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Keyboard input needs Giant locking, and that is not possible to do correctly here. Use mtx_trylock() and proceed unlocked as before if we can't acquire Giant (non-recursively), except in kdb mode don't even try to acquire Giant. Everything here is a hack, but it often works. Even if mtx_trylock() succeeds, this might be a LOR. Keyboard input also needs screen locking, to handle screen updates and switches. Add this, using the same simplistic screen locking as for sc_cnputc(). Giant must be acquired before the screen lock, and the screen lock must be dropped when calling the keyboard driver (else it would get a harmless LOR if it tries to acquire Giant). It was intended that sc cn open/close hide the locking calls, and they do for i/o functions functions except for this complication. Non-console keyboard input is still only Giant-locked, with screen locking in some called functions. This is correct for the keyboard parts only. When Giant cannot be acquired properly, atkbd and kbdmux tend to race and work (they assume that the caller acquired Giant properly and don't try to acquire it again or check that it has been acquired, and the races rarely matter), while ukbd tends to deadlock or panic (since it does the opposite, and has other usb threads to deadlock with). The keyboard (Giant) locking here does very little, but the screen locking completes screen locking for console mode except for not detecting or handling deadlock.
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