firewire: Fix for broken configrom updates in quick succession

Current implementation of ohci_set_config_rom() uses a deferred
bus reset via fw_schedule_bus_reset(). If clients add multiple
unit descriptors to the config_rom in quick succession, the
deferred bus reset may not have fired before succeeding update
requests have come in. This can lead to an incorrect partial
update of the config_rom for both addition and removal of
config_rom descriptors, as the ohci_set_config_rom() routine
will return -EBUSY if a previous pending update has not been
completed yet; the requested update just gets dropped on the floor.

This patch recognizes that the "in-flight" update can be modified
until it has been processed by the bus-reset, and the locking
in the bus_reset_tasklet ensures that the update is done atomically
with respect to modifications made by ohci_set_config_rom(). The
-EBUSY error case is simply removed.

[Stefan R:  The bug always existed at least theoretically.  But it
became easy to trigger since 2.6.36 commit 02d37bed "firewire: core:
integrate software-forced bus resets with bus management" which
introduced long mandatory delays between janitorial bus resets.]
Signed-off-by: Benjamin Buchalter <bj@mhlabs.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (trivial style changes)
Cc: <stable@kernel.org> # 2.6.36.y and newer

drivers/firewire/ohci.c

@@ -2199,7 +2199,6 @@ static int ohci_set_config_rom(struct fw_card *card,
 {
 	struct fw_ohci *ohci;
 	unsigned long flags;
-	int ret = -EBUSY;
 	__be32 *next_config_rom;
 	dma_addr_t uninitialized_var(next_config_rom_bus);
 
@@ -2240,22 +2239,37 @@ static int ohci_set_config_rom(struct fw_card *card,
 
 	spin_lock_irqsave(&ohci->lock, flags);
 
+	/*
+	 * If there is not an already pending config_rom update,
+	 * push our new allocation into the ohci->next_config_rom
+	 * and then mark the local variable as null so that we
+	 * won't deallocate the new buffer.
+	 *
+	 * OTOH, if there is a pending config_rom update, just
+	 * use that buffer with the new config_rom data, and
+	 * let this routine free the unused DMA allocation.
+	 */
+
 	if (ohci->next_config_rom == NULL) {
 		ohci->next_config_rom = next_config_rom;
 		ohci->next_config_rom_bus = next_config_rom_bus;
+		next_config_rom = NULL;
+	}
 
 	copy_config_rom(ohci->next_config_rom, config_rom, length);
 
 	ohci->next_header = config_rom[0];
 	ohci->next_config_rom[0] = 0;
 
-		reg_write(ohci, OHCI1394_ConfigROMmap,
-			  ohci->next_config_rom_bus);
-		ret = 0;
-	}
+	reg_write(ohci, OHCI1394_ConfigROMmap, ohci->next_config_rom_bus);
 
 	spin_unlock_irqrestore(&ohci->lock, flags);
 
+	/* If we didn't use the DMA allocation, delete it. */
+	if (next_config_rom != NULL)
+		dma_free_coherent(ohci->card.device, CONFIG_ROM_SIZE,
+				  next_config_rom, next_config_rom_bus);
+
 	/*
 	 * Now initiate a bus reset to have the changes take
 	 * effect. We clean up the old config rom memory and DMA
@@ -2263,13 +2277,10 @@ static int ohci_set_config_rom(struct fw_card *card,
 	 * controller could need to access it before the bus reset
 	 * takes effect.
 	 */
-	if (ret == 0)
+
 	fw_schedule_bus_reset(&ohci->card, true, true);
-	else
-		dma_free_coherent(ohci->card.device, CONFIG_ROM_SIZE,
-				  next_config_rom, next_config_rom_bus);
 
-	return ret;
+	return 0;
 }
 
 static void ohci_send_request(struct fw_card *card, struct fw_packet *packet)