Re: [PATCH] mce: fix RCU lockdep from mce_log()

[Davidlohr Bueso <dave@gnu.org>]

On Sun, 2010-11-07 at 05:39 -0800, Paul E. McKenney wrote:
> On Sat, Nov 06, 2010 at 07:53:50PM +0100, Andi Kleen wrote:
> > On Fri, Nov 05, 2010 at 06:44:59PM -0300, Davidlohr Bueso wrote:
> > > Hi,
> > > 
> > > Please review this patch, I am not very familiar with MCE/RCU so I'm not sure that this is the correct fix (otherwise consider it a bug report :)).
> > > This does "fix" the message though and I can use MCE normally.
> > 
> > The patch is certainly not correct. The variable needs to be read
> > independently of the mutex.
> 
> This code is simply checking the value of the pointer, and therefore
> need not protect any actual dereferences.  So why not replace the
> rcu_dereference_check_mce() with rcu_access_pointer()?  If this is
> OK, please see the patch below.
> 
> BTW, assigning the value returned by rcu_access_pointer() into a
> variable often indicates a bug.  ;-)
> 
> 							Thanx, Paul
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index 7a35b72..4d29d50 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -1625,7 +1625,7 @@ out:
>  static unsigned int mce_poll(struct file *file, poll_table *wait)
>  {
>  	poll_wait(file, &mce_wait, wait);
> -	if (rcu_dereference_check_mce(mcelog.next))
> +	if (rcu_access_pointer(mcelog.next))

this doesn't compile (mcelog.next is an index):

arch/x86/kernel/cpu/mcheck/mce.c: In function ‘mce_poll’:
arch/x86/kernel/cpu/mcheck/mce.c:1628: error: invalid type argument of
‘unary *’ (have ‘unsigned int’)
arch/x86/kernel/cpu/mcheck/mce.c:1628: warning: type defaults to ‘int’
in declaration of ‘_________p1’
arch/x86/kernel/cpu/mcheck/mce.c:1628: error: invalid type argument of
‘unary *’ (have ‘unsigned int’)
arch/x86/kernel/cpu/mcheck/mce.c:1628: warning: type defaults to ‘int’
in declaration of ‘type name’
arch/x86/kernel/cpu/mcheck/mce.c:1628: warning: cast to pointer from
integer of different size
arch/x86/kernel/cpu/mcheck/mce.c:1628: error: invalid type argument of
‘unary *’ (have ‘unsigned int’)
arch/x86/kernel/cpu/mcheck/mce.c:1628: warning: type defaults to ‘int’
in declaration of ‘type name’
make[4]: *** [arch/x86/kernel/cpu/mcheck/mce.o] Error 1


Since the mutex is independent, what about this patch?

 Signed-off-by: Davidlohr Bueso <dave@gnu.org>

---
 arch/x86/kernel/cpu/mcheck/mce.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c
b/arch/x86/kernel/cpu/mcheck/mce.c
index 7a35b72..cc1c673 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -1625,7 +1625,7 @@ out:
 static unsigned int mce_poll(struct file *file, poll_table *wait)
 {
 	poll_wait(file, &mce_wait, wait);
-	if (rcu_dereference_check_mce(mcelog.next))
+	if (rcu_dereference_index_check(mcelog.next,
rcu_read_lock_sched_held()))
 		return POLLIN | POLLRDNORM;
 	if (!mce_apei_read_done && apei_check_mce())
 		return POLLIN | POLLRDNORM;
-- 
1.7.1