linux-kernel - Re: [PATCH] locking/lockdep: Dump state of percpu

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Date:   Tue, 10 Jul 2018 11:30:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] locking/lockdep: Dump state of percpu_rwsem upon hung up.

On Tue, Jul 10, 2018 at 03:10:51PM +0900, Tetsuo Handa wrote:
> From 2642b4a1904259384f2018ea8df03ac49509c57a Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SKAURA.ne.jp>
> Date: Tue, 10 Jul 2018 15:01:20 +0900
> Subject: [PATCH] locking/rwsem: Convert the other sem->count to 'atomic_long_t'
> 
> Since "locking/rwsem: Convert sem->count to 'atomic_long_t'" forgot to
> convert "struct rw_semaphore"->count in linux/rwsem-spinlock.h which is
> used when CONFIG_RWSEM_GENERIC_SPINLOCK=y, trying to access it in MIPS
> architecture causes build error due to type mismatch.
> 
>   error: passing argument 1 of 'atomic_long_read' from incompatible pointer type [-Werror=incompatible-pointer-types]
>   note: expected 'const atomic_long_t * {aka const struct <anonymous> *}' but argument is of type '__s32 * {aka int *}'
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SKAURA.ne.jp>
> Reported-by: kbuild test robot <lkp@...el.com>
> Fixes: 8ee62b1870be8e63 ("locking/rwsem: Convert sem->count to 'atomic_long_t'")
> Cc: stable <stable@...r.kernel.org> # v4.8+
> Cc: Jason Low <jason.low2@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  include/linux/rwsem-spinlock.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rwsem-spinlock.h b/include/linux/rwsem-spinlock.h
> index e475683..1164965 100644
> --- a/include/linux/rwsem-spinlock.h
> +++ b/include/linux/rwsem-spinlock.h
> @@ -22,7 +22,7 @@
>   * - if wait_list is not empty, then there are processes waiting for the semaphore
>   */
>  struct rw_semaphore {
> -	__s32			count;
> +	atomic_long_t		count;

Uhhh... how does this even build? rwsem-spinlock.c doesn't use
atomic_long primitives to change count.

And the atomic_long count usage is completely private to rwsem-xadd.c,
nothing outside of that should use that field, ever.