timer_process_alarm function of esp_timer holds a spinlock for the
entire duration of its operation, except for the time when timer
callback function is called. It is possible that when
timer_process_alarm releases the spinlock, a higher priority task may
run and delete the timer. Then the execution will return to
timer_process_alarm, and this will either cause a crash, or undesired
execution of callback after the timer has been stopped or deleted.
To solve this problem, add a mutex which will prevent deletion of timers
while callbacks are being dispatched.
Timer callback can delete the timer. If CONFIG_ESP_TIMER_PROFILING was
enabled, this caused an access to invalid (freed) memory.
This fix adds a pointer to track the timer while executing the callback.
This is needed so that we can check if callback deletes the timer,
in which case we won't try updating profiling counters for this timer
after the callback is done.
Moves the ets_timer_arm() / ets_timer_disarm() code paths to RAM
Overhead is 740 bytes of IRAM, 0 bytes DRAM
(For comparison: If all of esp_timer.c is moved to RAM, overhead is 1068 bytes IRAM and 480 bytes DRAM.)
Since timestamps are 64-bit, loosing one bit of range due to sign does
not present an issue, however for applications doing calculations on
timestamps, signed return type is more convenient.
