Merge google-perftools r109 (the current contents of third_party/tcmalloc/vendor)

third_party/tcmalloc/chromium/src/base/spinlock.cc

Index: third_party/tcmalloc/chromium/src/base/spinlock.cc
===================================================================
--- third_party/tcmalloc/chromium/src/base/spinlock.cc	(revision 88335)
+++ third_party/tcmalloc/chromium/src/base/spinlock.cc	(working copy)
@@ -32,47 +32,28 @@
  */
 
 #include <config.h>
-#include <time.h>       /* For nanosleep() */
-#ifdef HAVE_SCHED_H
-#include <sched.h>      /* For sched_yield() */
-#endif
-#ifdef HAVE_UNISTD_H
-#include <unistd.h>     /* For read() */
-#endif
-#include <fcntl.h>      /* for open(), O_RDONLY */
-#include <string.h>     /* for strncmp */
-#include <errno.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() */
 
-// We can do contention-profiling of SpinLocks, but the code is in
-// mutex.cc, which is not always linked in with spinlock.  Hence we
-// provide this weak definition, which is used if mutex.cc isn't linked in.
-ATTRIBUTE_WEAK extern void SubmitSpinLockProfileData(const void *, int64);
-void SubmitSpinLockProfileData(const void *, int64) {}
+// 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;
 
-// The OS-specific header included below must provide two calls:
-// Wait until *w becomes zero, atomically set it to 1 and return.
-//    static void SpinLockWait(volatile Atomic32 *w);
-// 
-// Hint that a thread waiting in SpinLockWait() could now make progress.  May
-// do nothing.  This call may not read or write *w; it must use only the
-// address.
-//    static void SpinLockWake(volatile Atomic32 *w);
-#if defined(_WIN32)
-#include "base/spinlock_win32-inl.h"
-#elif defined(__linux__)
-#include "base/spinlock_linux-inl.h"
-#else
-#include "base/spinlock_posix-inl.h"
-#endif
-
 namespace {
 struct SpinLock_InitHelper {
   SpinLock_InitHelper() {
@@ -91,36 +72,111 @@
 
 }  // 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() {
-  int c = adaptive_spin_count;
+  // 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);
 
-  // Spin a few times in the hope that the lock holder releases the lock
-  while ((c > 0) && (lockword_ != 0)) {
-    c--;
-  }
+  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 aquire 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
+      }
+    }
 
-  if (lockword_ == 1) {
-    int32 now = (CycleClock::Now() >> PROFILE_TIMESTAMP_SHIFT);
-    // Don't loose the lock: make absolutely sure "now" is not zero
-    now |= 1;
-    // Atomically replace the value of lockword_ with "now" if
-    // lockword_ is 1, thereby remembering the first timestamp to
-    // be recorded.
-    base::subtle::NoBarrier_CompareAndSwap(&lockword_, 1, now);
-    // base::subtle::NoBarrier_CompareAndSwap() returns:
-    //   0: the lock is/was available; nothing stored
-    //   1: our timestamp was stored
-    //   > 1: an older timestamp is already in lockword_; nothing stored
+    // 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);
   }
-
-  SpinLockWait(&lockword_);  // wait until lock acquired; OS specific
 }
 
-void SpinLock::SlowUnlock(int64 wait_timestamp) {
-  SpinLockWake(&lockword_);  // wake waiter if necessary; OS specific 
+// 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 };
 
-  // Collect contentionz profile info.  Subtract one from wait_timestamp as
-  // antidote to "now |= 1;" in SlowLock().
-  SubmitSpinLockProfileData(this, wait_timestamp - 1);
+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;
+}