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important detail that sc_cngetc() now opens and closes the keyboard on every call again. This was moved from sc_cngetc() to scn_cngrab/ ungrab() in r228644, but the change wasn't quite complete. After fixes for nesting in kbdd_poll() in ukbd and kbdmux, these opens and closes should have no significant effect if done while grabbed. They fix unusual cases when cngetc() is called while not grabbed. This commit is the main fix for screen locking in sc_cnputc(): detect deadlock or likely-deadlock and handle it by buffering the output atomically and printing it later if the deadlock condition clears (and sc_cnputc() is called). The most common deadlock is when the screen lock is held by ourself. Then it would be safe to acquire the lock recursively if the console driver is calling printf() in a safe context, but we don't know when that is. It is not safe to ignore the lock even in kdb or panic mode. But ignore it in panic mode. The only other known case of deadlock is when another thread holds the lock but is running on a stopped CPU. Detect that case approximately by using trylock and retrying for 1000 usec. On a 4 GHz CPU, 100 usec is almost long enough -- screen switches take slightly longer than that. Not retrying at all is good enough except for stress tests, and planned future versions will extend the timeout so that the stress tests work better. To see the behaviour when deadlock is detected, single step through sctty_outwakeup() (or sc_puts() to start with deadlock). Another (serial) console is needed to the buffered-only output, but the keyboard works in this context to continue or step out of the deadlocked region. The buffer is not large enough to hold all the output for this.
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