zircon/kernel/kernel/brwlock.cc - fuchsia - Git at Google

 // Copyright 2018 The Fuchsia Authors
 //
 // Use of this source code is governed by a MIT-style
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT

 #include "kernel/brwlock.h"

 #include <lib/zircon-internal/macros.h>

 #include <kernel/auto_preempt_disabler.h>
 #include <kernel/thread_lock.h>
 #include <ktl/limits.h>

 namespace internal {

 template <BrwLockEnablePi PI>
 BrwLock<PI>::~BrwLock() {
   DEBUG_ASSERT(state_.state_.load(ktl::memory_order_relaxed) == 0);
 }

 template <BrwLockEnablePi PI>
 void BrwLock<PI>::Block(bool write) {
   zx_status_t ret;

   auto reason = write ? ResourceOwnership::Normal : ResourceOwnership::Reader;

   if constexpr (PI == BrwLockEnablePi::Yes) {
     ret = wait_.BlockAndAssignOwner(Deadline::infinite(),
                                     state_.writer_.load(ktl::memory_order_relaxed), reason,
                                     Interruptible::No);
   } else {
     ret = wait_.BlockEtc(Deadline::infinite(), 0, reason, Interruptible::No);
   }

   if (unlikely(ret < ZX_OK)) {
     panic(
         "BrwLock<%d>::Block: Block returned with error %d lock %p, thr %p, "
         "sp %p\n",
         static_cast<bool>(PI), ret, this, Thread::Current::Get(), __GET_FRAME());
   }
 }

 template <BrwLockEnablePi PI>
 ResourceOwnership BrwLock<PI>::Wake() {
   if constexpr (PI == BrwLockEnablePi::Yes) {
     using Action = OwnedWaitQueue::Hook::Action;
     struct Context {
       ResourceOwnership ownership;
       BrwLockState<PI>& state;
     };
     Context context = {ResourceOwnership::Normal, state_};
     auto cbk = [](Thread* woken, void* ctx) -> Action {
       Context* context = reinterpret_cast<Context*>(ctx);

       if (context->ownership == ResourceOwnership::Normal) {
         // Check if target is blocked for writing and not reading
         if (woken->state() == THREAD_BLOCKED) {
           context->state.writer_.store(woken, ktl::memory_order_relaxed);
           context->state.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter,
                                           ktl::memory_order_acq_rel);
           return Action::SelectAndAssignOwner;
         }
         // If not writing then we must be blocked for reading
         DEBUG_ASSERT(woken->state() == THREAD_BLOCKED_READ_LOCK);
         context->ownership = ResourceOwnership::Reader;
       }
       // Our current ownership is ResourceOwnership::Reader otherwise we would
       // have returned early
       DEBUG_ASSERT(context->ownership == ResourceOwnership::Reader);
       if (woken->state() == THREAD_BLOCKED_READ_LOCK) {
         // We are waking readers and we found a reader, so we can wake them up and
         // search for me.
         context->state.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader,
                                         ktl::memory_order_acq_rel);
         return Action::SelectAndKeepGoing;
       } else {
         // We are waking readers but we have found a writer. To preserve fairness we
         // immediately stop and do not wake this thread or any others.
         return Action::Stop;
       }
     };

     bool resched = wait_.WakeThreads(ktl::numeric_limits<uint32_t>::max(), {cbk, &context});
     if (resched) {
       Scheduler::Reschedule();
     }
     return context.ownership;
   } else {
     Thread* next = wait_.Peek();
     DEBUG_ASSERT(next != NULL);
     if (next->state() == THREAD_BLOCKED_READ_LOCK) {
       while (!wait_.IsEmpty()) {
         Thread* next = wait_.Peek();
         if (next->state() != THREAD_BLOCKED_READ_LOCK) {
           break;
         }
         state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader, ktl::memory_order_acq_rel);
         wait_.UnblockThread(next, ZX_OK);
       }
       return ResourceOwnership::Reader;
     } else {
       state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter, ktl::memory_order_acq_rel);
       wait_.UnblockThread(next, ZX_OK);
       return ResourceOwnership::Normal;
     }
   }
 }

