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Commit 93728af0 authored by Michalis Kokologiannakis's avatar Michalis Kokologiannakis Committed by Paul E. McKenney
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doc: Update the comparisons rule in rcu_dereference.txt 



When an RCU-protected pointer is fetched but never dereferenced
rcu_access_pointer() should be used in place of rcu_dereference().
This commit explicitly records this very fact in Documentation/
RCU/rcu_dereference.txt, in order to prevent the usage of
rcu_dereference() in comparisons.

Signed-off-by: default avatarMichalis Kokologiannakis <mixaskok@gmail.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 066bb1c8

Documentation/RCU/rcu_dereference.txt

+9 −0
Original line number Diff line number Diff line
@@ -138,6 +138,15 @@ o Be very careful about comparing pointers obtained from 
		This sort of comparison occurs frequently when scanning

		RCU-protected circular linked lists.



		Note that if checks for being within an RCU read-side

		critical section are not required and the pointer is never

		dereferenced, rcu_access_pointer() should be used in place

		of rcu_dereference(). The rcu_access_pointer() primitive

		does not require an enclosing read-side critical section,

		and also omits the smp_read_barrier_depends() included in

		rcu_dereference(), which in turn should provide a small

		performance gain in some CPUs (e.g., the DEC Alpha).



	o	The comparison is against a pointer that references memory

		that was initialized "a long time ago."  The reason

		this is safe is that even if misordering occurs, the