atari_scsi: Fix atari_scsi deadlocks on Falcon

Don't disable irqs when waiting for the ST DMA "lock"; its release may
require an interrupt.

Introduce stdma_try_lock() for use in soft irq context. atari_scsi now tells
the SCSI mid-layer to defer queueing a command if the ST DMA lock is not
available, as per Michael's patch:
http://marc.info/?l=linux-m68k&m=139095335824863&w=2

The falcon_got_lock variable is race prone: we can't disable IRQs while
waiting to acquire the lock, so after acquiring it there must be some
interval during which falcon_got_lock remains false. Introduce
stdma_is_locked_by() to replace falcon_got_lock.

The falcon_got_lock tests in the EH handlers are incorrect these days. It
can happen that an EH handler is called after a command completes normally.
Remove these checks along with falcon_got_lock.

Also remove the complicated and racy fairness wait queues. If fairness is an
issue (when SCSI competes with IDE for the ST DMA interrupt), the solution
is likely to be a lower value for host->can_queue.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

2 files changed, 42 insertions, 23 deletions

diff --git a/arch/m68k/atari/stdma.c b/arch/m68k/atari/stdma.c
index ddbf43ca8858..e5a66596b116 100644
--- a/arch/m68k/atari/stdma.c
+++ b/arch/m68k/atari/stdma.c
@@ -59,6 +59,31 @@ static irqreturn_t stdma_int (int irq, void *dummy);
 /************************* End of Prototypes **************************/
 
 
+/**
+ * stdma_try_lock - attempt to acquire ST DMA interrupt "lock"
+ * @handler: interrupt handler to use after acquisition
+ *
+ * Returns !0 if lock was acquired; otherwise 0.
+ */
+
+int stdma_try_lock(irq_handler_t handler, void *data)
+{
+        unsigned long flags;
+
+        local_irq_save(flags);
+        if (stdma_locked) {
+                local_irq_restore(flags);
+                return 0;
+        }
+
+        stdma_locked   = 1;
+        stdma_isr      = handler;
+        stdma_isr_data = data;
+        local_irq_restore(flags);
+        return 1;
+}
+EXPORT_SYMBOL(stdma_try_lock);
+
 
 /*
  * Function: void stdma_lock( isrfunc isr, void *data )
@@ -78,19 +103,10 @@ static irqreturn_t stdma_int (int irq, void *dummy);
 
 void stdma_lock(irq_handler_t handler, void *data)
 {
-        unsigned long flags;
-
-        local_irq_save(flags);          /* protect lock */
-
         /* Since the DMA is used for file system purposes, we
          have to sleep uninterruptible (there may be locked
          buffers) */
-        wait_event(stdma_wait, !stdma_locked);
-
-        stdma_locked   = 1;
-        stdma_isr      = handler;
-        stdma_isr_data = data;
-        local_irq_restore(flags);
+        wait_event(stdma_wait, stdma_try_lock(handler, data));
 }
 EXPORT_SYMBOL(stdma_lock);
 
@@ -122,22 +138,25 @@ void stdma_release(void)
 EXPORT_SYMBOL(stdma_release);
 
 
-/*
- * Function: int stdma_others_waiting( void )
- *
- * Purpose: Check if someone waits for the ST-DMA lock.
- *
- * Inputs: none
- *
- * Returns: 0 if no one is waiting, != 0 otherwise
+/**
+ * stdma_is_locked_by - allow lock holder to check whether it needs to release.
+ * @handler: interrupt handler previously used to acquire lock.
  *
+ * Returns !0 if locked for the given handler; 0 otherwise.
  */
 
-int stdma_others_waiting(void)
+int stdma_is_locked_by(irq_handler_t handler)
 {
-        return waitqueue_active(&stdma_wait);
+        unsigned long flags;
+        int result;
+
+        local_irq_save(flags);
+        result = stdma_locked && (stdma_isr == handler);
+        local_irq_restore(flags);
+
+        return result;
 }
-EXPORT_SYMBOL(stdma_others_waiting);
+EXPORT_SYMBOL(stdma_is_locked_by);
 
 
 /*
diff --git a/arch/m68k/include/asm/atari_stdma.h b/arch/m68k/include/asm/atari_stdma.h
index 8e389b7fa70c..d24e34d870dc 100644
--- a/arch/m68k/include/asm/atari_stdma.h
+++ b/arch/m68k/include/asm/atari_stdma.h
@@ -8,11 +8,11 @@
 
 /***************************** Prototypes *****************************/
 
+int stdma_try_lock(irq_handler_t, void *);
 void stdma_lock(irq_handler_t handler, void *data);
 void stdma_release( void );
-int stdma_others_waiting( void );
 int stdma_islocked( void );
-void *stdma_locked_by( void );
+int stdma_is_locked_by(irq_handler_t);
 void stdma_init( void );
 
 /************************* End of Prototypes **************************/