 template <BrwLockEnablePi PI>
 void BrwLock<PI>::ContendedReadAcquire() {
   // In the case where we wake other threads up we need them to not run until we're finished
   // holding the thread_lock, so disable local rescheduling.
   AutoPreemptDisabler preempt_disable;
   {
     Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};
     // Remove our optimistic reader from the count, and put a waiter on there instead.
     uint64_t prev =
         state_.state_.fetch_add(-kBrwLockReader + kBrwLockWaiter, ktl::memory_order_relaxed);
     // If there is a writer then we just block, they will wake us up
     if (prev & kBrwLockWriter) {
       Block(false);
       return;
     }
     // If we raced and there is in fact no one waiting then we can switch to
     // having the lock
     if ((prev & kBrwLockWaiterMask) == 0) {
       state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader, ktl::memory_order_acquire);
       return;
     }
     // If there are no current readers then we need to wake somebody up
     if ((prev & kBrwLockReaderMask) == 1) {
       if (Wake() == ResourceOwnership::Reader) {
         // Join the reader pool.
         state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader, ktl::memory_order_acquire);
         return;
       }
     }

     Block(false);
   }
 }

 template <BrwLockEnablePi PI>
 void BrwLock<PI>::ContendedWriteAcquire() {
   // In the case where we wake other threads up we need them to not run until we're finished
   // holding the thread_lock, so disable local rescheduling.
   AutoPreemptDisabler preempt_disable;
   {
     Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};
     // Mark ourselves as waiting
     uint64_t prev = state_.state_.fetch_add(kBrwLockWaiter, ktl::memory_order_relaxed);
     // If there is a writer then we just block, they will wake us up
     if (prev & kBrwLockWriter) {
       Block(true);
       return;
     }
     if ((prev & kBrwLockReaderMask) == 0) {
       if ((prev & kBrwLockWaiterMask) == 0) {
         if constexpr (PI == BrwLockEnablePi::Yes) {
           state_.writer_.store(Thread::Current::Get(), ktl::memory_order_relaxed);
         }
         // Must have raced previously as turns out there's no readers or
         // waiters, so we can convert to having the lock
         state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter, ktl::memory_order_acquire);
         return;
       } else {
         // There's no readers, but someone already waiting, wake up someone
         // before we ourselves block
         Wake();
       }
     }
     Block(true);
   }
 }

 template <BrwLockEnablePi PI>
 void BrwLock<PI>::WriteRelease() {
   canary_.Assert();

 #if LK_DEBUGLEVEL > 0
   if constexpr (PI == BrwLockEnablePi::Yes) {
     Thread* holder = state_.writer_.load(ktl::memory_order_relaxed);
     Thread* ct = Thread::Current::Get();
     if (unlikely(ct != holder)) {
       panic(
           "BrwLock<PI>::WriteRelease: thread %p (%s) tried to release brwlock %p it "
           "doesn't "
           "own. Ownedby %p (%s)\n",
           ct, ct->name(), this, holder, holder ? holder->name() : "none");
     }
   }
 #endif

   // For correct PI handling we need to ensure that up until a higher priority
   // thread can acquire the lock we will correctly be considered the owner.
   // Other threads are able to acquire the lock *after* we call ReleaseWakeup,
   // prior to that we could be racing with a higher priority acquirer and it
   // could be our responsibility to wake them up, and so up until ReleaseWakeup
   // is called they must be able to observe us as the owner.
   //
   // If we hold off on changing writer_ till after ReleaseWakeup we will then be
   // racing with others who may be acquiring, or be granted the write lock in
   // ReleaseWakeup, and so we would have to CAS writer_ to not clobber the new
   // holder. CAS is much more expensive than just a 'store', so to avoid that
   // we instead disable preemption. Disabling preemption effectively gives us the
   // highest priority, and so it is fine if acquirers observe writer_ to be null
   // and 'fail' to treat us as the owner.
   if constexpr (PI == BrwLockEnablePi::Yes) {
     Thread::Current::preemption_state().PreemptDisable();

