lkml.org 
[lkml]   [2016]   [Oct]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)
On Mon, Oct 17, 2016 at 08:55:52PM +0200, Christoph Hellwig wrote:
> On Mon, Oct 17, 2016 at 02:19:47PM -0400, Jeff Moyer wrote:
> > This ends up being a call to __sb_end_write:
> >
> > void __sb_end_write(struct super_block *sb, int level)
> > {
> > percpu_up_read(sb->s_writers.rw_sem + level-1);
> > }
> >
> > Nothing guarantees that submission and completion happen on the same
> > CPU. Is this safe?
>
> Good point. From my reading of the percpu_rwsem implementation it
> is not safe to release it from a different CPU. Which makes me
> wonder how we can protect aio writes properly here..

percpu-rwsem has the same semantics as regular rwsems, so preemptible
and 'owner' stuff.

Therefore we must support doing up from a different cpu than we did down
on; the owner could've been migrated while we held it.

And while there's a metric ton of tricky in the implementation, this
part is actually fairly straight forward. We only care about the direct
sum of the per-cpu counter, see readers_active_check() -> per_cpu_sum().

So one cpu doing an inc and another doing a dec summed is still 0.

\
 
 \ /
  Last update: 2016-10-17 21:58    [W:0.149 / U:0.164 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site