linux-kernel - Re: [External] Re: [PATCH] riscv: Fix local irq restore when flags indicates irq disabled

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Date:   Wed, 9 Aug 2023 14:58:11 +0800
From:   旭路 <luxu.kernel@...edance.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH] riscv: Fix local irq restore when flags
 indicates irq disabled

On Wed, Aug 9, 2023 at 2:05 PM Palmer Dabbelt <palmer@...belt.com> wrote:
>
> On Tue, 25 Jul 2023 00:05:49 PDT (-0700), luxu.kernel@...edance.com wrote:
> > When arch_local_irq_restore() is called with flags indicating irqs
> > disabled, we need to clear SR_IE bit in CSR_STATUS, whereas current
> > implementation based on csr_set() function only sets SR_IE bit of
> > CSR_STATUS when SR_IE bit of flags is high and does nothing when
> > SR_IE bit of flags is low.
> >
> > This commit supplies csr clear operation when calling irq restore
> > function with flags indicating irq disabled.
> >
> > Fixes: 6d60b6ee0c97 ("RISC-V: Device, timer, IRQs, and the SBI")
> > Signed-off-by: Xu Lu <luxu.kernel@...edance.com>
> > ---
> >  arch/riscv/include/asm/irqflags.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/include/asm/irqflags.h b/arch/riscv/include/asm/irqflags.h
> > index 08d4d6a5b7e9..7c31fc3c3559 100644
> > --- a/arch/riscv/include/asm/irqflags.h
> > +++ b/arch/riscv/include/asm/irqflags.h
> > @@ -49,7 +49,10 @@ static inline int arch_irqs_disabled(void)
> >  /* set interrupt enabled status */
> >  static inline void arch_local_irq_restore(unsigned long flags)
> >  {
> > -     csr_set(CSR_STATUS, flags & SR_IE);
> > +     if (flags & SR_IE)
> > +             csr_set(CSR_STATUS, SR_IE);
> > +     else
> > +             csr_clear(CSR_STATUS, SR_IE);
>
> Unless I'm missing something, the original version is correct:
> local_irq_restore() must be paired with a local_irq_save(), so we can
> only get here with interrupts disabled.

Yes, it is correct if local_irq_save() is called when irq is enabled before.
The flags returned will be SR_IE. And it is safe to call local_irq_restore()
with flag SR_IE in any case.

However, if local_irq_save() is called when irq is disabled. The SR_IE bit of
flag returned is clear. If some code between local_irq_save() and
local_irq_restore() reenables irq, causing the SR_IE bit of CSR_STATUS
back to 1, then local_irq_restore() can not restore irq status back to disabled.

Here is an example in existing driver (may not belong to riscv arch) in
drivers/net/wireless/realtek/rtlwifi/rtl8723ae/hw.c. The pseudo code
behaves like:

int rtl8723e_hw_init(struct ieee80211_hw *hw)
{
     int err;
     unsigned long flags;
     ...
     local_irq_save_flags(flags);
     local_irq_enable();
     ...
     local_irq_restore(flags);
     ...
     return err;
}


>
> >  }
> >
> >  #endif /* _ASM_RISCV_IRQFLAGS_H */