Re: [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree_sensitive()

Michal Hocko
2020-06-17T12:56:05Z

On Wed 17-06-20 05:23:21, Matthew Wilcox wrote:
> On Wed, Jun 17, 2020 at 01:31:57PM +0200, Michal Hocko wrote:
> > On Wed 17-06-20 04:08:20, Matthew Wilcox wrote:
> > > If you call vfree() under
> > > a spinlock, you're in trouble. in_atomic() only knows if we hold a
> > > spinlock for CONFIG_PREEMPT, so it's not safe to check for in_atomic()
> > > in __vfree(). So we need the warning in order that preempt people can
> > > tell those without that there is a bug here.
> >
> > ... Unless I am missing something in_interrupt depends on preempt_count() as
> > well so neither of the two is reliable without PREEMPT_COUNT configured.
>
> preempt_count() always tracks whether we're in interrupt context,
> regardless of CONFIG_PREEMPT. The difference is that CONFIG_PREEMPT
> will track spinlock acquisitions as well.

Right you are! Thanks for the clarification. I find the situation
around preempt_count quite confusing TBH. Looking at existing users
of in_atomic() (e.g. a random one zd_usb_iowrite16v_async which check
in_atomic and then does GFP_KERNEL allocation which would be obviously
broken on !PREEMPT if the function can be called from an atomic
context), I am wondering whether it would make sense to track atomic
context also for !PREEMPT. This check is just terribly error prone.

--
Michal Hocko
SUSE Labs