Re: [PATCH 0/2] del_timer_sync: proof of concept

[Oleg Nesterov <oleg@tv-sign.ru>]

Andrew Morton wrote:
>
> If we're prepared to rule that a timer handler is not allowed to do
> add_timer_on() then a recurring timer is permanently pinned to a CPU, isn't
> it?
>
> That should make things simpler?

I think that current inplementation of del_timer_sync() don't like
add_timer_on() too.

Consider the timer running on CPU_0. It sets timer->expires = jiffies,
and calls add_timer_on(1). Now it is possible that local timer interrupt
on CPU_1 happens and starts that timer before timer->function returns on
CPU_0.

del_timer_sync() detects that timer is running on CPU_0, waits while
->running_timer == timer, and returns. The timer still runs on CPU_1.

Oleg.