Messages in this thread | | | Date | Thu, 21 Dec 2017 04:36:30 -0800 | From | Matthew Wilcox <> | Subject | Re: [PATCH] Move kfree_call_rcu() to slab_common.c |
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On Thu, Dec 21, 2017 at 12:19:47AM -0800, rao.shoaib@oracle.com wrote: > This patch moves kfree_call_rcu() and related macros out of rcu code. A new > function __call_rcu_lazy() is created for calling __call_rcu() with the lazy > flag.
Something you probably didn't know ... there are two RCU implementations in the kernel; Tree and Tiny. It looks like you've only added __call_rcu_lazy() to Tree and you'll also need to add it to Tiny.
> Also moving macros generated following checkpatch noise. I do not know > how to silence checkpatch as there is nothing wrong. > > CHECK: Macro argument reuse 'offset' - possible side-effects? > #91: FILE: include/linux/slab.h:348: > +#define __kfree_rcu(head, offset) \ > + do { \ > + BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \ > + kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \ > + } while (0)
What checkpatch is warning you about here is that somebody might call
__kfree_rcu(p, a++);
and this would expand into
do { \ BUILD_BUG_ON(!__is_kfree_rcu_offset(a++)); \ kfree_call_rcu(p, (rcu_callback_t)(unsigned long)(a++)); \ } while (0)
which would increment 'a' twice, and cause pain and suffering.
That's pretty unlikely usage of __kfree_rcu(), but I suppose it's not impossible. We have various hacks to get around this kind of thing; for example I might do this as::
#define __kfree_rcu(head, offset) \ do { \ unsigned long __o = offset; BUILD_BUG_ON(!__is_kfree_rcu_offset(__o)); \ kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(__o)); \ } while (0)
Now offset is only evaluated once per invocation of the macro. The other two warnings are the same problem.
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