linux-kernel - Re: [PATCH] Move kfree_call_rcu() to slab

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Message-Id: <20171222013937.GA7829@linux.vnet.ibm.com>
Date:   Thu, 21 Dec 2017 17:39:37 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Rao Shoaib <rao.shoaib@...cle.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        brouer@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH] Move kfree_call_rcu() to slab_common.c

On Thu, Dec 21, 2017 at 09:31:23AM -0800, Rao Shoaib wrote:
> 
> 
> On 12/21/2017 04:36 AM, Matthew Wilcox wrote:
> >On Thu, Dec 21, 2017 at 12:19:47AM -0800, rao.shoaib@...cle.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.
> I left it out on purpose because the call in tiny is a little different
> 
> rcutiny.h:
> 
> static inline void kfree_call_rcu(struct rcu_head *head,
>                   void (*func)(struct rcu_head *rcu))
> {
>     call_rcu(head, func);
> }
> 
> tree.c:
> 
> void kfree_call_rcu(struct rcu_head *head,
>             void (*func)(struct rcu_head *rcu))
> {
>     __call_rcu(head, func, rcu_state_p, -1, 1);
> }
> EXPORT_SYMBOL_GPL(kfree_call_rcu);
> 
> If we want the code to be exactly same I can create a lazy version
> for tiny as well. However,  I don not know where to move
> kfree_call_rcu() from it's current home in rcutiny.h though. Any
> thoughts ?

I might be missing something subtle here, but in case I am not, my
suggestion is to simply rename rcutiny.h's kfree_call_rcu() and otherwise
leave it as is.  If you want to update the type of the second argument,
which got missed back in the day, there is always this:

static inline void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func)
{
	call_rcu(head, func);
}

The reason that Tiny RCU doesn't handle laziness specially is because
Tree RCU's handling of laziness is a big no-op on the single CPU systems
on which Tiny RCU runs.  So Tiny RCU need do nothing special to support
laziness.

							Thanx, Paul

> >>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.
> >
> Thanks. I was not sure if I was required to fix the noise or based
> on inspection the noise could be ignored. I will make the change and
> resubmit.
> 
> Shoaib
>