Re: [PATCH] parisc: Fix extraction of hash lock bits in syscall.S

[Helge Deller <deller@gmx.de>]

On 11/18/21 18:03, John David Anglin wrote:
> The extru instruction leaves the most significant 32 bits of the target register in an undefined
> state on PA 2.0 systems.  If any of these bits are nonzero, this will break the calculation of the
> lock pointer.
>
> Fix by using extrd,u instruction on 64-bit kernels.

Good catch!!
Did you checked if it actually happened that the most
significant 32 bits were non-zero?
If so, could this be one of the reasons we saw strange
issues or even memory corruption?

Sadly I sent a pull request to Linus a few hours ago,
otherwise I would have added this patch...

Helge

> Signed-off-by: John David Anglin <dave.anglin@bell.net>
> ---
> diff --git a/arch/parisc/kernel/syscall.S b/arch/parisc/kernel/syscall.S
> index 3f24a0af1e04..3f70528622eb 100644
> --- a/arch/parisc/kernel/syscall.S
> +++ b/arch/parisc/kernel/syscall.S
> @@ -572,7 +572,11 @@ lws_compare_and_swap:
>      ldo    R%lws_lock_start(%r20), %r28
>
>      /* Extract eight bits from r26 and hash lock (Bits 3-11) */
> +#ifdef CONFIG_64BIT
> +    extrd,u  %r26, 60, 8, %r20
> +#else
>      extru  %r26, 28, 8, %r20
> +#endif
>
>      /* Find lock to use, the hash is either one of 0 to
>         15, multiplied by 16 (keep it 16-byte aligned)
> @@ -762,7 +761,11 @@ cas2_lock_start:
>      ldo    R%lws_lock_start(%r20), %r28
>
>      /* Extract eight bits from r26 and hash lock (Bits 3-11) */
> +#ifdef CONFIG_64BIT
> +    extrd,u  %r26, 60, 8, %r20
> +#else
>      extru  %r26, 28, 8, %r20
> +#endif
>
>      /* Find lock to use, the hash is either one of 0 to
>         15, multiplied by 16 (keep it 16-byte aligned)