lkml.org 
[lkml]   [2011]   [Aug]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: pstore: change mutex locking to spin_locks
On Fri, 12 Aug 2011 10:54:51 -0700
"Luck, Tony" <tony.luck@intel.com> wrote:

> From: Don Zickus <dzickus@redhat.com>
>
> pstore was using mutex locking to protect read/write access to the
> backend plug-ins. This causes problems when pstore is executed in
> an NMI context through panic() -> kmsg_dump().
>
> This patch changes the mutex to a spin_lock_irqsave then also checks to
> see if we are in an NMI context. If we are in an NMI and can't get the
> lock, just print a message stating that and blow by the locking.
>
> All this is probably a hack around the bigger locking problem but it
> solves my current situation of trying to sleep in an NMI context.
>
> Tested by loading the lkdtm module and executing a HARDLOCKUP which
> will cause the machine to panic inside the nmi handler.
>
> ...
>
> + if (in_nmi()) {
> + is_locked = spin_trylock(&psinfo->buf_lock);
> + if (!is_locked)
> + pr_err("pstore dump routine blocked in NMI, may corrupt error record\n");
> + } else
> + spin_lock_irqsave(&psinfo->buf_lock, flags);
> oopscount++;
> while (total < kmsg_bytes) {
> dst = psinfo->buf;
> @@ -123,7 +131,11 @@ static void pstore_dump(struct kmsg_dumper *dumper,
> total += l1_cpy + l2_cpy;
> part++;
> }
> - mutex_unlock(&psinfo->buf_mutex);
> + if (in_nmi()) {
> + if (is_locked)
> + spin_unlock(&psinfo->buf_lock);
> + } else
> + spin_unlock_irqrestore(&psinfo->buf_lock, flags);
> }

It's still bad if lockdep is enabled. See
kernel/lockdep.c:lock_acquire() and lock_release(). They aren't
NMI-safe.

One approach would be to switch to bit_spin_lock(). Which will break
if/when bit spinlocks get lockdep-enabled, so don't do that.

A better approach would be to use the underlying spinlock functions
which bypass lockdep, but I cannot immediately locate those amongst
the misama of spinlock interface mess.

This problem of locking-vs-NMIs has been "solved" several times before
but I don't recall any standardized approach being developed. Does
anyone have a favorite implementation to look at?


\
 
 \ /
  Last update: 2011-08-17 23:25    [from the cache]
©2003-2011 Jasper Spaans