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Re: [dm-devel] [RFC, PATCH] remove signal handling from dm-io.c, sync_io()



On Friday 11 June 2004 11:48 am, Kevin Corry wrote:
> Upon further review of dm-io.c::sync_io(), I have a couple more comments.
>
> First, I'm not certain whether the signal handling needs to be there or
> not. Joe, perhaps you can comment on that. If we don't need it, then we can
> use Dave's suggestion and just get rid of the signal_pending() call. If we
> do need it, then instead of declaring a struct io on the stack in
> sync_io(), we need to allocate one (async_io() already does this using a
> mempool) and use dec_count() to deallocate it. This ought to prevent the
> stack corruption Dave described. I'll put together a patch in a bit to
> demonstrate how this would work.

After talking to Alasdair a bit, it seems like there still might be a reason
for using the interruptible wait in sync_io(). The point he brought up was
that if you are using dm-io to do I/O to another DM device, and that DM device
is suspended, then the I/O may never complete if the device doesn't get
unsuspended (for some reason). In this case, you'd like to be able to
interrupt the process that's waiting for that I/O so it isn't stuck in D-state
forever.

So this means we need to allocate a struct io from the mempool in sync_io()
instead of declaring it on the stack. Here's a patch that should accomplish
this. It ain't pretty, but I think it gets the job done. It requires some
changes in dec_count(), and I've also replaced the io_schedule() loop with a
call to wait_event_interruptible(). Now everyone take a look and tell me how
ugly this is. :)  And if anyone can think of something more elegant, please
let me know.

-- 
Kevin Corry
kevcorry us ibm com
http://evms.sourceforge.net/


--- diff/drivers/md/dm-io.c	2004-06-11 12:16:24.729145072 +0000
+++ source/drivers/md/dm-io.c	2004-06-11 13:56:02.626366560 +0000
@@ -317,17 +317,19 @@
 		set_bit(region, &io->error);
 
 	if (atomic_dec_and_test(&io->count)) {
-		if (io->sleeper)
-			wake_up_process(io->sleeper);
+		int r = io->error;
+		io_notify_fn fn = io->callback;
+		void *context = io->context;
 
-		else {
-			int r = io->error;
-			io_notify_fn fn = io->callback;
-			void *context = io->context;
+		mempool_free(io, _io_pool);
 
-			mempool_free(io, _io_pool);
+		if (fn) {
 			fn(r, context);
 		}
+	} else {
+		struct task_struct *sleeper = io->sleeper;
+		if (sleeper)
+			wake_up_process(sleeper);
 	}
 }
 
@@ -535,33 +537,34 @@
 static int sync_io(unsigned int num_regions, struct io_region *where,
 	    int rw, struct dpages *dp, unsigned long *error_bits)
 {
-	struct io io;
+	DECLARE_WAIT_QUEUE_HEAD(wq);
+	struct io *io;
+	int rc;
 
 	if (num_regions > 1 && rw != WRITE) {
 		return -EIO;
 	}
 
-	io.error = 0;
-	atomic_set(&io.count, 1); /* see dispatch_io() */
-	io.sleeper = current;
-
-	dispatch_io(rw, num_regions, where, dp, &io, 1);
-
-	while (1) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
-
-		if (!atomic_read(&io.count) || signal_pending(current))
-			break;
-
-		io_schedule();
+	io = mempool_alloc(_io_pool, GFP_NOIO);
+	memset(io, 0, sizeof(*io));
+	atomic_set(&io->count, 2);	/* One for us, one for dispatch_io(). */
+	io->sleeper = current;
+
+	dispatch_io(rw, num_regions, where, dp, io, 1);
+
+	wait_event_interruptible(wq, (atomic_read(&io->count) == 1));
+
+	if (atomic_read(&io->count) > 1) {
+		rc = -EINTR;
+	} else {
+		*error_bits = io->error;
+		rc = io->error ? -EIO : 0;
 	}
-	set_current_state(TASK_RUNNING);
 
-	if (atomic_read(&io.count))
-		return -EINTR;
+	io->sleeper = NULL;
+	dec_count(io, 0, rc);
 
-	*error_bits = io.error;
-	return io.error ? -EIO : 0;
+	return rc;
 }
 
 static int async_io(unsigned int num_regions, struct io_region *where, int rw,

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