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Subject[PATCH 06/11] uml ubd driver: ubd_io_lock usage fixup
From: Paolo 'Blaisorblade' Giarrusso <>

Add some comments about requirements for ubd_io_lock and expand its use.

When an irq signals that the "controller" (i.e. another thread on the host,
which does the actual requests and is the only one blocked on I/O on the host)
has done some work, we call again the request function ourselves

We now do that with ubd_io_lock held - that's useful to protect against
concurrent calls to elv_next_request and so on.

XXX: Maybe we shouldn't call at all the request function. Input needed on this.
Are we supposed to plug and unplug the queue? That code "indirectly" does that
by setting a flag, called do_ubd, which makes the request function return (it's
a residual of 2.4 block layer interface).

Meanwhile, however, merge this patch, which improves things.

Cc: Jens Axboe <>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <>

arch/um/drivers/ubd_kern.c | 8 +++++++-
1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index e4fd29a..1202592 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -106,6 +106,8 @@ static inline void ubd_set_bit(__u64 bit

#define DRIVER_NAME "uml-blkdev"

+/* Can be taken in interrupt context, and is passed to the block layer to lock
+ * the request queue. Kernel side code knows that. */
static DEFINE_SPINLOCK(ubd_io_lock);

static DEFINE_MUTEX(ubd_lock);
@@ -497,6 +499,8 @@ static void __ubd_finish(struct request
end_request(req, 1);

+/* Callable only from interrupt context - otherwise you need to do
+ * spin_lock_irq()/spin_lock_irqsave() */
static inline void ubd_finish(struct request *req, int error)
@@ -504,7 +508,7 @@ static inline void ubd_finish(struct req

-/* Called without ubd_io_lock held */
+/* Called without ubd_io_lock held, and only in interrupt context. */
static void ubd_handler(void)
struct io_thread_req req;
@@ -525,7 +529,9 @@ static void ubd_handler(void)

ubd_finish(rq, req.error);
reactivate_fd(thread_fd, UBD_IRQ);
+ spin_lock(&ubd_io_lock);
+ spin_unlock(&ubd_io_lock);

static irqreturn_t ubd_intr(int irq, void *dev)
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