Re: [Xen-devel] [PATCH v4 1/5] xen/spinlocks: in debug builds store cpu holding the lock

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.09.2019 16:31, Juergen Gross wrote:
> --- a/xen/include/xen/spinlock.h
> +++ b/xen/include/xen/spinlock.h
> @@ -5,15 +5,24 @@
>  #include <asm/spinlock.h>
>  #include <asm/types.h>
>  
> +#define SPINLOCK_CPU_BITS  12
> +
>  #ifndef NDEBUG
> -struct lock_debug {
> -    s16 irq_safe; /* +1: IRQ-safe; 0: not IRQ-safe; -1: don't know yet */
> +union lock_debug {
> +    uint16_t val;
> +#define LOCK_DEBUG_INITVAL 0xffff
> +    struct {
> +        uint16_t cpu:SPINLOCK_CPU_BITS;
> +        uint16_t :(14 - SPINLOCK_CPU_BITS);

I'm sorry that I realize this only now that I see this and ...

> +        bool irq_safe:1;
> +        bool unseen:1;
> +    };
>  };
> -#define _LOCK_DEBUG { -1 }
> +#define _LOCK_DEBUG { LOCK_DEBUG_INITVAL }
>  void spin_debug_enable(void);
>  void spin_debug_disable(void);
>  #else
> -struct lock_debug { };
> +union lock_debug { };
>  #define _LOCK_DEBUG { }
>  #define spin_debug_enable() ((void)0)
>  #define spin_debug_disable() ((void)0)
> @@ -138,11 +147,12 @@ typedef union {
>  
>  typedef struct spinlock {
>      spinlock_tickets_t tickets;
> -    u16 recurse_cpu:12;
> -#define SPINLOCK_NO_CPU 0xfffu
> -    u16 recurse_cnt:4;
> -#define SPINLOCK_MAX_RECURSE 0xfu
> -    struct lock_debug debug;
> +    u16 recurse_cpu:SPINLOCK_CPU_BITS;
> +#define SPINLOCK_NO_CPU        ((1u << SPINLOCK_CPU_BITS) - 1)
> +#define SPINLOCK_RECURSE_BITS  (16 - SPINLOCK_CPU_BITS)
> +    u16 recurse_cnt:SPINLOCK_RECURSE_BITS;
> +#define SPINLOCK_MAX_RECURSE   ((1u << SPINLOCK_RECURSE_BITS) - 1)

... this: These subtractions are prone to yield de-generate
bitfields when the difference ends up zero (leading to
presumably very strange breakage, albeit one would hope that
it also would be very obvious that _something_ is broken). I
think we need BUILD_BUG_ON()s checking that neither
difference actually is zero.

Jan

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