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#include <Common/SharedMutex.h>
#ifdef OS_LINUX /// Because of futex
#include <bit>
#include <Common/futex.h>
namespace DB
{
SharedMutex::SharedMutex()
: state(0)
, waiters(0)
{}
void SharedMutex::lock()
{
UInt64 value = state.load();
while (true)
{
if (value & writers)
{
waiters++;
futexWaitUpperFetch(state, value);
waiters--;
}
else if (state.compare_exchange_strong(value, value | writers))
break;
}
value |= writers;
while (value & readers)
futexWaitLowerFetch(state, value);
}
bool SharedMutex::try_lock()
{
UInt64 value = 0;
return state.compare_exchange_strong(value, writers);
}
void SharedMutex::unlock()
{
state.store(0);
if (waiters)
futexWakeUpperAll(state);
}
void SharedMutex::lock_shared()
{
UInt64 value = state.load();
while (true)
{
if (value & writers)
{
waiters++;
futexWaitUpperFetch(state, value);
waiters--;
}
else if (state.compare_exchange_strong(value, value + 1))
break;
}
}
bool SharedMutex::try_lock_shared()
{
UInt64 value = state.load();
while (true)
{
if (value & writers)
return false;
if (state.compare_exchange_strong(value, value + 1))
break;
// Concurrent try_lock_shared() should not fail, so we have to retry CAS, but avoid blocking wait
}
return true;
}
void SharedMutex::unlock_shared()
{
UInt64 value = state.fetch_sub(1) - 1;
if (value == writers)
futexWakeLowerOne(state); // Wake writer
}
}
#endif
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