Commit-ID: 8db70b132dd57696cfc7560203a72e90c51bfdda
Gitweb: http://git.kernel.org/tip/8db70b132dd57696cfc7560203a72e90c51bfdda
Author: Patrick Marlier <patrick.marlier@xxxxxxxxx>
AuthorDate: Fri, 11 Sep 2015 15:50:35 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 6 Oct 2015 11:16:18 -0700
rculist: Make list_entry_rcu() use lockless_dereference()
The current list_entry_rcu() implementation copies the pointer to a stack
variable, then invokes rcu_dereference_raw() on it. This results in an
additional store-load pair. Now, most compilers will emit normal store
and load instructions, which might seem to be of negligible overhead,
but this results in a load-hit-store situation that can cause surprisingly
long pipeline stalls, even on modern microprocessors. The problem is
that it takes time for the store to get the store buffer updated, which
can delay the subsequent load, which immediately follows.
This commit therefore switches to the lockless_dereference() primitive,
which does not expect the __rcu annotations (that are anyway not present
in the list_head structure) and which, like rcu_dereference_raw(),
does not check for an enclosing RCU read-side critical section.
Most importantly, it does not copy the pointer, thus avoiding the
load-hit-store overhead.
Signed-off-by: Patrick Marlier <patrick.marlier@xxxxxxxxx>
[ paulmck: Switched to lockless_dereference() to suppress sparse warnings. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---
include/linux/rculist.h | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)
diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 17c6b1f..5ed5409 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -247,10 +247,7 @@ static inline void list_splice_init_rcu(struct list_head *list,
* primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
*/
#define list_entry_rcu(ptr, type, member) \
-({ \
- typeof(*ptr) __rcu *__ptr = (typeof(*ptr) __rcu __force *)ptr; \
- container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \
-})
+ container_of(lockless_dereference(ptr), type, member)
/**
* Where are list_empty_rcu() and list_first_entry_rcu()?
--
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