Re: pstore/core: drop cmpxchg based updates

[Guenter Roeck]

On Wed, Aug 24, 2016 at 03:09:35PM +0200, Sebastian Andrzej Siewior wrote:
> I have here a FPGA behind PCIe which exports SRAM which I use for
> pstore. Now it seems that the FPGA no longer supports cmpxchg based
> updates and writes back 0xffâff and returns the same.  This leads to
> crash during crash rendering pstore useless.
> Since I doubt that there is much benefit from using cmpxchg() here, I am
> dropping this atomic access and use the spinlock based version.
> 
> Cc: Anton Vorontsov <anton@xxxxxxxxxx>
> Cc: Colin Cross <ccross@xxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Tested-by: Rabin Vincent <rabinv@xxxxxxxx>

Great, one item taken off my task list.

As Rabin suggested, this patch is needed for at least some ARM chips,
and should be applied to stable releases.

Reviewed-by: Guenter Roeck <linux@xxxxxxxxxxxx>

> ---
>  fs/pstore/ram_core.c | 43 ++-----------------------------------------
>  1 file changed, 2 insertions(+), 41 deletions(-)
> 
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index 76c3f80efdfa..4bae54bb61cd 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -47,39 +47,6 @@ static inline size_t buffer_start(struct persistent_ram_zone *prz)
>  	return atomic_read(&prz->buffer->start);
>  }
>  
> -/* increase and wrap the start pointer, returning the old value */
> -static size_t buffer_start_add_atomic(struct persistent_ram_zone *prz, size_t a)
> -{
> -	int old;
> -	int new;
> -
> -	do {
> -		old = atomic_read(&prz->buffer->start);
> -		new = old + a;
> -		while (unlikely(new >= prz->buffer_size))
> -			new -= prz->buffer_size;
> -	} while (atomic_cmpxchg(&prz->buffer->start, old, new) != old);
> -
> -	return old;
> -}
> -
> -/* increase the size counter until it hits the max size */
> -static void buffer_size_add_atomic(struct persistent_ram_zone *prz, size_t a)
> -{
> -	size_t old;
> -	size_t new;
> -
> -	if (atomic_read(&prz->buffer->size) == prz->buffer_size)
> -		return;
> -
> -	do {
> -		old = atomic_read(&prz->buffer->size);
> -		new = old + a;
> -		if (new > prz->buffer_size)
> -			new = prz->buffer_size;
> -	} while (atomic_cmpxchg(&prz->buffer->size, old, new) != old);
> -}
> -
>  static DEFINE_RAW_SPINLOCK(buffer_lock);
>  
>  /* increase and wrap the start pointer, returning the old value */
> @@ -124,9 +91,6 @@ static void buffer_size_add_locked(struct persistent_ram_zone *prz, size_t a)
>  	raw_spin_unlock_irqrestore(&buffer_lock, flags);
>  }
>  
> -static size_t (*buffer_start_add)(struct persistent_ram_zone *, size_t) = buffer_start_add_atomic;
> -static void (*buffer_size_add)(struct persistent_ram_zone *, size_t) = buffer_size_add_atomic;
> -
>  static void notrace persistent_ram_encode_rs8(struct persistent_ram_zone *prz,
>  	uint8_t *data, size_t len, uint8_t *ecc)
>  {
> @@ -338,9 +302,9 @@ int notrace persistent_ram_write(struct persistent_ram_zone *prz,
>  		c = prz->buffer_size;
>  	}
>  
> -	buffer_size_add(prz, c);
> +	buffer_size_add_locked(prz, c);
>  
> -	start = buffer_start_add(prz, c);
> +	start = buffer_start_add_locked(prz, c);
>  
>  	rem = prz->buffer_size - start;
>  	if (unlikely(rem < c)) {
> @@ -426,9 +390,6 @@ static void *persistent_ram_iomap(phys_addr_t start, size_t size,
>  		return NULL;
>  	}
>  
> -	buffer_start_add = buffer_start_add_locked;
> -	buffer_size_add = buffer_size_add_locked;
> -
>  	if (memtype)
>  		va = ioremap(start, size);
>  	else