Kernel Timers: hrtimer vs legacy timer_list

Two timer APIs, two callback contexts, one shared trap: a callback that runs after the data it touches is gone. Load references/README.md before writing any mod_timer / del_timer_sync / hrtimer_start / hrtimer_cancel path.

When to use

• Writing or reviewing any kernel timer: retries, timeouts, periodic polls, rate limiting, hardware watchdog re-arms, delayed deactivation.
• Choosing between timer_list (jiffies/HZ) and hrtimer (nanoseconds).
• Determining what context a timer callback runs in and what is legal there.
• Restarting a periodic timer from inside its own callback.
• Tearing timers down at module unload or device close/release.

When not to use