Re: [RFC PATCH] x86, mce: change the mce notifier to 'blocking' from 'atomic'

[Vishal Verma]

On 04/12, Borislav Petkov wrote:
> On Tue, Apr 11, 2017 at 04:44:57PM -0600, Vishal Verma wrote:
> > The NFIT MCE handler callback (for handling media errors on NVDIMMs)
> > takes a mutex to add the location of a memory error to a list. But since
> > the notifier call chain for machine checks (x86_mce_decoder_chain) is
> > atomic, we get a lockdep splat like:
> > 
> >   BUG: sleeping function called from invalid context at kernel/locking/mutex.c:620
> >   in_atomic(): 1, irqs_disabled(): 0, pid: 4, name: kworker/0:0
> >   [..]
> >   Call Trace:
> >    dump_stack+0x86/0xc3
> >    ___might_sleep+0x178/0x240
> >    __might_sleep+0x4a/0x80
> >    mutex_lock_nested+0x43/0x3f0
> >    ? __lock_acquire+0xcbc/0x1290
> >    nfit_handle_mce+0x33/0x180 [nfit]
> >    notifier_call_chain+0x4a/0x70
> >    atomic_notifier_call_chain+0x6e/0x110
> >    ? atomic_notifier_call_chain+0x5/0x110
> >    mce_gen_pool_process+0x41/0x70
> > 
> > Commit 648ed94038c030245a06e4be59744fd5cdc18c40
> >       x86/mce: Provide a lockless memory pool to save error records
> > Changes the mce notifier callbacks to be run in a process context, and
> > this can allow us to use the 'blocking' type notifier, where we can take
> > mutexes etc. in the call chain functions.
> > 
> > Reported-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Tony Luck <tony.luck@xxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/mcheck/mce-genpool.c  | 2 +-
> >  arch/x86/kernel/cpu/mcheck/mce-internal.h | 2 +-
> >  arch/x86/kernel/cpu/mcheck/mce.c          | 8 ++++----
> >  3 files changed, 6 insertions(+), 6 deletions(-)
> > 
> > While this patch almost solves the problem, I think it is not quite right.
> > The x86_mce_decoder_chain is also called from print_mce for fatal machine
> > checks, and that is, afaict, still from an atomic context. One thing Tony
> > suggested was splitting the notifier chain into two distinct chains, one
> > for regular logging and recoverable actions that allows blocking, the
> > other from the panic path.
> 
> Well, if Mohammad won't come to the mountain...
> 
> So the NFIT handler has:
> 
>         /* We only care about memory errors */
>         if (!(mce->status & MCACOD))
>                 return NOTIFY_DONE;
> 
> what severity are we talking here? Errors which can be reported on the
> panic path, i.e., in atomic context or only AO/AR ones which don't raise
> an #MC exception?

I don't think we can do anything about the panic path errors. The NFIT
handler takes the recoverable machine checks, and essentially, adds the
location to a list.

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
> --