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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Nov 08, 2010 at 08:30:19AM -0300, Davidlohr Bueso wrote:
> 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?

Looks good to me!

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

>  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
> 
> 
>