Re: [PATCH v3 1/2] wait_bit: do read barrier after testing a bit

[Alan Stern]

On Sun, Jul 31, 2022 at 04:40:59PM -0400, Mikulas Patocka wrote:
> wait_on_bit tests the bit without any memory barriers, consequently the
> code that follows wait_on_bit may be moved before testing the bit on
> architectures with weak memory ordering. When the code tests for some
> event using wait_on_bit and then performs a load operation, the load may
> be unexpectedly moved before wait_on_bit and it may return data that
> existed before the event occurred.
> 
> Such bugs exist in fs/buffer.c:__wait_on_buffer,
> drivers/md/dm-bufio.c:new_read,
> drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:dvb_usb_start_feed,
> drivers/bluetooth/btusb.c:btusb_mtk_hci_wmt_sync
> and perhaps in other places.
> 
> We fix this class of bugs by adding a read barrier after test_bit().
> 
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> 
> Index: linux-2.6/include/linux/wait_bit.h
> ===================================================================
> --- linux-2.6.orig/include/linux/wait_bit.h
> +++ linux-2.6/include/linux/wait_bit.h
> @@ -71,8 +71,10 @@ static inline int
>  wait_on_bit(unsigned long *word, int bit, unsigned mode)
>  {
>  	might_sleep();
> -	if (!test_bit(bit, word))
> +	if (!test_bit(bit, word)) {
> +		smp_rmb();

Any new code using smp_rmb or an acquire access should always include a 
comment that explains where the matching smp_wmb or release access is.

Alan Stern

>  		return 0;
> +	}
>  	return out_of_line_wait_on_bit(word, bit,
>  				       bit_wait,
>  				       mode);
> @@ -96,8 +98,10 @@ static inline int
>  wait_on_bit_io(unsigned long *word, int bit, unsigned mode)
>  {
>  	might_sleep();
> -	if (!test_bit(bit, word))
> +	if (!test_bit(bit, word)) {
> +		smp_rmb();
>  		return 0;
> +	}
>  	return out_of_line_wait_on_bit(word, bit,
>  				       bit_wait_io,
>  				       mode);
> @@ -123,8 +127,10 @@ wait_on_bit_timeout(unsigned long *word,
>  		    unsigned long timeout)
>  {
>  	might_sleep();
> -	if (!test_bit(bit, word))
> +	if (!test_bit(bit, word)) {
> +		smp_rmb();
>  		return 0;
> +	}
>  	return out_of_line_wait_on_bit_timeout(word, bit,
>  					       bit_wait_timeout,
>  					       mode, timeout);
> @@ -151,8 +157,10 @@ wait_on_bit_action(unsigned long *word,
>  		   unsigned mode)
>  {
>  	might_sleep();
> -	if (!test_bit(bit, word))
> +	if (!test_bit(bit, word)) {
> +		smp_rmb();
>  		return 0;
> +	}
>  	return out_of_line_wait_on_bit(word, bit, action, mode);
>  }
>  
> Index: linux-2.6/kernel/sched/wait_bit.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/wait_bit.c
> +++ linux-2.6/kernel/sched/wait_bit.c
> @@ -51,6 +51,8 @@ __wait_on_bit(struct wait_queue_head *wq
>  
>  	finish_wait(wq_head, &wbq_entry->wq_entry);
>  
> +	smp_rmb();
> +
>  	return ret;
>  }
>  EXPORT_SYMBOL(__wait_on_bit);
>