This commit expands on the ordering properties of rcu_assign_pointer() and rcu_dereference(), outlining their constraints on CPUs and compilers.
Reported-by: Rao Shoaib <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> --- Documentation/RCU/whatisRCU.rst | 30 +++++++++++++++++++----------- 1 file changed, 19 insertions(+), 11 deletions(-) diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst index 94838c65c7d97..d585a5490aeec 100644 --- a/Documentation/RCU/whatisRCU.rst +++ b/Documentation/RCU/whatisRCU.rst @@ -250,21 +250,25 @@ rcu_assign_pointer() ^^^^^^^^^^^^^^^^^^^^ void rcu_assign_pointer(p, typeof(p) v); - Yes, rcu_assign_pointer() **is** implemented as a macro, though it - would be cool to be able to declare a function in this manner. - (Compiler experts will no doubt disagree.) + Yes, rcu_assign_pointer() **is** implemented as a macro, though + it would be cool to be able to declare a function in this manner. + (And there has been some discussion of adding overloaded functions + to the C language, so who knows?) The updater uses this spatial macro to assign a new value to an RCU-protected pointer, in order to safely communicate the change in value from the updater to the reader. This is a spatial (as opposed to temporal) macro. It does not evaluate to an rvalue, - but it does execute any memory-barrier instructions required - for a given CPU architecture. Its ordering properties are that - of a store-release operation. - - Perhaps just as important, it serves to document (1) which - pointers are protected by RCU and (2) the point at which a - given structure becomes accessible to other CPUs. That said, + but it does provide any compiler directives and memory-barrier + instructions required for a given compile or CPU architecture. + Its ordering properties are that of a store-release operation, + that is, any prior loads and stores required to initialize the + structure are ordered before the store that publishes the pointer + to that structure. + + Perhaps just as important, rcu_assign_pointer() serves to document + (1) which pointers are protected by RCU and (2) the point at which + a given structure becomes accessible to other CPUs. That said, rcu_assign_pointer() is most frequently used indirectly, via the _rcu list-manipulation primitives such as list_add_rcu(). @@ -283,7 +287,11 @@ rcu_dereference() executes any needed memory-barrier instructions for a given CPU architecture. Currently, only Alpha needs memory barriers within rcu_dereference() -- on other CPUs, it compiles to a - volatile load. + volatile load. However, no mainstream C compilers respect + address dependencies, so rcu_dereference() uses volatile casts, + which, in combination with the coding guidelines listed in + rcu_dereference.rst, prevent current compilers from breaking + these dependencies. Common coding practice uses rcu_dereference() to copy an RCU-protected pointer to a local variable, then dereferences -- 2.40.1
