Why JSRunLoopTimerCallBack executes in the main thread?

Hi,all.

My application used the JavaScriptCore framework.

What I'm confused about is:

Instance of JSContext was created in the work queue, and there was no code that access JSCore in the main thread,but JSC::JSRunLoopTimer::Manager::timerDidFire executed in the main thread.

Sometimes,I got a dead-lock.

The call stack of the main thread:

#0 0x0000000194a4dc94 in __psynch_cvwait ()

#1 0x000000019496acf8 in _pthread_cond_wait$VARIANT$mp ()

#2 0x00000001a399595c in ***::ThreadCondition::timedWait(***::Mutex&, ***::WallTime) ()

#3 0x00000001a397b848 in ***::ParkingLot::parkConditionallyImpl(void const*, ***::ScopedLambda<bool ()> const&, ***::ScopedLambda<void ()> const&, ***::TimeWithDynamicClockType const&) ()

#4 0x00000001a396e6e4 in ***::LockAlgorithm<unsigned char, (unsigned char)1, (unsigned char)2, ***::EmptyLockHooks<unsigned char> >::lockSlow(***::Atomic<unsigned char>&) ()

#5 0x00000001a4379f68 in JSC::JSLock::lock(long) ()

#6 0x00000001a43a4b44 in JSC::JSRunLoopTimer::timerDidFire() ()

#7 0x00000001a43a47c8 in JSC::JSRunLoopTimer::Manager::timerDidFire() ()

#8 0x0000000194bd503c in __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ ()

#9 0x0000000194bd4d78 in __CFRunLoopDoTimer ()

#10 0x0000000194bd4448 in __CFRunLoopDoTimers ()

#11 0x0000000194bcf584 in __CFRunLoopRun ()

#12 0x0000000194bceadc in CFRunLoopRunSpecific ()

#13 0x0000000194f0e7f4 in -[NSRunLoop(NSRunLoop) runMode:beforeDate:] ()

device:iPhone 6s

iOS:13.3

xcode:11.3

Thanks!