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#include <Common/CancelableSharedMutex.h>
#ifdef OS_LINUX /// Because of futex
#include <Common/futex.h>
namespace DB
{
namespace
{
inline bool cancelableWaitUpperFetch(std::atomic<UInt64> & address, UInt64 & value)
{
bool res = CancelToken::local().wait(upperHalfAddress(&address), upperHalf(value));
value = address.load();
return res;
}
inline bool cancelableWaitLowerFetch(std::atomic<UInt64> & address, UInt64 & value)
{
bool res = CancelToken::local().wait(lowerHalfAddress(&address), lowerHalf(value));
value = address.load();
return res;
}
}
CancelableSharedMutex::CancelableSharedMutex()
: state(0)
, waiters(0)
{}
void CancelableSharedMutex::lock()
{
UInt64 value = state.load();
while (true)
{
if (value & writers)
{
waiters++;
if (!cancelableWaitUpperFetch(state, value))
{
waiters--;
CancelToken::local().raise();
}
else
waiters--;
}
else if (state.compare_exchange_strong(value, value | writers))
break;
}
value |= writers;
while (value & readers)
{
if (!cancelableWaitLowerFetch(state, value))
{
state.fetch_and(~writers);
futexWakeUpperAll(state);
CancelToken::local().raise();
}
}
}
bool CancelableSharedMutex::try_lock()
{
UInt64 value = state.load();
return (value & (readers | writers)) == 0 && state.compare_exchange_strong(value, value | writers);
}
void CancelableSharedMutex::unlock()
{
state.fetch_and(~writers);
if (waiters)
futexWakeUpperAll(state);
}
void CancelableSharedMutex::lock_shared()
{
UInt64 value = state.load();
while (true)
{
if (value & writers)
{
waiters++;
if (!cancelableWaitUpperFetch(state, value))
{
waiters--;
CancelToken::local().raise();
}
else
waiters--;
}
else if (state.compare_exchange_strong(value, value + 1)) // overflow is not realistic
break;
}
}
bool CancelableSharedMutex::try_lock_shared()
{
UInt64 value = state.load();
while (true)
{
if (value & writers)
return false;
if (state.compare_exchange_strong(value, value + 1)) // overflow is not realistic
break;
// Concurrent try_lock_shared() should not fail, so we have to retry CAS, but avoid blocking wait
}
return true;
}
void CancelableSharedMutex::unlock_shared()
{
UInt64 value = state.fetch_sub(1) - 1;
if ((value & (writers | readers)) == writers) // If writer is waiting and no more readers
futexWakeLowerOne(state); // Wake writer
}
}
#endif
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