From ee7c29be3695996536395f647e8a2ed6b1ab3a0d Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 7 Apr 2015 12:45:41 -0700 Subject: documentation: Update rcu_dereference.txt based on WG21 discussions This commit provides another caveat for the care and feeding of pointers returned by rcu_dereference() that was pointed out in discussions within the C++ standards committee. Signed-off-by: Paul E. McKenney Reviewed-by: Mathieu Desnoyers --- Documentation/RCU/rcu_dereference.txt | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'Documentation/RCU') diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt index ceb05da5a5ac..2d05c9241a33 100644 --- a/Documentation/RCU/rcu_dereference.txt +++ b/Documentation/RCU/rcu_dereference.txt @@ -193,6 +193,11 @@ o Be very careful about comparing pointers obtained from pointer. Note that the volatile cast in rcu_dereference() will normally prevent the compiler from knowing too much. + However, please note that if the compiler knows that the + pointer takes on only one of two values, a not-equal + comparison will provide exactly the information that the + compiler needs to deduce the value of the pointer. + o Disable any value-speculation optimizations that your compiler might provide, especially if you are making use of feedback-based optimizations that take data collected from prior runs. Such -- cgit v1.2.3 From ed38446424dd531f1b7a167677232a6d400d69d5 Mon Sep 17 00:00:00 2001 From: Milos Vyletel Date: Fri, 17 Apr 2015 16:38:04 +0200 Subject: documentation: State that rcu_dereference() reloads pointer Make a note stating that repeated calls of rcu_dereference() may not return the same pointer if update happens while in critical section. Reported-by: Jeff Haran Signed-off-by: Milos Vyletel Reviewed-by: Steven Rostedt Signed-off-by: Paul E. McKenney --- Documentation/RCU/whatisRCU.txt | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'Documentation/RCU') diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 88dfce182f66..16622c9e86b5 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -256,7 +256,9 @@ rcu_dereference() If you are going to be fetching multiple fields from the RCU-protected structure, using the local variable is of course preferred. Repeated rcu_dereference() calls look - ugly and incur unnecessary overhead on Alpha CPUs. + ugly, do not guarantee that the same pointer will be returned + if an update happened while in the critical section, and incur + unnecessary overhead on Alpha CPUs. Note that the value returned by rcu_dereference() is valid only within the enclosing RCU read-side critical section. -- cgit v1.2.3