Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT | linux-arm-kernel

(off-list ancestor, not in this archive)

On 2021-09-22 13:38:20 [+0200], Frederic Weisbecker wrote:
The part with rcutree.use_softirq=0 on RT does not make it any better,
right?
The rcuc kthread disables softirqs before calling rcu_core(), so it behaves
pretty much the same as a softirq. Or am I missing something?
Oh, no you don't.

So you rely on some implicit behaviour which breaks with RT such as:

                              CPU 0
           -----------------------------------------------
           RANDOM TASK-A                      RANDOM TASK-B
           ------                             -----------
           int *X = &per_cpu(CPUX, 0)         int *X = &per_cpu(CPUX, 0)
           int A, B;                          
					      spin_lock(&D);
           spin_lock(&C);
	   				      WRITE_ONCE(*X, 0);
           A = READ_ONCE(*X);
                                              WRITE_ONCE(*X, 1);
           B = READ_ONCE(*X);

while spinlock C and D are just random locks not related to CPUX but it
just happens that they are held at that time. So for !RT you guarantee
that A == B while it is not the case on RT.
Not sure which spinlocks you are referring to here. Also most RCU spinlocks
are raw.
I was bringing an example where you also could rely on implicit locking
provided by spin_lock() which breaks on RT.

Sebastian

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