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SubjectRe: [PATCH 31/36] security,rcu: convert call_rcu(user_update_rcu_disposal) to kfree_rcu()
Date
Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> commit 316b740440ea0a54615a3516df536fbf9a4c11b8
> Author: Lai Jiangshan <laijs@cn.fujitsu.com>
> Date: Fri Mar 18 11:15:47 2011 +0800
>
> rcu: introduce kfree_rcu()
>
> Many rcu callbacks functions just call kfree() on the base structure.
> These functions are trivial, but their size adds up, and furthermore
> when they are used in a kernel module, that module must invoke the
> high-latency rcu_barrier() function at module-unload time.
>
> The kfree_rcu() function introduced by this commit addresses this issue.
> Rather than encoding a function address in the embedded rcu_head
> structure, kfree_rcu() instead encodes the offset of the rcu_head
> structure within the base structure. Because the functions are not
> allowed in the low-order 4096 bytes of kernel virtual memory, offsets
> up to 4095 bytes can be accommodated. If the offset is larger than
> 4095 bytes, a compile-time error will be generated in __kfree_rcu().
> If this error is triggered, you can either fall back to use of call_rcu()
> or rearrange the structure to position the rcu_head structure into the
> first 4096 bytes.
>
> Note that the allowable offset might decrease in the future, for example,
> to allow something like kmem_cache_free_rcu().
>
> The new kfree_rcu() function can replace code as follows:
>
> call_rcu(&p->rcu, simple_kfree_callback);
>
> where "simple_kfree_callback()" might be defined as follows:
>
> void simple_kfree_callback(struct rcu_head *p)
> {
> struct foo *q = container_of(p, struct foo, rcu);
>
> kfree(q);
> }
>
> with the following:
>
> kfree_rcu(&p->rcu, rcu);
>
> Note that the "rcu" is the name of a field in the structure being
> freed. The reason for using this rather than passing in a pointer
> to the base structure is that the above approach allows better type
> checking.
>
> This commit is based on earlier work by Lai Jiangshan and Manfred Spraul:
>
> Lai's V1 patch: http://lkml.org/lkml/2008/9/18/1
> Manfred's patch: http://lkml.org/lkml/2009/1/2/115
>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-and-sneakiness-appreciated-by: David Howells <dhowells@redhat.com>


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