• Paul E. McKenney's avatar
    rcu: Teach RCU that idle task is not quiscent state at boot · a6826048
    Paul E. McKenney authored
    This patch fixes a bug located by Vegard Nossum with the aid of
    kmemcheck, updated based on review comments from Nick Piggin,
    Ingo Molnar, and Andrew Morton.  And cleans up the variable-name
    and function-name language.  ;-)
    The boot CPU runs in the context of its idle thread during boot-up.
    During this time, idle_cpu(0) will always return nonzero, which will
    fool Classic and Hierarchical RCU into deciding that a large chunk of
    the boot-up sequence is a big long quiescent state.  This in turn causes
    RCU to prematurely end grace periods during this time.
    This patch changes the rcutree.c and rcuclassic.c rcu_check_callbacks()
    function to ignore the idle task as a quiescent state until the
    system has started up the scheduler in rest_init(), introducing a
    new non-API function rcu_idle_now_means_idle() to inform RCU of this
    transition.  RCU maintains an internal rcu_idle_cpu_truthful variable
    to track this state, which is then used by rcu_check_callback() to
    determine if it should believe idle_cpu().
    Because this patch has the effect of disallowing RCU grace periods
    during long stretches of the boot-up sequence, this patch also introduces
    Josh Triplett's UP-only optimization that makes synchronize_rcu() be a
    no-op if num_online_cpus() returns 1.  This allows boot-time code that
    calls synchronize_rcu() to proceed normally.  Note, however, that RCU
    callbacks registered by call_rcu() will likely queue up until later in
    the boot sequence.  Although rcuclassic and rcutree can also use this
    same optimization after boot completes, rcupreempt must restrict its
    use of this optimization to the portion of the boot sequence before the
    scheduler starts up, given that an rcupreempt RCU read-side critical
    section may be preeempted.
    In addition, this patch takes Nick Piggin's suggestion to make the
    system_state global variable be __read_mostly.
    Changes since v4:
    o	Changes the name of the introduced function and variable to
    	be less emotional.  ;-)
    Changes since v3:
    o	WARN_ON(nr_context_switches() > 0) to verify that RCU
    	switches out of boot-time mode before the first context
    	switch, as suggested by Nick Piggin.
    Changes since v2:
    o	Created rcu_blocking_is_gp() internal-to-RCU API that
    	determines whether a call to synchronize_rcu() is itself
    	a grace period.
    o	The definition of rcu_blocking_is_gp() for rcuclassic and
    	rcutree checks to see if but a single CPU is online.
    o	The definition of rcu_blocking_is_gp() for rcupreempt
    	checks to see both if but a single CPU is online and if
    	the system is still in early boot.
    	This allows rcupreempt to again work correctly if running
    	on a single CPU after booting is complete.
    o	Added check to rcupreempt's synchronize_sched() for there
    	being but one online CPU.
    Tested all three variants both SMP and !SMP, booted fine, passed a short
    rcutorture test on both x86 and Power.
    Located-by: default avatarVegard Nossum <vegard.nossum@gmail.com>
    Tested-by: default avatarVegard Nossum <vegard.nossum@gmail.com>
    Tested-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
main.c 21.1 KB