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BeefRT/gperftools/src/base/spinlock.cc

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+/* Copyright (c) 2006, Google Inc.
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ * 
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * ---
+ * Author: Sanjay Ghemawat
+ */
+
+#include <config.h>
+#include "base/spinlock.h"
+#include "base/synchronization_profiling.h"
+#include "base/spinlock_internal.h"
+#include "base/cycleclock.h"
+#include "base/sysinfo.h"   /* for NumCPUs() */
+
+// NOTE on the Lock-state values:
+//
+//   kSpinLockFree represents the unlocked state
+//   kSpinLockHeld represents the locked state with no waiters
+//
+// Values greater than kSpinLockHeld represent the locked state with waiters,
+// where the value is the time the current lock holder had to
+// wait before obtaining the lock.  The kSpinLockSleeper state is a special
+// "locked with waiters" state that indicates that a sleeper needs to
+// be woken, but the thread that just released the lock didn't wait.
+
+static int adaptive_spin_count = 0;
+
+const base::LinkerInitialized SpinLock::LINKER_INITIALIZED =
+    base::LINKER_INITIALIZED;
+
+namespace {
+struct SpinLock_InitHelper {
+  SpinLock_InitHelper() {
+    // On multi-cpu machines, spin for longer before yielding
+    // the processor or sleeping.  Reduces idle time significantly.
+    if (NumCPUs() > 1) {
+      adaptive_spin_count = 1000;
+    }
+  }
+};
+
+// Hook into global constructor execution:
+// We do not do adaptive spinning before that,
+// but nothing lock-intensive should be going on at that time.
+static SpinLock_InitHelper init_helper;
+
+}  // unnamed namespace
+
+// Monitor the lock to see if its value changes within some time period
+// (adaptive_spin_count loop iterations).  A timestamp indicating
+// when the thread initially started waiting for the lock is passed in via
+// the initial_wait_timestamp value.  The total wait time in cycles for the
+// lock is returned in the wait_cycles parameter.  The last value read
+// from the lock is returned from the method.
+Atomic32 SpinLock::SpinLoop(int64 initial_wait_timestamp,
+                            Atomic32* wait_cycles) {
+  int c = adaptive_spin_count;
+  while (base::subtle::NoBarrier_Load(&lockword_) != kSpinLockFree && --c > 0) {
+  }
+  Atomic32 spin_loop_wait_cycles = CalculateWaitCycles(initial_wait_timestamp);
+  Atomic32 lock_value =
+      base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree,
+                                           spin_loop_wait_cycles);
+  *wait_cycles = spin_loop_wait_cycles;
+  return lock_value;
+}
+
+void SpinLock::SlowLock() {
+  // The lock was not obtained initially, so this thread needs to wait for
+  // it.  Record the current timestamp in the local variable wait_start_time
+  // so the total wait time can be stored in the lockword once this thread
+  // obtains the lock.
+  int64 wait_start_time = CycleClock::Now();
+  Atomic32 wait_cycles;
+  Atomic32 lock_value = SpinLoop(wait_start_time, &wait_cycles);
+
+  int lock_wait_call_count = 0;
+  while (lock_value != kSpinLockFree) {
+    // If the lock is currently held, but not marked as having a sleeper, mark
+    // it as having a sleeper.
+    if (lock_value == kSpinLockHeld) {
+      // Here, just "mark" that the thread is going to sleep.  Don't store the
+      // lock wait time in the lock as that will cause the current lock
+      // owner to think it experienced contention.
+      lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_,
+                                                        kSpinLockHeld,
+                                                        kSpinLockSleeper);
+      if (lock_value == kSpinLockHeld) {
+        // Successfully transitioned to kSpinLockSleeper.  Pass
+        // kSpinLockSleeper to the SpinLockWait routine to properly indicate
+        // the last lock_value observed.
+        lock_value = kSpinLockSleeper;
+      } else if (lock_value == kSpinLockFree) {
+        // Lock is free again, so try and acquire it before sleeping.  The
+        // new lock state will be the number of cycles this thread waited if
+        // this thread obtains the lock.
+        lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_,
+                                                          kSpinLockFree,
+                                                          wait_cycles);
+        continue;  // skip the delay at the end of the loop
+      }
+    }
+
+    // Wait for an OS specific delay.
+    base::internal::SpinLockDelay(&lockword_, lock_value,
+                                  ++lock_wait_call_count);
+    // Spin again after returning from the wait routine to give this thread
+    // some chance of obtaining the lock.
+    lock_value = SpinLoop(wait_start_time, &wait_cycles);
+  }
+}
+
+// The wait time for contentionz lock profiling must fit into 32 bits.
+// However, the lower 32-bits of the cycle counter wrap around too quickly
+// with high frequency processors, so a right-shift by 7 is performed to
+// quickly divide the cycles by 128.  Using these 32 bits, reduces the
+// granularity of time measurement to 128 cycles, and loses track
+// of wait time for waits greater than 109 seconds on a 5 GHz machine
+// [(2^32 cycles/5 Ghz)*128 = 109.95 seconds]. Waits this long should be
+// very rare and the reduced granularity should not be an issue given
+// processors in the Google fleet operate at a minimum of one billion
+// cycles/sec.
+enum { PROFILE_TIMESTAMP_SHIFT = 7 };
+
+void SpinLock::SlowUnlock(uint64 wait_cycles) {
+  base::internal::SpinLockWake(&lockword_, false);  // wake waiter if necessary
+
+  // Collect contentionz profile info, expanding the wait_cycles back out to
+  // the full value.  If wait_cycles is <= kSpinLockSleeper, then no wait
+  // was actually performed, so don't record the wait time.  Note, that the
+  // CalculateWaitCycles method adds in kSpinLockSleeper cycles
+  // unconditionally to guarantee the wait time is not kSpinLockFree or
+  // kSpinLockHeld.  The adding in of these small number of cycles may
+  // overestimate the contention by a slight amount 50% of the time.  However,
+  // if this code tried to correct for that addition by subtracting out the
+  // kSpinLockSleeper amount that would underestimate the contention slightly
+  // 50% of the time.  Both ways get the wrong answer, so the code
+  // overestimates to be more conservative. Overestimating also makes the code
+  // a little simpler.
+  //
+  if (wait_cycles > kSpinLockSleeper) {
+    base::SubmitSpinLockProfileData(this,
+                                    wait_cycles << PROFILE_TIMESTAMP_SHIFT);
+  }
+}
+
+inline int32 SpinLock::CalculateWaitCycles(int64 wait_start_time) {
+  int32 wait_cycles = ((CycleClock::Now() - wait_start_time) >>
+                       PROFILE_TIMESTAMP_SHIFT);
+  // The number of cycles waiting for the lock is used as both the
+  // wait_cycles and lock value, so it can't be kSpinLockFree or
+  // kSpinLockHeld.  Make sure the value returned is at least
+  // kSpinLockSleeper.
+  wait_cycles |= kSpinLockSleeper;
+  return wait_cycles;
+}