Re: Copying to loop device hangs up everything

[Momchil Velikov]

>>>>> "Momchil" == Momchil Velikov <[e-mail]> writes:

>>>>> "David" == David Gomez <[e-mail]> writes:
David> On 16 Dec 2001, Momchil Velikov wrote:

>>> [...]
>>> 
David> Thanks ;), this patch solves the problem and copying a lot of data to the
David> loop device now doesn't hang the computer.
>>> 
David> Is this patch going to be applied to the stable kernel ? Marcelo ?
>>> 
>>> I've had exactly the same hangups with or without the patch.

David> I've tested several times after applying the loop-deadlock patch and the
David> bug seems to be fixed. No more hangups while copying a lot of data to
David> loopback devices. Post more info about your hangups, maybe is another
David> different loop device deadlock.

Momchil> Maybe it's different I don't know. Looks like I've found a fix and in
Momchil> a minute I'll test _without_ the Andrea's patch and post whatever
Momchil> comes out of it.

It turned out that Andrea's patch is needed and it needs to be
augmented slightly.  The loop_thread can do the following:

loop_thread
-> do_bh_filebacked
-> lo_send
-> ...
->  kmem_cache_alloc
-> ...
-> shrink_cache
-> try_to_release_page
-> try_to_free_buffers
-> sync_page_buffers
-> __wait_on_buffer

And if the buffer must be flushed to the loopback device we deadlock.

The following patch is the Andrea's one + one additional change -- we
don't allow the loop_thread to wait in sync_page_buffers.

Regards,
-velco

diff -Nru a/drivers/block/loop.c b/drivers/block/loop.c
--- a/drivers/block/loop.c	Sun Dec 16 23:50:25 2001
+++ b/drivers/block/loop.c	Sun Dec 16 23:50:25 2001
@@ -578,6 +578,8 @@
 	atomic_inc(&lo->lo_pending);
 	spin_unlock_irq(&lo->lo_lock);
 
+	current->flags |= PF_NOIO;
+
 	/*
 	 * up sem, we are running
 	 */
diff -Nru a/fs/buffer.c b/fs/buffer.c
--- a/fs/buffer.c	Sun Dec 16 23:50:25 2001
+++ b/fs/buffer.c	Sun Dec 16 23:50:25 2001
@@ -1045,7 +1045,7 @@
 
 	/* First, check for the "real" dirty limit. */
 	if (dirty > soft_dirty_limit) {
-		if (dirty > hard_dirty_limit)
+		if (dirty > hard_dirty_limit && !(current->flags & PF_NOIO))
 			return 1;
 		return 0;
 	}
@@ -2448,6 +2448,8 @@
 		/* Second time through we start actively writing out.. */
 		if (test_and_set_bit(BH_Lock, &bh->b_state)) {
 			if (!test_bit(BH_launder, &bh->b_state))
+				continue;
+			if (current->flags & PF_NOIO)
 				continue;
 			wait_on_buffer(bh);
 			tryagain = 1;
diff -Nru a/include/linux/sched.h b/include/linux/sched.h
--- a/include/linux/sched.h	Sun Dec 16 23:50:25 2001
+++ b/include/linux/sched.h	Sun Dec 16 23:50:25 2001
@@ -426,6 +426,7 @@
 #define PF_MEMALLOC	0x00000800	/* Allocating memory */
 #define PF_MEMDIE	0x00001000	/* Killed for out-of-memory */
 #define PF_FREE_PAGES	0x00002000	/* per process page freeing */
+#define PF_NOIO		0x00004000	/* avoid generating further I/O */
 
 #define PF_USEDFPU	0x00100000	/* task used FPU this quantum (SMP) */