ataflop: fix sleep_on races

sleep_on() is inherently racy, and has been deprecated for a long time.
This fixes two instances in the atari floppy driver:

* fdc_wait/fdc_busy becomes an open-coded mutex. We cannot use the
  regular mutex since it gets released in interrupt context. The
  open-coded version using wait_event() and cmpxchg() is equivalent
  to the existing code but does the checks atomically, and we can
  now safely check the condition with irqs enabled.

* format_wait becomes a completion, which is the natural structure
  here. The format ioctl waits for the background task to either
  complete or abort.

This does not attempt to fix the preexisting bug of calling schedule
with local interrupts disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael Schmitz <schmitz@biophys.uni-duesseldorf.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/ataflop.c

@@ -68,6 +68,8 @@
 #include <linux/init.h>
 #include <linux/blkdev.h>
 #include <linux/mutex.h>
+#include <linux/completion.h>
+#include <linux/wait.h>
 
 #include <asm/atafd.h>
 #include <asm/atafdreg.h>
@@ -301,7 +303,7 @@ module_param_array(UserSteprate, int, NULL, 0);
 /* Synchronization of FDC access. */
 static volatile int fdc_busy = 0;
 static DECLARE_WAIT_QUEUE_HEAD(fdc_wait);
-static DECLARE_WAIT_QUEUE_HEAD(format_wait);
+static DECLARE_COMPLETION(format_wait);
 
 static unsigned long changed_floppies = 0xff, fake_change = 0;
 #define	CHECK_CHANGE_DELAY	HZ/2
@@ -608,7 +610,7 @@ static void fd_error( void )
 	if (IsFormatting) {
 		IsFormatting = 0;
 		FormatError = 1;
-		wake_up( &format_wait );
+		complete(&format_wait);
 		return;
 	}
 
@@ -650,9 +652,8 @@ static int do_format(int drive, int type, struct atari_format_descr *desc)
 	DPRINT(("do_format( dr=%d tr=%d he=%d offs=%d )\n",
 		drive, desc->track, desc->head, desc->sect_offset ));
 
+	wait_event(fdc_wait, cmpxchg(&fdc_busy, 0, 1) == 0);
 	local_irq_save(flags);
-	while( fdc_busy ) sleep_on( &fdc_wait );
-	fdc_busy = 1;
 	stdma_lock(floppy_irq, NULL);
 	atari_turnon_irq( IRQ_MFP_FDC ); /* should be already, just to be sure */
 	local_irq_restore(flags);
@@ -706,7 +707,7 @@ static int do_format(int drive, int type, struct atari_format_descr *desc)
 	ReqSide  = desc->head;
 	do_fd_action( drive );
 
-	sleep_on( &format_wait );
+	wait_for_completion(&format_wait);
 
 	redo_fd_request();
 	return( FormatError ? -EIO : 0 );	
@@ -1229,7 +1230,7 @@ static void fd_writetrack_done( int status )
 		goto err_end;
 	}
 
-	wake_up( &format_wait );
+	complete(&format_wait);
 	return;
 
   err_end:
@@ -1497,8 +1498,7 @@ repeat:
 void do_fd_request(struct request_queue * q)
 {
 	DPRINT(("do_fd_request for pid %d\n",current->pid));
-	while( fdc_busy ) sleep_on( &fdc_wait );
-	fdc_busy = 1;
+	wait_event(fdc_wait, cmpxchg(&fdc_busy, 0, 1) == 0);
 	stdma_lock(floppy_irq, NULL);
 
 	atari_disable_irq( IRQ_MFP_FDC );