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Date: Fri, 12 May 2023 12:40:13 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Arve Hjønnevåg <arve@...roid.com>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/wait: Fix a kthread_park race with wait_woken()
On Thu, May 11, 2023 at 05:52:24PM -0700, John Stultz wrote:
> On Thu, May 11, 2023 at 3:31 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, May 11, 2023 at 09:41:30PM +0000, John Stultz wrote:
> > > From: Arve Hjønnevåg <arve@...roid.com>
> > >
> > > kthread_park and wait_woken have a similar race that kthread_stop and
> > > wait_woken used to have before it was fixed in
> > > cb6538e740d7543cd989128625cf8cac4b471e0a. Extend that fix to also cover
> >
> > cb6538e740d7 ("sched/wait: Fix a kthread race with wait_woken()")
> >
> > > kthread_park.
> > >
> > > Cc: Ingo Molnar <mingo@...hat.com>
> > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > Cc: Juri Lelli <juri.lelli@...hat.com>
> > > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > Cc: Ben Segall <bsegall@...gle.com>
> > > Cc: Mel Gorman <mgorman@...e.de>
> > > Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> > > Cc: Valentin Schneider <vschneid@...hat.com>
> > > Signed-off-by: Arve Hjønnevåg <arve@...roid.com>
> > > Signed-off-by: John Stultz <jstultz@...gle.com>
> > > ---
> > > This seemingly slipped by, so I wanted to resend it
> > > for review.
> > > ---
> > > kernel/sched/wait.c | 6 +++---
> > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> > > index 133b74730738..a9cf49da884b 100644
> > > --- a/kernel/sched/wait.c
> > > +++ b/kernel/sched/wait.c
> > > @@ -425,9 +425,9 @@ int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, i
> > > }
> > > EXPORT_SYMBOL(autoremove_wake_function);
> > >
> > > -static inline bool is_kthread_should_stop(void)
> > > +static inline bool is_kthread_should_stop_or_park(void)
> > > {
> > > - return (current->flags & PF_KTHREAD) && kthread_should_stop();
> > > + return (current->flags & PF_KTHREAD) && (kthread_should_stop() || kthread_should_park());
> > > }
> > >
> > > /*
> >
> > That's a bit sad; that two function calls for checking two consecutive
> > bits in the same word :-(
> >
> > If we move this to kthread.c and write it like:
> >
> > kthread = __to_kthread(current);
> > if (!kthread)
> > return false;
> >
> > return test_bit(KTHREAD_SHOULD_STOP, &kthread->flags) ||
> > test_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
> >
> > Then the compiler should be able to merge the two bits in a single load
> > and test due to constant_test_bit() -- do check though.
>
> Hrm. Apologies, as it's been awhile since I've looked at disassembled
> asm, so I may be wrong, but I don't think that's happening here.
>
> With the logic above I'm seeing it build as:
> 0000000000000a50 <kthread_should_stop_or_park>:
> a50: 65 48 8b 14 25 00 00 mov %gs:0x0,%rdx
> a57: 00 00
> a59: 48 8b 8a 78 0a 00 00 mov 0xa78(%rdx),%rcx
> a60: 31 c0 xor %eax,%eax
> a62: 48 85 c9 test %rcx,%rcx
> a65: 74 11 je a78 <kthread_should_stop_or_park+0x28>
> a67: f6 42 2e 20 testb $0x20,0x2e(%rdx)
> a6b: 74 0b je a78 <kthread_should_stop_or_park+0x28>
> a6d: 48 8b 01 mov (%rcx),%rax
> a70: 48 d1 e8 shr %rax
> a73: 83 e0 01 and $0x1,%eax
> a76: 74 05 je a7d <kthread_should_stop_or_park+0x2d>
> a78: e9 00 00 00 00 jmp a7d <kthread_should_stop_or_park+0x2d>
> a7d: 48 8b 01 mov (%rcx),%rax
> a80: 48 c1 e8 02 shr $0x2,%rax
> a84: 83 e0 01 and $0x1,%eax
> a87: e9 00 00 00 00 jmp a8c <kthread_should_stop_or_park+0x3c>
> a8c: 0f 1f 40 00 nopl 0x0(%rax)
Moo, where is that optimization pass when you need it ;-)
If we forgo test_bit() and write it plainly it seems to work:
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 7e6751b29101..36f94616cae5 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -164,6 +164,15 @@ bool __kthread_should_park(struct task_struct *k)
}
EXPORT_SYMBOL_GPL(__kthread_should_park);
+bool kthread_should_stop_or_park(void)
+{
+ struct kthread *kthread = __to_kthread(current);
+ if (!kthread)
+ return false;
+
+ return kthread->flags & (BIT(KTHREAD_SHOULD_STOP) | BIT(KTHREAD_SHOULD_PARK));
+}
+
/**
* kthread_should_park - should this kthread park now?
*
0000000000001850 <kthread_should_stop_or_park>:
1850: f3 0f 1e fa endbr64
1854: 65 48 8b 04 25 00 00 00 00 mov %gs:0x0,%rax 1859: R_X86_64_32S pcpu_hot
185d: 48 8b 88 08 06 00 00 mov 0x608(%rax),%rcx
1864: 31 d2 xor %edx,%edx
1866: 48 85 c9 test %rcx,%rcx
1869: 74 0c je 1877 <kthread_should_stop_or_park+0x27>
186b: f6 40 2e 20 testb $0x20,0x2e(%rax)
186f: 74 06 je 1877 <kthread_should_stop_or_park+0x27>
1871: f6 01 06 testb $0x6,(%rcx)
1874: 0f 95 c2 setne %dl
1877: 89 d0 mov %edx,%eax
1879: e9 00 00 00 00 jmp 187e <kthread_should_stop_or_park+0x2e> 187a: R_X86_64_PLT32 __x86_return_thunk-0x4
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