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From: Jens Axboe <[email protected]>
To: [email protected]
Cc: [email protected], [email protected]
Subject: Re: [PATCH 1/2] kernel: split task_work_add() into two separate helpers
Date: Mon, 10 Aug 2020 09:01:02 -0600	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 8/10/20 5:37 AM, [email protected] 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 <[email protected]>
>> Cc: [email protected] # v5.7+
>> Signed-off-by: Jens Axboe <[email protected]>
>> ---
>>  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


  reply	other threads:[~2020-08-10 15:01 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-08 18:34 [PATCHSET 0/2] io_uring: use TWA_SIGNAL more carefully Jens Axboe
2020-08-08 18:34 ` [PATCH 1/2] kernel: split task_work_add() into two separate helpers Jens Axboe
2020-08-10 11:37   ` peterz
2020-08-10 15:01     ` Jens Axboe [this message]
2020-08-10 15:28       ` peterz
2020-08-10 17:51       ` Jens Axboe
2020-08-10 19:53         ` Peter Zijlstra
2020-08-08 18:34 ` [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running Jens Axboe
2020-08-10 11:42   ` peterz
2020-08-10 15:02     ` Jens Axboe
2020-08-10 19:21       ` Jens Axboe
2020-08-10 20:12         ` Peter Zijlstra
2020-08-10 20:13           ` Jens Axboe
2020-08-10 20:25             ` Jens Axboe
2020-08-10 20:32               ` Peter Zijlstra
2020-08-10 20:35                 ` Jens Axboe
2020-08-10 20:35               ` Jann Horn
2020-08-10 21:06                 ` Jens Axboe
2020-08-10 21:10                   ` Peter Zijlstra
2020-08-10 21:12                     ` Jens Axboe
2020-08-10 21:26                       ` Jann Horn
2020-08-10 21:28                         ` Jens Axboe
2020-08-10 22:01                           ` Jens Axboe
2020-08-10 22:41                             ` Jann Horn
2020-08-11  1:25                               ` Jens Axboe
2020-08-11  6:45                               ` Oleg Nesterov
2020-08-11  6:56                                 ` Peter Zijlstra
2020-08-11  7:14                                   ` Oleg Nesterov
2020-08-11  7:26                                     ` Oleg Nesterov
2020-08-11  7:49                                       ` Peter Zijlstra
2020-08-11  7:45                                     ` Peter Zijlstra
2020-08-11  8:10                                       ` Oleg Nesterov
2020-08-11 13:06                                         ` Jens Axboe
2020-08-11 14:05                                           ` Oleg Nesterov
2020-08-11 14:12                                             ` Jens Axboe
2020-08-10 21:27                       ` Jens Axboe
2020-08-10 20:16           ` Jens Axboe
2020-08-13 16:25   ` Sasha Levin
2020-08-19 23:57   ` Sasha Levin
2020-08-19 23:59     ` Jens Axboe
2020-08-20  0:02       ` Jens Axboe

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