Messages in this thread | | | Date | Thu, 16 Feb 2012 11:17:14 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer() |
| |
On Sun, Feb 12, 2012 at 09:41:33AM -0800, Paul E. McKenney wrote: > On Mon, Feb 06, 2012 at 05:18:02PM +0100, Borislav Petkov wrote: > > On Fri, Jan 13, 2012 at 01:44:35PM -0800, Paul E. McKenney wrote: > > > Looks good to me, but I do need to defer to people who know this code > > > better than do I. The key thing that (from what I can see) makes > > > rcu_dereference() unnecessary is that the smp_rmb() used in conjunction > > > with polling the .finished field takes care of ordering. > > > > Right, this was me trying hard not to screw up touching mcelog.next, > > thus trying to use the rcu_dereference_index_check() primitive without > > thinking it through too much. But you're right, I'm polling the > > ->finished field 4 times (totally arbitrary, btw) which should suffice > > while the mce_log() routine above writes those entries. > > > > Although, the question still remains, since mce_log() accesses > > mcelog.next through the rcu_dereference_index_check() primitive, > > shouldn't I do it the same way? > > I don't claim to be an mce_log() expert, but when I looked it over, > I didn't see a need for rcu_dereference_index_check(). Unless I am > confused (quite possible), the memory barriers are sufficient. > > The rcu_read_lock() and rcu_read_unlock() seem to be needed to avoid > premature freeing, though.
Looka ere:
commit f56e8a0765cc4374e02f4e3a79e2427b5096b075 Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Date: Fri Mar 5 15:03:27 2010 -0800
x86/mce: Fix RCU lockdep splats
Create an rcu_dereference_check_mce() that checks for RCU-sched read side and mce_read_mutex being held on update side. Replace uses of rcu_dereference() in arch/x86/kernel/cpu/mcheck/mce.c with this new macro.
...
+#define rcu_dereference_check_mce(p) \ + rcu_dereference_check((p), \ + rcu_read_lock_sched_held() || \ + lockdep_is_held(&mce_read_mutex)) +
So I guess the check is to see that we're rather holding the mce_chrdev_read_mutex when accessing this from userspace (the mcelog userspace thing).
I can't comment on the RCU-sched thing because I don't know it that well. It has to do with extended CPU idle times but why do we require to be in RCU read-side critical section when accessing the index? Hmmm, I think a guy called Paul should know :-) ...
Thanks.
-- Regards/Gruss, Boris.
Advanced Micro Devices GmbH Einsteinring 24, 85609 Dornach GM: Alberto Bozzo Reg: Dornach, Landkreis Muenchen HRB Nr. 43632 WEEE Registernr: 129 19551
| |