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Date: Fri, 8 Jul 2022 15:13:29 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc: "frederic@...nel.org" <frederic@...nel.org>,
"quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
"rcu@...r.kernel.org" <rcu@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rcu: Make tiny RCU support leak callbacks for
debug-object errors
On Wed, Jul 06, 2022 at 04:24:03AM +0000, Zhang, Qiang1 wrote:
>
> On Wed, Jul 06, 2022 at 02:00:51AM +0000, Zhang, Qiang1 wrote:
> > On Fri, Jul 01, 2022 at 10:44:04AM +0800, Zqiang wrote:
> > > Currently, only tree RCU support leak callbacks setting when do
> > > duplicate call_rcu(). this commit add leak callbacks setting when
> > > fo duplicate call_rcu() for tiny RCU.
> > >
> > > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> >
> > >This does look plausible, thank you!
> > >
> > >What testing have you done?
> > >
> > >One important test for Tiny RCU is that the size of the kernel not
> > >grow without a very good reason. In this case, the added code should
> > >be dead code in a production build (CONFIG_DEBUG_OBJECTS_RCU_HEAD=n),
> > >but it is good to check.
> > >
> > >It is of course also good to check that the messages print as expected,
> > >which is what rcutorture.object_debug is there to help with.
> >
> > In the condition that the CONFIG_DEBUG_OBJECTS_RCU_HEAD=n, the function directly returns zero.
> >
> > #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
> > static inline int debug_rcu_head_queue(struct rcu_head *head)
> > {
> > return 0;
> > }
> >
> >Yes, like I said, the added code -should- be dead code. But there is
> >often a gap between "should" and "is", for example, compilers don't
> >always do what we would like them to. So please use the "size vmlinux"
> >command with and without your patch for a kernel built (both times)
> >with CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD==n.
> >
> >The rest of the test results look good, thank you!
>
> Hi Paul
>
> 1. CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
>
> Original:
> text data bss dec hex filename
> 26291319 20160143 15212544 61664006 3aceb06 vmlinux
>
> Applay patch:
> text data bss dec hex filename
> 26291319 20160431 15212544 61664294 3acec26 vmlinux
>
> 2. CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
>
> Original:
> text data bss dec hex filename
> 26290663 20159823 15212544 61663030 3ace736 vmlinux
>
> Applay patch:
> text data bss dec hex filename
> 26290663 20159823 15212544 61663030 3ace736 vmlinux
Much better, thank you!
Please see below for the commit updated with this information and
wordsmithed a bit. As always, please let me know if I messed something
up.
Thanx, Paul
------------------------------------------------------------------------
commit 88cea4e18ed430aa1187063450236fc00408eaac
Author: Zqiang <qiang1.zhang@...el.com>
Date: Fri Jul 1 10:44:04 2022 +0800
rcu: Make tiny RCU support leak callbacks for debug-object errors
Currently, only Tree RCU leaks callbacks setting when it detects a
duplicate call_rcu(). This commit causes Tiny RCU to also leak
callbacks in this situation.
Because this is Tiny RCU, kernel size is important:
1. CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
(Production kernel)
Original:
text data bss dec hex filename
26290663 20159823 15212544 61663030 3ace736 vmlinux
With this commit:
text data bss dec hex filename
26290663 20159823 15212544 61663030 3ace736 vmlinux
2. CONFIG_TINY_RCU=y and CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
(Debugging kernel)
Original:
text data bss dec hex filename
26291319 20160143 15212544 61664006 3aceb06 vmlinux
With this commit:
text data bss dec hex filename
26291319 20160431 15212544 61664294 3acec26 vmlinux
These results show that the kernel size is unchanged for production
kernels, as desired.
Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index f0561ee16b9c2..943d431b908f6 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -158,6 +158,10 @@ void synchronize_rcu(void)
}
EXPORT_SYMBOL_GPL(synchronize_rcu);
+static void tiny_rcu_leak_callback(struct rcu_head *rhp)
+{
+}
+
/*
* Post an RCU callback to be invoked after the end of an RCU grace
* period. But since we have but one CPU, that would be after any
@@ -165,9 +169,20 @@ EXPORT_SYMBOL_GPL(synchronize_rcu);
*/
void call_rcu(struct rcu_head *head, rcu_callback_t func)
{
+ static atomic_t doublefrees;
unsigned long flags;
- debug_rcu_head_queue(head);
+ if (debug_rcu_head_queue(head)) {
+ if (atomic_inc_return(&doublefrees) < 4) {
+ pr_err("%s(): Double-freed CB %p->%pS()!!! ", __func__, head, head->func);
+ mem_dump_obj(head);
+ }
+
+ if (!__is_kvfree_rcu_offset((unsigned long)head->func))
+ WRITE_ONCE(head->func, tiny_rcu_leak_callback);
+ return;
+ }
+
head->func = func;
head->next = NULL;
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