Re: frequent lockups in 3.18rc4

[Al Viro]

On Sat, Dec 13, 2014 at 03:09:59PM -0800, Linus Torvalds wrote:
> On Sat, Dec 13, 2014 at 2:59 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > The generic code does that mnt_want_write/mnt_drop_write
> > dance adound the call to setxattr, and that in turn does
> >
> >         while (ACCESS_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
> >                 cpu_relax();
> >
> > with preemption explicitly disabled.
> 
> Btw, I see no reason why mnt_want_write/mnt_drop_write disables
> preemption. They don't care, they just care about the ordering of the
> write counts and the MNT_WRITE_HOLD bit. It's the code that sets the
> bit that should care, afaik. But maybe I'm missing something.

Er...  There's much more direct reason - suppose we get a timer interrupt
right in the middle of mnt_drop_write().  And lost the timeslice.
On UP we have mnt->mnt_writers--, with no locks held.  On SMP we have
this_cpu_dec() instead, also without any locks.  You really don't want to
lose the timeslice in the middle of either...
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