maybe dumb question about RCU

[Jeff Haran]

-----Original Message-----
From: Rock Lee [mailto:rocklee_104 at outlook.com] 
Sent: Tuesday, April 07, 2015 7:09 PM
To: Jeff Haran
Cc: kernelnewbies
Subject: Re: maybe dumb question about RCU

>
> 256         If you are going to be fetching multiple fields from the
>
> 257         RCU-protected structure, using the local variable is of
>
> 258         course preferred.  Repeated rcu_dereference() calls look
>
> 259         ugly and incur unnecessary overhead on Alpha CPUs."
>
>  From lines 256 to 259 I conclude that reader()'s code is considered 
> ugly and wasteful,
>
> but a will always equal b.
>
> But looking at how rcu_dereference() and rcu_assign_pointer() are 
> implemented, I'm having a
>
> hard time seeing how reader() would always see a and b equal.
>


>This is the implementation of rcu_dereference(). It is a little old, but useful as well.
>
>#define rcu_dereference(p)     ({ \
>				typeof(p) _________p1 = ACCESS_ONCE(p); \
>				smp_read_barrier_depends(); \
>				(_________p1); \
>				})
>
>It uses memory barrier to guarantee the order of code execution.
>rcu_read_lock() actually disables preemption, so writer has no chance to modify critical section in the rcu_read_lock()/rcu_read_unlock() pair.

Thanks for getting back to me, Rock.

Disabling preemption would prevent a writer on the same core as the reader from changing the pointer in the read critical section.

But what happens if the writer is running on another core of a multi-core system?

Seems like a writer on another core could still get in there and change the value of the pointer between the two rcu_dereference() calls in the reader.

Jeff Haran