Re: [PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Jul 16, 2020 at 03:47:17AM +0100, Matthew Wilcox wrote:
> On Thu, Jul 16, 2020 at 11:46:56AM +1000, Dave Chinner wrote:
> > And why should we compromise performance on hundreds of millions of
> > modern systems to fix an extremely rare race on an extremely rare
> > platform that maybe only a hundred people world-wide might still
> > use?
> 
> I thought that wasn't the argument here.  It was that some future
> compiler might choose to do something absolutely awful that no current
> compiler does, and that rather than disable the stupid "optimisation",
> we'd be glad that we'd already stuffed the source code up so that it
> lay within some tortuous reading of the C spec.
> 
> The memory model is just too complicated.  Look at the recent exchange
> between myself & Dan Williams.  I spent literally _hours_ trying to
> figure out what rules to follow.
> 
> https://lore.kernel.org/linux-mm/CAPcyv4jgjoLqsV+aHGJwGXbCSwbTnWLmog5-rxD2i31vZ2rDNQ@mail.gmail.com/
> https://lore.kernel.org/linux-mm/CAPcyv4j2+7XiJ9BXQ4mj_XN0N+rCyxch5QkuZ6UsOBsOO1+2Vg@mail.gmail.com/
> 
> Neither Dan nor I are exactly "new" to Linux kernel development.  As Dave
> is saying here, having to understand the memory model is too high a bar.
> 
> Hell, I don't know if what we ended up with for v4 is actually correct.
> It lokos good to me, but *shrug*
> 
> https://lore.kernel.org/linux-mm/159009507306.847224.8502634072429766747.stgit@dwillia2-desk3.amr.corp.intel.com/

Looks like you still got it wrong :-(  It needs:

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 934c92dcb9ab..9a95fbe86e15 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -1029,7 +1029,7 @@ static int devmem_init_inode(void)
        }

        /* publish /dev/mem initialized */
-       WRITE_ONCE(devmem_inode, inode);
+       smp_store_release(&devmem_inode, inode);

        return 0;
 }

It seems one source of confusion is that READ_ONCE() and WRITE_ONCE() don't
actually pair with each other, unless no memory barriers are needed at all.

Instead, READ_ONCE() pairs with a primitive that has "release" semantics, e.g.
smp_store_release() or cmpxchg_release().  But READ_ONCE() is only correct if
there's no control flow dependency; if there is, it needs to be upgraded to a
primitive with "acquire" semantics, e.g. smp_load_acquire().

The best approach might be to just say that the READ_ONCE() + "release" pairing
should be avoided, and we should stick to "acquire" + "release".  (And I think
Dave may be saying he'd prefer that for ->s_dio_done_wq?)

- Eric