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

[Jens Axboe <axboe@kernel.dk>]

On 8/10/20 5:37 AM, peterz@infradead.org wrote:
> On Sat, Aug 08, 2020 at 12:34:38PM -0600, Jens Axboe wrote:
>> Some callers may need to make signaling decisions based on the state
>> of the targeted task, and that can only safely be done post adding
>> the task_work to the task. Split task_work_add() into:
>>
>> __task_work_add()	- adds the work item
>> __task_work_notify()	- sends the notification
>>
>> No functional changes in this patch.
> 
> Might be nice to mention __task_work_add() is now inline.

OK, will mention that.

>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: stable@vger.kernel.org # v5.7+
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>  include/linux/task_work.h | 19 ++++++++++++++++
>>  kernel/task_work.c        | 48 +++++++++++++++++++++------------------
>>  2 files changed, 45 insertions(+), 22 deletions(-)
> 
> 
> 
>> +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.

-- 
Jens Axboe