     state_.writer_.store(nullptr, ktl::memory_order_relaxed);
   }
   uint64_t prev = state_.state_.fetch_sub(kBrwLockWriter, ktl::memory_order_release);

   if (unlikely((prev & kBrwLockWaiterMask) != 0)) {
     // There are waiters, we need to wake them up
     ReleaseWakeup();
   }

   if constexpr (PI == BrwLockEnablePi::Yes) {
     Thread::Current::preemption_state().PreemptReenable();
   }
 }

 template <BrwLockEnablePi PI>
 void BrwLock<PI>::ReleaseWakeup() {
   // Don't reschedule whilst we're waking up all the threads as if there are
   // several readers available then we'd like to get them all out of the wait queue.
   AutoPreemptDisabler preempt_disable;
   {
     Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};
     uint64_t count = state_.state_.load(ktl::memory_order_relaxed);
     if ((count & kBrwLockWaiterMask) != 0 && (count & kBrwLockWriter) == 0 &&
         (count & kBrwLockReaderMask) == 0) {
       Wake();
     }
   }
 }

 template <BrwLockEnablePi PI>
 void BrwLock<PI>::ContendedReadUpgrade() {
   Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};

   // Convert our reading into waiting
   uint64_t prev =
       state_.state_.fetch_add(-kBrwLockReader + kBrwLockWaiter, ktl::memory_order_relaxed);
   if ((prev & ~kBrwLockWaiterMask) == kBrwLockReader) {
     if constexpr (PI == BrwLockEnablePi::Yes) {
       state_.writer_.store(Thread::Current::Get(), ktl::memory_order_relaxed);
     }
     // There are no writers or readers. There might be waiters, but as we
     // already have some form of lock we still have fairness even if we
     // bypass the queue, so we convert our waiting into writing
     state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter, ktl::memory_order_acquire);
   } else {
     Block(true);
   }
 }

 template class BrwLock<BrwLockEnablePi::Yes>;
 template class BrwLock<BrwLockEnablePi::No>;

 }  // namespace internal
	// Copyright 2018 The Fuchsia Authors
	//
	// Use of this source code is governed by a MIT-style
	// license that can be found in the LICENSE file or at
	// https://opensource.org/licenses/MIT

	#include "kernel/brwlock.h"

	#include <lib/zircon-internal/macros.h>

	#include <kernel/auto_preempt_disabler.h>
	#include <kernel/thread_lock.h>
	#include <ktl/limits.h>

	namespace internal {

	template <BrwLockEnablePi PI>
	BrwLock<PI>::~BrwLock() {
	DEBUG_ASSERT(state_.state_.load(ktl::memory_order_relaxed) == 0);
	}

	template <BrwLockEnablePi PI>
	void BrwLock<PI>::Block(bool write) {
	zx_status_t ret;

	auto reason = write ? ResourceOwnership::Normal : ResourceOwnership::Reader;

	if constexpr (PI == BrwLockEnablePi::Yes) {
	ret = wait_.BlockAndAssignOwner(Deadline::infinite(),
	state_.writer_.load(ktl::memory_order_relaxed), reason,
	Interruptible::No);
	} else {
	ret = wait_.BlockEtc(Deadline::infinite(), 0, reason, Interruptible::No);
	}

	if (unlikely(ret < ZX_OK)) {
	panic(
	"BrwLock<%d>::Block: Block returned with error %d lock %p, thr %p, "
	"sp %p\n",
	static_cast<bool>(PI), ret, this, Thread::Current::Get(), __GET_FRAME());
	}
	}

	template <BrwLockEnablePi PI>
	ResourceOwnership BrwLock<PI>::Wake() {
	if constexpr (PI == BrwLockEnablePi::Yes) {
	using Action = OwnedWaitQueue::Hook::Action;
	struct Context {
	ResourceOwnership ownership;
	BrwLockState<PI>& state;
	};
	Context context = {ResourceOwnership::Normal, state_};
	auto cbk = [](Thread* woken, void* ctx) -> Action {
	Context* context = reinterpret_cast<Context*>(ctx);

