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Message-ID: <ZWWzSZeyRuPDwBHZ@alley>
Date: Tue, 28 Nov 2023 10:30:49 +0100
From: Petr Mladek <pmladek@...e.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
Mike Christie <michael.christie@...cle.com>,
Zqiang <qiang1.zhang@...el.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] kthread: kthread_should_stop() checks if we're a kthread
Adding Andrew into Cc. He usually takes changes in kernel/kthread.c.
On Mon 2023-11-20 17:15:03, Kent Overstreet wrote:
> bcachefs has a fair amount of code that may or may not be running from a
> kthread (it may instead be called by a userspace ioctl); having
> kthread_should_stop() check if we're a kthread enables a fair bit of
> cleanup and makes it safer to use.
>
> Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> ---
> kernel/kthread.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 1eea53050bab..fe6090ddf414 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -155,7 +155,8 @@ void free_kthread_struct(struct task_struct *k)
> */
> bool kthread_should_stop(void)
> {
> - return test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
> + return (current->flags & PF_KTHREAD) &&
> + test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
> }
> EXPORT_SYMBOL(kthread_should_stop);
I agree that it makes the API more safe because &to_kthread(current)
is NULL when the process is not a kthread.
Well, I do not like the idea of quietly ignoring a misuse of
the kthread_*() API. I would personally prefer to do:
// define this in include/linux/kthread.h
static inline bool in_kthread(void)
{
return current->flags & PF_KTHREAD
}
// add WARN() into kthread_should_stop()
bool kthread_should_stop(void)
{
if (WARN_ON_ONCE(!in_kthread))
return false;
return test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
}
EXPORT_SYMBOL(kthread_should_stop);
And use the following in bcachefs() code:
if (in_kthread() && kthread_should_stop())
goto exit;
Is see several advantages:
+ It will warn when the API is misused.
+ It will be more clear that the bcachefs code might be
used in both kthread and userspace code.
+ in_kthread() might be used around other code which is
needed only when the process is a kthread.
+ Similar check and WARN() might be used also in the other
kthread() API.
Best Regards,
Petr
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