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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2008-02-06 10:37:25 +0100 |
|---|---|---|
| committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2008-02-06 19:41:06 +0100 |
| commit | d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d (patch) | |
| tree | 32e09d76cb46755d7420e6ad9a6e0802dab47963 | |
| parent | kill an unused PTR_ERR in bdev_cache_init() (diff) | |
| download | linux-d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d.tar.xz linux-d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d.zip | |
Remove rcu_assign_pointer() penalty for NULL pointers
The rcu_assign_pointer() primitive currently unconditionally executes a
memory barrier, even when a NULL pointer is being assigned. This has lead
some to avoid using rcu_assign_pointer() for NULL pointers, which loses the
self-documenting advantages of rcu_assign_pointer() This patch uses
__builtin_const_p() to omit needless memory barriers for NULL-pointer
assignments at compile time with no runtime penalty, as discussed in the
following thread:
http://www.mail-archive.com/netdev@vger.kernel.org/msg54852.html
Tested on x86_64 and ppc64, also compiled the four cases (NULL/non-NULL
and const/non-const) with gcc version 4.1.2, and hand-checked the
assembly output.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
| -rw-r--r-- | include/linux/rcupdate.h | 11 |
1 files changed, 7 insertions, 4 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index d32c14de270e..37a642c54871 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -174,10 +174,13 @@ struct rcu_head { * code. */ -#define rcu_assign_pointer(p, v) ({ \ - smp_wmb(); \ - (p) = (v); \ - }) +#define rcu_assign_pointer(p, v) \ + ({ \ + if (!__builtin_constant_p(v) || \ + ((v) != NULL)) \ + smp_wmb(); \ + (p) = (v); \ + }) /** * synchronize_sched - block until all CPUs have exited any non-preemptive |