	if (context->ownership == ResourceOwnership::Normal) {
	// Check if target is blocked for writing and not reading
	if (woken->state() == THREAD_BLOCKED) {
	context->state.writer_.store(woken, ktl::memory_order_relaxed);
	context->state.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter,
	ktl::memory_order_acq_rel);
	return Action::SelectAndAssignOwner;
	}
	// If not writing then we must be blocked for reading
	DEBUG_ASSERT(woken->state() == THREAD_BLOCKED_READ_LOCK);
	context->ownership = ResourceOwnership::Reader;
	}
	// Our current ownership is ResourceOwnership::Reader otherwise we would
	// have returned early
	DEBUG_ASSERT(context->ownership == ResourceOwnership::Reader);
	if (woken->state() == THREAD_BLOCKED_READ_LOCK) {
	// We are waking readers and we found a reader, so we can wake them up and
	// search for me.
	context->state.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader,
	ktl::memory_order_acq_rel);
	return Action::SelectAndKeepGoing;
	} else {
	// We are waking readers but we have found a writer. To preserve fairness we
	// immediately stop and do not wake this thread or any others.
	return Action::Stop;
	}
	};

	bool resched = wait_.WakeThreads(ktl::numeric_limits<uint32_t>::max(), {cbk, &context});
	if (resched) {
	Scheduler::Reschedule();
	}
	return context.ownership;
	} else {
	Thread* next = wait_.Peek();
	DEBUG_ASSERT(next != NULL);
	if (next->state() == THREAD_BLOCKED_READ_LOCK) {
	while (!wait_.IsEmpty()) {
	Thread* next = wait_.Peek();
	if (next->state() != THREAD_BLOCKED_READ_LOCK) {
	break;
	}
	state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader, ktl::memory_order_acq_rel);
	wait_.UnblockThread(next, ZX_OK);
	}
	return ResourceOwnership::Reader;
	} else {
	state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter, ktl::memory_order_acq_rel);
	wait_.UnblockThread(next, ZX_OK);
	return ResourceOwnership::Normal;
	}
	}
	}

	template <BrwLockEnablePi PI>
	void BrwLock<PI>::ContendedReadAcquire() {
	// In the case where we wake other threads up we need them to not run until we're finished
	// holding the thread_lock, so disable local rescheduling.
	AutoPreemptDisabler preempt_disable;
	{
	Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};
	// Remove our optimistic reader from the count, and put a waiter on there instead.
	uint64_t prev =
	state_.state_.fetch_add(-kBrwLockReader + kBrwLockWaiter, ktl::memory_order_relaxed);
	// If there is a writer then we just block, they will wake us up
	if (prev & kBrwLockWriter) {
	Block(false);
	return;
	}
	// If we raced and there is in fact no one waiting then we can switch to
	// having the lock
	if ((prev & kBrwLockWaiterMask) == 0) {
	state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader, ktl::memory_order_acquire);
	return;
	}
	// If there are no current readers then we need to wake somebody up
	if ((prev & kBrwLockReaderMask) == 1) {
	if (Wake() == ResourceOwnership::Reader) {
	// Join the reader pool.
	state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockReader, ktl::memory_order_acquire);
	return;
	}
	}

	Block(false);
	}
	}

	template <BrwLockEnablePi PI>
	void BrwLock<PI>::ContendedWriteAcquire() {
	// In the case where we wake other threads up we need them to not run until we're finished
	// holding the thread_lock, so disable local rescheduling.
	AutoPreemptDisabler preempt_disable;
	{
	Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};
	// Mark ourselves as waiting
	uint64_t prev = state_.state_.fetch_add(kBrwLockWaiter, ktl::memory_order_relaxed);
	// If there is a writer then we just block, they will wake us up
	if (prev & kBrwLockWriter) {
	Block(true);
	return;
	}
	if ((prev & kBrwLockReaderMask) == 0) {
	if ((prev & kBrwLockWaiterMask) == 0) {
	if constexpr (PI == BrwLockEnablePi::Yes) {
	state_.writer_.store(Thread::Current::Get(), ktl::memory_order_relaxed);
	}
	// Must have raced previously as turns out there's no readers or
	// waiters, so we can convert to having the lock
	state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter, ktl::memory_order_acquire);
	return;
	} else {
	// There's no readers, but someone already waiting, wake up someone
	// before we ourselves block
	Wake();
	}
	}
	Block(true);
	}
	}

	template <BrwLockEnablePi PI>
	void BrwLock<PI>::WriteRelease() {
	canary_.Assert();

