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// The OS-specific header included below must provide two calls:
// base::internal::SpinLockDelay() and base::internal::SpinLockWake().
// See spinlock_internal.h for the spec of SpinLockWake().
// void SpinLockDelay(volatile Atomic32 *w, int32 value, int loop)
// SpinLockDelay() generates an apprproate spin delay on iteration "loop" of a
// spin loop on location *w, whose previously observed value was "value".
// SpinLockDelay() may do nothing, may yield the CPU, may sleep a clock tick,
// or may wait for a delay that can be truncated by a call to SpinlockWake(w).
// In all cases, it must return in bounded time even if SpinlockWake() is not
// called.
#include "base/spinlock_internal.h"
// forward declaration for use by spinlock_*-inl.h
namespace base { namespace internal { static int SuggestedDelayNS(int loop); }}
#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 base {
namespace internal {
// See spinlock_internal.h for spec.
int32 SpinLockWait(volatile Atomic32 *w, int n,
const SpinLockWaitTransition trans[]) {
int32 v;
bool done = false;
for (int loop = 0; !done; loop++) {
v = base::subtle::Acquire_Load(w);
int i;
for (i = 0; i != n && v != trans[i].from; i++) {
}
if (i == n) {
SpinLockDelay(w, v, loop); // no matching transition
} else if (trans[i].to == v || // null transition
base::subtle::Acquire_CompareAndSwap(w, v, trans[i].to) == v) {
done = trans[i].done;
}
}
return v;
}
// Return a suggested delay in nanoseconds for iteration number "loop"
static int SuggestedDelayNS(int loop) {
// Weak pseudo-random number generator to get some spread between threads
// when many are spinning.
static base::subtle::Atomic64 rand;
uint64 r = base::subtle::NoBarrier_Load(&rand);
r = 0x5deece66dLL * r + 0xb; // numbers from nrand48()
base::subtle::NoBarrier_Store(&rand, r);
r <<= 16; // 48-bit random number now in top 48-bits.
if (loop < 0 || loop > 32) { // limit loop to 0..32
loop = 32;
}
// loop>>3 cannot exceed 4 because loop cannot exceed 32.
// Select top 20..24 bits of lower 48 bits,
// giving approximately 0ms to 16ms.
// Mean is exponential in loop for first 32 iterations, then 8ms.
// The futex path multiplies this by 16, since we expect explicit wakeups
// almost always on that path.
return r >> (44 - (loop >> 3));
}
} // namespace internal
} // namespace base