Re: [PATCH] rcu: Narrow early boot window of illegal synchronous grace periods

[Borislav Petkov]

On Thu, Jan 12, 2017 at 06:38:07PM -0800, Paul E. McKenney wrote:
> The current preemptible RCU implementation goes through three phases
> during bootup.  In the first phase, there is only one CPU that is running
> with preemption disabled, so that a no-op is a synchronous grace period.
> In the second mid-boot phase, the scheduler is running, but RCU has
> not yet gotten its kthreads spawned (and, for expedited grace periods,
> workqueues are not yet running.  During this time, any attempt to do
> a synchronous grace period will hang the system (or complain bitterly,
> depending).  In the third and final phase, RCU is fully operational and
> everything works normally.
> 
> This has been OK for some time, but there has recently been some
> synchronous grace periods showing up during the second mid-boot phase.
> This commit therefore reworks RCU to permit synchronous grace periods
> to proceed during this mid-boot phase.
> 
> This commit accomplishes this by setting a flag from the existing
> rcu_scheduler_starting() function which causes all synchronous grace
> periods to take the expedited path.  The expedited path now checks this
> flag, using the requesting task to drive the expedited grace period
> forward during the mid-boot phase.  Finally, this flag is updated by a
> core_initcall() function named rcu_exp_runtime_mode(), which causes the
> runtime codepaths to be used.
> 
> Note that this arrangement assumes that tasks are not sent POSIX signals
> (or anything similar) from the time that the first task is spawned
> through core_initcall() time.
> 
> Reported-by: "Zheng, Lv" <lv.zheng@xxxxxxxxx>
> Reported-by: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Tested on a bunch of boxes I have access to, looks good.

Tested-by: Borislav Petkov <bp@xxxxxxx>

-- 
Regards/Gruss,
    Boris.

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