linux-kernel - Re: usage of rcu_dereference

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Date:	Mon, 21 Apr 2014 18:47:34 -0400
From:	Pranith Kumar <pranith@...ech.edu>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org
Subject: Re: usage of rcu_dereference_raw


On Mon, Apr 21, 2014 at 6:03 PM, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>>   do most of the uses of rcu_dereference_raw() need to be changed to use other
>>   dereference functions or are there cases where its usage is valid?
>
> The second call from iwl_op_mode_dvm_stop() -might- be valid.  For it
> to be valid, there must be a grace period between the time that the
> field was made inaccessible to readers and the time that iwl_uninit_drv()
> was called.  Usually something like synchronize_rcu() waits for the
> needed grace period.

So there are valid use cases of the rcu_dereference_raw() in scenarios where
we can verify that a grace period has passed.

Thank you for the info. Mind adding it as a comment as in the patch below?

add comment for rcu_dereference_raw

Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
---
 include/linux/rcupdate.h |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 00a7fd6..af40a86 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -662,7 +662,13 @@ static inline void rcu_preempt_sleep_check(void)
     __rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \
                 __rcu)
 
-#define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
+/* rcu_dereference_raw() - rcu_dereference with no checking
+ * @p: The pointer to read, prior to dereferencing
+ *
+ * Use this to dereference a rcu pointer if you are sure that there exists a
+ * grace period between the time this pointer was made inaccessible to readers
+ */
+#define rcu_dereference_raw(p) rcu_dereference_check(p, 1)
 
 /*
  * The tracing infrastructure traces RCU (we want that), but unfortunately
-- 
1.7.9.5


-- 
Pranith
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