linux-kernel - Re: [PATCH 21/24] rcu/tiny: move kvfree_call

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Date:   Wed, 6 May 2020 20:29:02 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 21/24] rcu/tiny: move kvfree_call_rcu() out of header

Hello, Paul, Joel.

> > Move inlined kvfree_call_rcu() function out of the
> > header file. This step is a preparation for head-less
> > support.
> > 
> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> >  include/linux/rcutiny.h | 6 +-----
> >  kernel/rcu/tiny.c       | 6 ++++++
> >  2 files changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 0c6315c4a0fe..7eb66909ae1b 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -34,11 +34,7 @@ static inline void synchronize_rcu_expedited(void)
> >  	synchronize_rcu();
> >  }
> >  
> > -static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > -{
> > -	call_rcu(head, func);
> > -}
> > -
> > +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
> >  void rcu_qs(void);
> >  
> >  static inline void rcu_softirq_qs(void)
> > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> > index aa897c3f2e92..508c82faa45c 100644
> > --- a/kernel/rcu/tiny.c
> > +++ b/kernel/rcu/tiny.c
> > @@ -177,6 +177,12 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  }
> >  EXPORT_SYMBOL_GPL(call_rcu);
> >  
> > +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > +{
> > +	call_rcu(head, func);
> > +}
> > +EXPORT_SYMBOL_GPL(kvfree_call_rcu);
> 
> This increases the size of Tiny RCU.  Plus in Tiny RCU, the overhead of
> synchronize_rcu() is exactly zero.  So why not make the single-argument
> kvfree_call_rcu() just unconditionally do synchronize_rcu() followed by
> kvfree() or whatever?  That should go just fine into the header file.
> 
Seems it does not go well if i do it in header file:

<snip>
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 0c6315c4a0fe..76b7ad053218 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -13,6 +13,7 @@
 #define __LINUX_TINY_H
 
 #include <asm/param.h> /* for HZ */
+#include <linux/mm.h>
 
 /* Never flag non-existent other CPUs! */
 static inline bool rcu_eqs_special_set(int cpu) { return false; }
@@ -36,7 +37,15 @@ static inline void synchronize_rcu_expedited(void)
 
 static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
-       call_rcu(head, func);
+       if (head) {
+               call_rcu(head, func);
+               return;
+       }
+
+       // kvfree_rcu(one_arg) call.
+       might_sleep();
+       synchronize_rcu();
+       kvfree((void *) func);
 }
<snip> 

kvfree() is defined in <linux/mm.h> as extern void kvfree(const void *addr); 
If i just include <linux/mm.h> i get many errors related to "implicit declaration
of function" like:

<snip>
rcu_read_lock()
compound_mapcount_ptr()
rcu_assign_pointer()
...
<snip>

and many other messages like:

<snip>
warning: returning ‘int’ from a function with return type
error: unknown type name ‘vm_fault_t’; did you mean ‘pmdval_t’?
error: implicit declaration of function ‘RB_EMPTY_ROOT’
...
<snip>

Please see full log here: ftp://vps418301.ovh.net/incoming/include_mm_h_output.txt

I can fix it by adding the kvfree() declaration to the rcutiny.h also:
extern void kvfree(const void *addr);

what seems wired to me? Also it can be fixed if i move it to the tiny.c
so it will be aligned with the way how it is done for tree-RCU.

Any valuable proposals?

--
Vlad Rezki