_to_restore)
 {
     return process().big_lock().force_unlock_if_locked(lock_count_to_restore);
@@ -354,8 +342,8 @@ LockMode Thread::unlock_process_if_locked(u32& lock_count_to_restore)
 void Thread::relock_process(LockMode previous_locked, u32 lock_count_to_restore)
 {
     // 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
+    // flagged by calling 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);

Kernel/Thread.h

@@ -1337,7 +1337,6 @@ private:
     bool m_is_profiling_suppressed { false };
 
     void yield_without_holding_big_lock();
-    void donate_without_holding_big_lock(RefPtr<Thread>&, const char*);
     void yield_while_not_holding_big_lock();
     void drop_thread_count(bool);
 };

Userland/Libraries/LibC/unistd.cpp

@@ -695,12 +695,6 @@ int gettid()
     return cached_tid;
 }
 
-int donate(int tid)
-{
-    int rc = syscall(SC_donate, tid);
-    __RETURN_WITH_ERRNO(rc, rc, -1);
-}
-
 void sysbeep()
 {
     syscall(SC_beep);

Userland/Libraries/LibC/unistd.h

@@ -40,7 +40,6 @@ void dump_backtrace();
 int fsync(int fd);
 void sysbeep();
 int gettid();
-int donate(int tid);
 int getpagesize();
 pid_t fork();
 pid_t vfork();