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

[Borislav Petkov]

On Sat, Jan 14, 2017 at 12:00:22AM -0800, Paul E. McKenney wrote:
> It now looks like this:
> 
> ------------------------------------------------------------------------
> 
> Note that the code was buggy even before this commit, as it was subject
> to failure on real-time systems that forced all expedited grace periods
> to run as normal grace periods (for example, using the rcu_normal ksysfs
> parameter).  The callchain from the failure case is as follows:
> 
> early_amd_iommu_init()
> |-> acpi_put_table(ivrs_base);
> |-> acpi_tb_put_table(table_desc);
> |-> acpi_tb_invalidate_table(table_desc);
> |-> acpi_tb_release_table(...)
> |-> acpi_os_unmap_memory
> |-> acpi_os_unmap_iomem
> |-> acpi_os_map_cleanup
> |-> synchronize_rcu_expedited
> 
> The kernel showing this callchain was built with CONFIG_PREEMPT_RCU=y,
> which caused the code to try using workqueues before they were
> initialized, which did not go well.
> 
> ------------------------------------------------------------------------
> 
> Does that work?

Yap, thanks.

> Fair point, but this wording appears in almost all of my patches.  ;-)

:-)

> My rationale is that it provides a clear transition from describing the
> problem to introducing the solution.

Fair enough.

> Exactly, but yes, worth a comment.
> 
> The header comment for rcu_scheduler_starting() is now as follows:
> 
> /*
>  * During boot, we forgive RCU lockdep issues.  After this function is
>  * invoked, we start taking RCU lockdep issues seriously.  Note that unlike
>  * Tree RCU, Tiny RCU transitions directly from RCU_SCHEDULER_INACTIVE
>  * to RCU_SCHEDULER_RUNNING, skipping the RCU_SCHEDULER_INIT stage.
>  * The reason for this is that Tiny RCU does not need kthreads, so does
>  * not have to care about the fact that the scheduler is half-initialized
>  * at a certain phase of the boot process.
>  */

Good.

> I believe that this would not buy very much, but if this variable starts
> showing up on profiles, then perhaps a jump label would be appropriate.
> As a separate patch, though!

Yeah, let's keep that opportunity in the bag, just in case.

> Thank you for your review and comments!

Thanks for the fix.

Btw, I'll build one more test kernel for people with your final version here:

https://lkml.kernel.org/r/1484383554-18095-2-git-send-email-paulmck@xxxxxxxxxxxxxxxxxx

backported to 4.9.

I say 4.9 because the reports started then, probably because of

  8b355e3bc140 ("rcu: Drive expedited grace periods from workqueue")

Which means, you probably should tag your fix CC:stable and add

Fixes: 8b355e3bc140 ("rcu: Drive expedited grace periods from workqueue")

to it too.

Hmmm.

-- 
Regards/Gruss,
    Boris.

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