rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference when pointer==NULL
Subsystems (2)
RFC PATCH 1/1 rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference when pointer==NULL
Hi, Paul and Alpha hackers
memory barrier is expensive, I think it's worth that adding an if-statement to avoid it when pointer == NULL.
This patch change nothing in UP or in other architectures for compiler's optimization.
Thanks, Lai Jiangshan
---
include/linux/rcupdate.h +2 -1
@@ -143,7 +143,8 @@ struct rcu_head {#define rcu_dereference (p) ({ \ typeof(p) _________p1 = ACCESS_ONCE(p); \ - smp_read_barrier_depends(); \ + if (_________p1) \ + smp_read_barrier_depends(); \ (_________p1); \ })
Quoted 7 lines · Lai Jiangshan
Hi, Paul and Alpha hackers
memory barrier is expensive, I think it's worth that adding an if-statement to avoid it when pointer == NULL.
This patch change nothing in UP or in other architectures for compiler's optimization.
I suspect this optimization is invalid in the case where the loaded value is used as an offset (either pointer or array index). But I'm afraid I'll have to defer to Paul for explanations..
Quoted 27 lines
Thanks, Lai Jiangshan
Signed-off-by Lai Jiangshan <[email protected]>---
include/linux/rcupdate.h +2
-2@@ -143,7 +143,8 @@ struct rcu_head {#define rcu_dereference (p) ({ \ typeof(p) _________p1 = ACCESS_ONCE(p); \ - smp_read_barrier_depends(); \ + if (_________p1) \ + smp_read_barrier_depends(); \ (_________p1); \ }) --To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected]
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Quoted 6 lines · Peter Zijlstra
Quoted 5 lines · Lai Jiangshan
Hi, Paul and Alpha hackers
memory barrier is expensive, I think it's worth that adding an if-statement to avoid it when pointer == NULL.
Quoted 6 lines
Quoted 2 lines
This patch change nothing in UP or in other architectures for compiler's optimization.
I suspect this optimization is invalid in the case where the loaded value is used as an offset (either pointer or array index). But I'm afraid I'll have to defer to Paul for explanations..
What Peter said!
There are cases where the argument to rcu_dereference() is an array index rather than a pointer. In that case, zero is a perfectly valid array index and requires a memory barrier on Alpha. Given that the smp_read_barrier_depends() primitive is very nearly (but not quite) a no-op on non-Alpha machines, it will also be necessary to carefully check the generated code to make sure that we are not slowing down the common non-Alpha machines to help out the very rare Alpha machines.
Some months ago, we attempted to add a similar check to rcu_assign_pointer(), but we were unable to find an approach that handled all the cases currently in the Linux kernel, even when we introduced a special-case primitive for array indexes. Please note that we did not prove it impossible, but rather decided that it was consuming more time and energy than was warranted.
Thanx, Paul
PS. I have not forgotten about your earlier patch to the list structures
used by classic RCU -- but I am still curious to know the results of the more-harsh testing that I suggested.
Quoted 28 lines
Quoted 27 lines
Thanks, Lai Jiangshan
Signed-off-by Lai Jiangshan <[email protected]>---
include/linux/rcupdate.h +2
-2@@ -143,7 +143,8 @@ struct rcu_head {#define rcu_dereference (p) ({ \ typeof(p) _________p1 = ACCESS_ONCE(p); \ - smp_read_barrier_depends(); \ + if (_________p1) \ + smp_read_barrier_depends(); \ (_________p1); \ }) --To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/