Re: [PATCH 1/2] kernel: split task_work_add() into two separate helpers

[peterz@infradead.org]

On Mon, Aug 10, 2020 at 09:01:02AM -0600, Jens Axboe wrote:

> >> +struct callback_head work_exited = {
> >> +	.next = NULL	/* all we need is ->next == NULL */
> >> +};
> > 
> > Would it make sense to make this const ? Esp. with the thing exposed,
> > sticking it in R/O memory might avoid a mistake somewhere.
> 
> That was my original intent, but that makes 'head' in task_work_run()
> const as well, and cmpxchg() doesn't like that:
> 
> kernel/task_work.c: In function ‘task_work_run’:
> ./arch/x86/include/asm/cmpxchg.h:89:29: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>    89 |  __typeof__(*(ptr)) __new = (new);    \
>       |                             ^
> ./arch/x86/include/asm/cmpxchg.h:134:2: note: in expansion of macro ‘__raw_cmpxchg’
>   134 |  __raw_cmpxchg((ptr), (old), (new), (size), LOCK_PREFIX)
>       |  ^~~~~~~~~~~~~
> ./arch/x86/include/asm/cmpxchg.h:149:2: note: in expansion of macro ‘__cmpxchg’
>   149 |  __cmpxchg(ptr, old, new, sizeof(*(ptr)))
>       |  ^~~~~~~~~
> ./include/asm-generic/atomic-instrumented.h:1685:2: note: in expansion of macro ‘arch_cmpxchg’
>  1685 |  arch_cmpxchg(__ai_ptr, __VA_ARGS__);    \
>       |  ^~~~~~~~~~~~
> kernel/task_work.c:126:12: note: in expansion of macro ‘cmpxchg’
>   126 |   } while (cmpxchg(&task->task_works, work, head) != work);
>       |            ^~~~~~~
> 
> which is somewhat annoying. Because there's really no good reason why it
> can't be const, it'll just require the changes to dig a bit deeper.

Bah! Best I can come up with is casting the const away there, what a
mess :-(