Re: lock_kernel called under spinlock in NFS

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Fri, 2006-06-02 at 16:24 -0400, Joe Korty wrote:
> On Thu, Jun 01, 2006 at 04:13:39PM -0400, Trond Myklebust wrote:
> > On Thu, 2006-06-01 at 15:55 -0400, Joe Korty wrote:
> >> Tree 5fdccf2354269702f71beb8e0a2942e4167fd992
> >> 
> >>         [PATCH] vfs: *at functions: core
> >> 
> >> introduced a bug where lock_kernel() can be called from
> >> under a spinlock.  To trigger the bug one must have
> >> CONFIG_PREEMPT_BKL=y and be using NFS heavily.  It is
> >> somewhat rare and, so far, haven't traced down the userland
> >> sequence that causes the fatal path to be taken.
> >> 
> >> The bug was caused by the insertion into do_path_lookup()
> >> of a call to file_permission().  do_path_lookup()
> >> read-locks current->fs->lock for most of its operation.
> >> file_permission() calls permission() which calls
> >> nfs_permission(), which has one path through it
> >> that uses lock_kernel().
> 
> > Nowhere should anyone be calling file_permission() under a spinlock.
> > 
> > Why would you need to read-protect current->fs in the case where you are
> > starting from a file? The correct thing to do there would appear to be
> > to read_protect only the cases where (*name=='/') and (dfd == AT_FDCWD).
> > 
> > Something like the attached patch...
> 
> 
> Hi Trond,
> I've been running with the patch for the last few hours, on an nfs-rooted
> system, and it has been working fine.  Any plans to submit this for 2.6.17?

It probably ought to be, given the nature of the sin. Andrew?

Cheers,
  Trond