maybe dumb question about RCU

Jeff Haran Jeff.Haran at
Tue Apr 7 22:16:53 EDT 2015

-----Original Message-----
From: Rock Lee [mailto:rocklee_104 at] 
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

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