Messages in this thread Patch in this message | | | Date | Fri, 12 May 2023 12:40:13 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/wait: Fix a kthread_park race with wait_woken() |
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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@infradead.org> wrote: > > > > On Thu, May 11, 2023 at 09:41:30PM +0000, John Stultz wrote: > > > From: Arve Hjønnevåg <arve@android.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@redhat.com> > > > Cc: Peter Zijlstra <peterz@infradead.org> > > > Cc: Juri Lelli <juri.lelli@redhat.com> > > > Cc: Vincent Guittot <vincent.guittot@linaro.org> > > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> > > > Cc: Steven Rostedt <rostedt@goodmis.org> > > > Cc: Ben Segall <bsegall@google.com> > > > Cc: Mel Gorman <mgorman@suse.de> > > > Cc: Daniel Bristot de Oliveira <bristot@redhat.com> > > > Cc: Valentin Schneider <vschneid@redhat.com> > > > Signed-off-by: Arve Hjønnevåg <arve@android.com> > > > Signed-off-by: John Stultz <jstultz@google.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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