[PATCHv4 01/12] atomic/tty: Fix up atomic abuse in ldsem

On Tue, Jul 24, 2018 at 11:20:36AM +0200, Peter Zijlstra wrote:

On Tue, Jul 24, 2018 at 09:15:18AM +0200, Ingo Molnar wrote:

quoted

* Mark Rutland [off-list ref] wrote:

quoted

From: Peter Zijlstra <peterz@infradead.org>

Mark found ldsem_cmpxchg() needed an (atomic_long_t *) cast to keep
working after making the atomic_long interface type safe.

Needing casts is bad form, which made me look at the code. There are no
ld_semaphore::count users outside of these functions so there is no
reason why it can not be an atomic_long_t in the first place, obviating
the need for this cast.

That also ensures the loads use atomic_long_read(), which implies (at
least) READ_ONCE() in order to guarantee single-copy-atomic loads.

When using atomic_long_try_cmpxchg() the ldsem_cmpxchg() wrapper gets
very thin (the only difference is not changing *old on success, which
most callers don't seem to care about).

So rework the whole thing to use atomic_long_t and its accessors
directly.

While there, fixup all the horrible comment styles.

Cc: Peter Hurley <redacted>
Acked-by: Will Deacon <redacted>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andy Shevchenko <redacted>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 drivers/tty/tty_ldsem.c   | 82 ++++++++++++++++++++---------------------------
 include/linux/tty_ldisc.h |  4 +--
 2 files changed, 37 insertions(+), 49 deletions(-)

Note: Greg has queued this via the in the tty tree for v4.19, which can be seen at: 

https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?h=tty-next&id=5fd691afdf929061c391d897fa627822c3b2fd5a

Can this patch be skipped, or do the others depend on it?

IIRC it depends on it, without this patch you get build issues due to
atomic_long_cmpxchg() getting picky about it's arguments (type safety
improved).

Yup. Without this patch, there will be a build regression at patch 9,
when we move to generated atomic_long_*() wrappers.

Thanks,
Mark.