	#if LK_DEBUGLEVEL > 0
	if constexpr (PI == BrwLockEnablePi::Yes) {
	Thread* holder = state_.writer_.load(ktl::memory_order_relaxed);
	Thread* ct = Thread::Current::Get();
	if (unlikely(ct != holder)) {
	panic(
	"BrwLock<PI>::WriteRelease: thread %p (%s) tried to release brwlock %p it "
	"doesn't "
	"own. Ownedby %p (%s)\n",
	ct, ct->name(), this, holder, holder ? holder->name() : "none");
	}
	}
	#endif

	// For correct PI handling we need to ensure that up until a higher priority
	// thread can acquire the lock we will correctly be considered the owner.
	// Other threads are able to acquire the lock after we call ReleaseWakeup,
	// prior to that we could be racing with a higher priority acquirer and it
	// could be our responsibility to wake them up, and so up until ReleaseWakeup
	// is called they must be able to observe us as the owner.
	//
	// If we hold off on changing writer_ till after ReleaseWakeup we will then be
	// racing with others who may be acquiring, or be granted the write lock in
	// ReleaseWakeup, and so we would have to CAS writer_ to not clobber the new
	// holder. CAS is much more expensive than just a 'store', so to avoid that
	// we instead disable preemption. Disabling preemption effectively gives us the
	// highest priority, and so it is fine if acquirers observe writer_ to be null
	// and 'fail' to treat us as the owner.
	if constexpr (PI == BrwLockEnablePi::Yes) {
	Thread::Current::preemption_state().PreemptDisable();

	state_.writer_.store(nullptr, ktl::memory_order_relaxed);
	}
	uint64_t prev = state_.state_.fetch_sub(kBrwLockWriter, ktl::memory_order_release);

	if (unlikely((prev & kBrwLockWaiterMask) != 0)) {
	// There are waiters, we need to wake them up
	ReleaseWakeup();
	}

	if constexpr (PI == BrwLockEnablePi::Yes) {
	Thread::Current::preemption_state().PreemptReenable();
	}
	}

	template <BrwLockEnablePi PI>
	void BrwLock<PI>::ReleaseWakeup() {
	// Don't reschedule whilst we're waking up all the threads as if there are
	// several readers available then we'd like to get them all out of the wait queue.
	AutoPreemptDisabler preempt_disable;
	{
	Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};
	uint64_t count = state_.state_.load(ktl::memory_order_relaxed);
	if ((count & kBrwLockWaiterMask) != 0 && (count & kBrwLockWriter) == 0 &&
	(count & kBrwLockReaderMask) == 0) {
	Wake();
	}
	}
	}

	template <BrwLockEnablePi PI>
	void BrwLock<PI>::ContendedReadUpgrade() {
	Guard<MonitoredSpinLock, IrqSave> guard{ThreadLock::Get(), SOURCE_TAG};

	// Convert our reading into waiting
	uint64_t prev =
	state_.state_.fetch_add(-kBrwLockReader + kBrwLockWaiter, ktl::memory_order_relaxed);
	if ((prev & ~kBrwLockWaiterMask) == kBrwLockReader) {
	if constexpr (PI == BrwLockEnablePi::Yes) {
	state_.writer_.store(Thread::Current::Get(), ktl::memory_order_relaxed);
	}
	// There are no writers or readers. There might be waiters, but as we
	// already have some form of lock we still have fairness even if we
	// bypass the queue, so we convert our waiting into writing
	state_.state_.fetch_add(-kBrwLockWaiter + kBrwLockWriter, ktl::memory_order_acquire);
	} else {
	Block(true);
	}
	}

	template class BrwLock<BrwLockEnablePi::Yes>;
	template class BrwLock<BrwLockEnablePi::No>;

	} // namespace internal