Messages in this thread | | | Subject | Re: [PATCH v3 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state | From | Julien Grall <> | Date | Thu, 25 Apr 2019 18:12:59 +0100 |
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Hi Dave,
On 25/04/2019 17:39, Dave Martin wrote: > On Thu, Apr 25, 2019 at 04:57:26PM +0100, Julien Grall wrote: >> Hi Dave, >> >> On 24/04/2019 14:17, Dave Martin wrote: >>> On Tue, Apr 23, 2019 at 02:57:19PM +0100, Julien Grall wrote: >>>> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c >>>> index 5313aa257be6..6168d06bbd20 100644 >>>> --- a/arch/arm64/kernel/fpsimd.c >>>> +++ b/arch/arm64/kernel/fpsimd.c >>>> @@ -92,7 +92,8 @@ >>>> * To prevent this from racing with the manipulation of the task's FPSIMD state >>>> * from task context and thereby corrupting the state, it is necessary to >>>> * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE >>>> - * flag with local_bh_disable() unless softirqs are already masked. >>>> + * flag with {, __}get_cpu_fpsimd_context(). This will still allow softirqs to >>>> + * run but prevent them to use FPSIMD. >>>> * >>>> * For a certain task, the sequence may look something like this: >>>> * - the task gets scheduled in; if both the task's fpsimd_cpu field >>>> @@ -155,6 +156,56 @@ extern void __percpu *efi_sve_state; >>>> #endif /* ! CONFIG_ARM64_SVE */ >>>> +DEFINE_PER_CPU(bool, fpsimd_context_busy); >>>> +EXPORT_PER_CPU_SYMBOL(fpsimd_context_busy); >>>> + >>>> +static void __get_cpu_fpsimd_context(void) >>>> +{ >>>> + bool busy = __this_cpu_xchg(fpsimd_context_busy, true); >>>> + >>>> + WARN_ON(busy); >>>> +} >>>> + >>>> +/* >>>> + * Claim ownership of the CPU FPSIMD context for use by the calling context. >>>> + * >>>> + * The caller may freely modify FPSIMD context until *put_cpu_fpsimd_context() >>>> + * is called. >>> >>> Nit: it may be better to say "freely manipulate the FPSIMD context >>> metadata". >>> >>> get_cpu_fpsimd_context() isn't enough to allow the FPSIMD regs to be >>> safely trashed, because they may still contain live data (or an up to >>> date copy) for some task. >> >> Good point, I will update the comment. >> >>> >>> (For that you also need fpsimd_save_and_flush_cpu_state(), or just use >>> kernel_neon_begin() instead.) >>> >>> [...] >>> >>>> @@ -922,6 +971,8 @@ void fpsimd_thread_switch(struct task_struct *next) >>>> if (!system_supports_fpsimd()) >>>> return; >>>> + __get_cpu_fpsimd_context(); >>>> + >>>> /* Save unsaved fpsimd state, if any: */ >>>> fpsimd_save(); >>>> @@ -936,6 +987,8 @@ void fpsimd_thread_switch(struct task_struct *next) >>>> update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE, >>>> wrong_task || wrong_cpu); >>>> + >>>> + __put_cpu_fpsimd_context(); >>> >>> There should be a note in the commit message explaining why these are >>> here. >>> >>> Are they actually needed, other than to keep >>> WARN_ON(have_cpu_fpsimd_context()) happy elsewhere? >> >> It depends on how fpsimd_thread_switch() is called. I will answer more below. >> >>> >>> Does PREEMPT_RT allow non-threaded softirqs to execute while we're in >>> this code? >> >> This has nothing to do with PREEMPT_RT. Softirqs might be executed after >> handling interrupt (see irq_exit()). >> >> A call to preempt_disable() will not be enough to prevent softirqs, you >> actually need to either mask interrupts or have BH disabled. >> >> fpsimd_thread_switch() seems to be only called from the context switch code. >> AFAICT, interrupt will be masked. Therefore, holding the FPSIMD CPU is not >> necessary. However... >> >>> >>> >>> OTOH, if the overall effect on performance remains positive, we can >>> probably argue that these operations make the code more self-describing >>> and help guard against mistakes during future maintanence, even if >>> they're not strictly needed today. >> >> .... I think it would help guard against mistakes. The more I haven't seen >> any performance impact in the benchmark. > > Which generally seems a good thing. The commit message should explain > that these are being added for hygiene rather than necessity here, > though.
I will update the commit message.
> >> [...] >> >>>> -/* >>>> - * Save the FPSIMD state to memory and invalidate cpu view. >>>> - * This function must be called with softirqs (and preemption) disabled. >>>> - */ >>>> +/* Save the FPSIMD state to memory and invalidate cpu view. */ >>>> void fpsimd_save_and_flush_cpu_state(void) >>>> { >>>> + get_cpu_fpsimd_context(); >>>> fpsimd_save(); >>>> fpsimd_flush_cpu_state(); >>>> + put_cpu_fpsimd_context(); >>>> } >>> >>> Again, are these added just to keep WARN_ON()s happy? >> >> !preemptible() is not sufficient to prevent softirq running. You also need >> to have either interrupt masked or BH disabled. > > So, why was the code safe before this series? (In fact, _was_ it safe?) > > AFAICT, we have local_irq_disable() around context switch, which covers > preempt notifiers (where kvm_arch_vcpu_put_fp() gets called) and > fpsimd_thread_switch(): this is what prevents softirqs from firing.
That's correct, both callers of the function will have IRQs masked. Also, the function fpsimd_save() contained: WARN_ON(!in_softirq() && !irqs_disabled());
So we were covered in case of misuse.
> > So, while it's clean to have get/put here, I still don't see why they're > required. > > I think the arguments are basically similar to fpsimd_thread_switch(). > Since fpsimd_save_and_flush_cpu_state() and fpsimd_thread_switch() are > called from similar contexts, is makes sense to keep them aligned.
Yes, this is for hygiene rather than a real bug (thought the WARN_ON() in fpsimd_save() would fire). I will update the message accordingly.
Cheers,
-- Julien Grall
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