Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[Thomas Gleixner]

Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:
>> On Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote:
>>> Now RCU creates a new thing which enforces to make page allocation in
>>> atomic context possible on RT. What for?
>>> 
>>> What's the actual use case in truly atomic context for this new thing on
>>> an RT kernel?
>>
>> It is not just RT kernels.  CONFIG_PROVE_RAW_LOCK_NESTING=y propagates
>> this constraint to all configurations, and a patch in your new favorite
>> subsystem really did trigger this lockdep check in a non-RT kernel.
>>
>>> The actual RCU code disabling interrupts is an implementation detail
>>> which can easily be mitigated with a local lock.
>>
>> In this case, we are in raw-spinlock context on entry to kfree_rcu().
>
> Where?

And aside of the where, wasn't kfree_rcu() from within raw spinlock held
regions possible all the time? Either I'm missing something or you are
fundamentally changing RCU internals. kfree_rcu() saved RT in various
ways where invoking kfree() was just not an option. Confused...

Thanks,

        tglx