timers: Add a function to start/reduce a timer

Add a function, similar to mod_timer(), that will start a timer if it isn't
running and will modify it if it is running and has an expiry time longer
than the new time.  If the timer is running with an expiry time that's the
same or sooner, no change is made.

The function looks like:

	int timer_reduce(struct timer_list *timer, unsigned long expires);

This can be used by code such as networking code to make it easier to share
a timer for multiple timeouts.  For instance, in upcoming AF_RXRPC code,
the rxrpc_call struct will maintain a number of timeouts:

	unsigned long	ack_at;
	unsigned long	resend_at;
	unsigned long	ping_at;
	unsigned long	expect_rx_by;
	unsigned long	expect_req_by;
	unsigned long	expect_term_by;

each of which is set independently of the others.  With timer reduction
available, when the code needs to set one of the timeouts, it only needs to
look at that timeout and then call timer_reduce() to modify the timer,
starting it or bringing it forward if necessary.  There is no need to refer
to the other timeouts to see which is earliest and no need to take any lock
other than, potentially, the timer lock inside timer_reduce().

Note, that this does not protect against concurrent invocations of any of
the timer functions.

As an example, the expect_rx_by timeout above, which terminates a call if
we don't get a packet from the server within a certain time window, would
be set something like this:

	unsigned long now = jiffies;
	unsigned long expect_rx_by = now + packet_receive_timeout;
	WRITE_ONCE(call->expect_rx_by, expect_rx_by);
	timer_reduce(&call->timer, expect_rx_by);

The timer service code (which might, say, be in a work function) would then
check all the timeouts to see which, if any, had triggered, deal with
those:

	t = READ_ONCE(call->ack_at);
	if (time_after_eq(now, t)) {
		cmpxchg(&call->ack_at, t, now + MAX_JIFFY_OFFSET);
		set_bit(RXRPC_CALL_EV_ACK, &call->events);
	}

and then restart the timer if necessary by finding the soonest timeout that
hasn't yet passed and then calling timer_reduce().

The disadvantage of doing things this way rather than comparing the timers
each time and calling mod_timer() is that you *will* take timer events
unless you can finish what you're doing and delete the timer in time.

The advantage of doing things this way is that you don't need to use a lock
to work out when the next timer should be set, other than the timer's own
lock - which you might not have to take.

[ tglx: Fixed weird formatting and adopted it to pending changes ]

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: keyrings@vger.kernel.org
Cc: linux-afs@lists.infradead.org
Link: https://lkml.kernel.org/r/151023090769.23050.1801643667223880753.stgit@warthog.procyon.org.uk

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