Re: [PATCH] floppy: annotate data-races around command_status and floppy_work_fn

["Denis Efremov (Oracle)" <efremov@linux.com>]

Hello,

thank you for the patch. Some comments below.

On 16/03/2026 13:08, Cen Zhang wrote:
> command_status is accessed by multiple contexts:
>  - generic_done() and do_wakeup() write it from the floppy
>    workqueue context.
>  - wait_til_done() reads it in wait_event conditions without
>    holding fdc_busy.
>  - is_alive() reads it locklessly to check driver liveness.
>  - floppy_queue_rq(), lock_fdc(), and unlock_fdc() write it
>    during FDC ownership transitions.
> 
> There is currently no LKMM annotation for these concurrent accesses
> since command_status relies on the deprecated volatile qualifier.
> Remove volatile and use READ_ONCE()/WRITE_ONCE() on every access
> to command_status to provide proper LKMM data-race annotations.
> 
> Also annotate floppy_work_fn with WRITE_ONCE() in schedule_bh()
> and READ_ONCE() in floppy_work_workfn(), and current_type[drive]
> with READ_ONCE() in drive_no_geom() where it can be read without
> holding the FDC lock.

Please, inline a KCSAN report if you used it.

> 
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
> ---
>  drivers/block/floppy.c | 38 +++++++++++++++++++++-----------------
>  1 file changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
> index 92e446a64371..1da7947fe042 100644
> --- a/drivers/block/floppy.c
> +++ b/drivers/block/floppy.c
> @@ -502,7 +502,7 @@ static int probing;
>  #define FD_COMMAND_ERROR	2
>  #define FD_COMMAND_OKAY		3
>  
> -static volatile int command_status = FD_COMMAND_NONE;
> +static int command_status = FD_COMMAND_NONE;
>  static unsigned long fdc_busy;
>  static DECLARE_WAIT_QUEUE_HEAD(fdc_wait);
>  static DECLARE_WAIT_QUEUE_HEAD(command_done);
> @@ -601,7 +601,8 @@ static inline void fdc_outb(unsigned char value, int fdc, int reg)
>  
>  static inline bool drive_no_geom(int drive)
>  {
> -	return !current_type[drive] && !ITYPE(drive_state[drive].fd_device);
> +	return !READ_ONCE(current_type[drive]) &&
> +	       !ITYPE(drive_state[drive].fd_device);
>  }
>  
>  #ifndef fd_eject
> @@ -640,7 +641,7 @@ static const char *timeout_message;
>  static void is_alive(const char *func, const char *message)
>  {
>  	/* this routine checks whether the floppy driver is "alive" */
> -	if (test_bit(0, &fdc_busy) && command_status < 2 &&
> +	if (test_bit(0, &fdc_busy) && READ_ONCE(command_status) < 2 &&
>  	    !delayed_work_pending(&fd_timeout)) {
>  		DPRINT("%s: timeout handler died.  %s\n", func, message);
>  	}
> @@ -892,7 +893,7 @@ static int lock_fdc(int drive)
>  	if (wait_event_interruptible(fdc_wait, !test_and_set_bit(0, &fdc_busy)))
>  		return -EINTR;
>  
> -	command_status = FD_COMMAND_NONE;
> +	WRITE_ONCE(command_status, FD_COMMAND_NONE);
>  
>  	reschedule_timeout(drive, "lock fdc");
>  	set_fdc(drive);
> @@ -906,7 +907,7 @@ static void unlock_fdc(void)
>  		DPRINT("FDC access conflict!\n");
>  
>  	raw_cmd = NULL;
> -	command_status = FD_COMMAND_NONE;
> +	WRITE_ONCE(command_status, FD_COMMAND_NONE);
>  	cancel_delayed_work(&fd_timeout);
>  	do_floppy = NULL;
>  	cont = NULL;
> @@ -990,7 +991,9 @@ static void (*floppy_work_fn)(void);
>  
>  static void floppy_work_workfn(struct work_struct *work)
>  {
> -	floppy_work_fn();
> +	void (*fn)(void) = READ_ONCE(floppy_work_fn);
> +
> +	fn();
>  }
>  
>  static DECLARE_WORK(floppy_work, floppy_work_workfn);
> @@ -999,7 +1002,7 @@ static void schedule_bh(void (*handler)(void))
>  {
>  	WARN_ON(work_pending(&floppy_work));
>  
> -	floppy_work_fn = handler;
> +	WRITE_ONCE(floppy_work_fn, handler);
>  	queue_work(floppy_wq, &floppy_work);
>  }
>  
> @@ -1864,7 +1867,7 @@ static void show_floppy(int fdc)
>  
>  	pr_info("cont=%p\n", cont);
>  	pr_info("current_req=%p\n", current_req);
> -	pr_info("command_status=%d\n", command_status);
> +	pr_info("command_status=%d\n", READ_ONCE(command_status));
>  	pr_info("\n");
>  }
>  
> @@ -1991,7 +1994,7 @@ static void do_wakeup(void)
>  {
>  	reschedule_timeout(MAXTIMEOUT, "do wakeup");
>  	cont = NULL;
> -	command_status += 2;
> +	WRITE_ONCE(command_status, command_status + 2);

WRITE_ONCE(command_status, READ_ONCE(command_status) + 2); ?

or with a temporary variable.

>  	wake_up(&command_done);
>  }
>  
> @@ -2019,11 +2022,12 @@ static int wait_til_done(void (*handler)(void), bool interruptible)
>  	schedule_bh(handler);
>  
>  	if (interruptible)
> -		wait_event_interruptible(command_done, command_status >= 2);
> +		wait_event_interruptible(command_done,
> +					 READ_ONCE(command_status) >= 2);
>  	else
> -		wait_event(command_done, command_status >= 2);
> +		wait_event(command_done, READ_ONCE(command_status) >= 2);
>  
> -	if (command_status < 2) {
> +	if (READ_ONCE(command_status) < 2) {
>  		cancel_activity();
>  		cont = &intr_cont;
>  		reset_fdc();
> @@ -2031,18 +2035,18 @@ static int wait_til_done(void (*handler)(void), bool interruptible)
>  	}
>  
>  	if (fdc_state[current_fdc].reset)
> -		command_status = FD_COMMAND_ERROR;
> -	if (command_status == FD_COMMAND_OKAY)
> +		WRITE_ONCE(command_status, FD_COMMAND_ERROR);
> +	if (READ_ONCE(command_status) == FD_COMMAND_OKAY)
>  		ret = 0;
>  	else
>  		ret = -EIO;
> -	command_status = FD_COMMAND_NONE;
> +	WRITE_ONCE(command_status, FD_COMMAND_NONE);
>  	return ret;
>  }
>  
>  static void generic_done(int result)
>  {
> -	command_status = result;
> +	WRITE_ONCE(command_status, result);
>  	cont = &wakeup_cont;
>  }
>  
> @@ -2873,7 +2877,7 @@ static blk_status_t floppy_queue_rq(struct blk_mq_hw_ctx *hctx,
>  	list_add_tail(&bd->rq->queuelist, &floppy_reqs);
>  	spin_unlock_irq(&floppy_lock);
>  
> -	command_status = FD_COMMAND_NONE;
> +	WRITE_ONCE(command_status, FD_COMMAND_NONE);
>  	__reschedule_timeout(MAXTIMEOUT, "fd_request");
>  	set_fdc(0);
>  	process_fd_request();