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SubjectRe: [PATCH] 2.4.18 raid1 - fix SMP locking/interrupt errors, fix resync counter errors
On Monday March 25, kernel@steeleye.com wrote:
> Neil,
>
> Thanks for your feedback. Replies below...
>

ditto :-)

> >
> > I can believe that there could be extra contention because of the dual
> > use of this spin lock. Do you have lockmeter numbers at all?
>
> No, I'm not familiar with that. How do I get those? Is it fairly
> simple?

I've never tried myself, so I don't know of simple it is. The relevant
home page seems to be:
http://oss.sgi.com/projects/lockmeter/

>
> I wasn't so much concerned about extra contention as the (in my mind)
> logical separation of these two different tasks, and the fact that
> the lack of separation had led to a deadlock.
>

Certainly these are logically distinct uses of the same lock, though
there are still closely related.
There is often a tension between splitting a lock to reduce
granularity and keeping the number of spinlocks to a minimum.
In general I would only split a lock if their was either a clear
semantic need, or measureable performance impact.

If a deadlock were a consequence of not splitting, that would be a
strong argument, but I think we can avoid the deadlock by other means.

>
> > However I cannot see how it would cause a deadlock. Could you please
> > give details?
>
> raid1_diskop() calls close_sync() -- close_sync() schedules itself out
> to wait for pending I/O to quiesce so that the resync can end...
> meanwhile #CPUs (in my case, 2) tasks enter into any of the memory
> (de)allocation routines and spin on the device_lock forever...

Ahhhh.... Thanks....
close_sync() definately shouldn't be called with a spinlock held. In
the patch below I have moved it out of the locked region.
> > You are definately right that we should not be calling kmalloc with a
> > spinlock held - my bad.
> > However I don't think your fix is ideal. The relevant code is
> > "raid1_grow_buffers" which allocates a bunch of buffers and attaches
> > them to the device structure.
> > The lock is only realy needed for the attachment. A better fix would
> > be to build a separate list, and then just claim the lock while
> > attaching that list to the structure.
>
> Unfortunately, this won't work, because the segment_lock is also held
> while this code is executing (see raid1_sync_request).
>

Good point, thanks. I think this means that the call to
raid1_grow_buffers needs to be moved out from inside the locked
region. This is done in the following patch.

>
> > >
> > > 3) incorrect enabling/disabling of interrupts during locking
> > >
...
> >
> > I don't believe that this is true.
> > The save/restore versions are only needed if the code might be called
> > from interrupt context. However the routines where you made this
> > change: raid1_grow_buffers, raid1_shrink_buffers, close_sync,
> > are only ever called from process context, with interrupts enabled.
> > Or am I missing something?
>
> please see my other e-mail reply to Andrew Morton regarding this...

OK, I understand now. spin_lock_irq is being called while
spin_lock_irq is already in-force. I think this is best fixed by
moving calls outside of locked regions as mentioned above.


>
>
> > >
> > > 4) incorrect setting of conf->cnt_future and conf->phase resync counters
> > >
> > > The symptoms of this problem were that, if I/O was occurring when a
> > > resync ended (or was aborted), the resync would hang and never complete.
> > > This eventually would cause all I/O to the md device to hang.
> >
> > I'll have to look at this one a bit more closely. I'll let you know
> > what I think of it.
>
> OK. If you come up with something better, please let me know.

OK, I've had a look, and I see what the problem is:

conf->start_future = mddev->sb->size+1;

start_future is in sectors. sb->size is in Kibibytes :-(
Should be
conf->start_future = (mddev->sb->size<<1)+1;

This error would explain your symptom.


Below is a patch which I believe should address all of your symptoms
and the bugs that they expose. If your are able to test it and let me
know how it works for you I would appreciate it.

Thanks
NeilBrown



----------- Diffstat output ------------
./drivers/md/raid1.c | 54 +++++++++++++++++++++++++++++++++++++--------------
1 files changed, 40 insertions(+), 14 deletions(-)
--- ./drivers/md/raid1.c 2002/03/25 21:53:59 1.1
+++ ./drivers/md/raid1.c 2002/03/26 01:47:56 1.2
@@ -269,8 +269,9 @@
static int raid1_grow_buffers (raid1_conf_t *conf, int cnt)
{
int i = 0;
+ struct raid1_bh *head = NULL, **tail;
+ tail = &head;

- md_spin_lock_irq(&conf->device_lock);
while (i < cnt) {
struct raid1_bh *r1_bh;
struct page *page;
@@ -287,10 +288,18 @@
memset(r1_bh, 0, sizeof(*r1_bh));
r1_bh->bh_req.b_page = page;
r1_bh->bh_req.b_data = page_address(page);
- r1_bh->next_r1 = conf->freebuf;
- conf->freebuf = r1_bh;
+ *tail = r1_bh;
+ r1_bh->next_r1 = NULL;
+ tail = & r1_bh->next_r1;
i++;
}
+ /* this lock probably isn't needed, as at the time when
+ * we are allocating buffers, nobody else will be touching the
+ * freebuf list. But it doesn't hurt....
+ */
+ md_spin_lock_irq(&conf->device_lock);
+ *tail = conf->freebuf;
+ conf->freebuf = head;
md_spin_unlock_irq(&conf->device_lock);
return i;
}
@@ -825,7 +834,7 @@
conf->start_ready = conf->start_pending;
wait_event_lock_irq(conf->wait_ready, !conf->cnt_pending, conf->segment_lock);
conf->start_active =conf->start_ready = conf->start_pending = conf->start_future;
- conf->start_future = mddev->sb->size+1;
+ conf->start_future = (mddev->sb->size<<1)+1;
conf->cnt_pending = conf->cnt_future;
conf->cnt_future = 0;
conf->phase = conf->phase ^1;
@@ -849,6 +858,14 @@
mdk_rdev_t *spare_rdev, *failed_rdev;

print_raid1_conf(conf);
+
+ switch (state) {
+ case DISKOP_SPARE_ACTIVE:
+ case DISKOP_SPARE_INACTIVE:
+ /* need to wait for pending sync io before locking device */
+ close_sync(conf);
+ }
+
md_spin_lock_irq(&conf->device_lock);
/*
* find the disk ...
@@ -951,7 +968,11 @@
* Deactivate a spare disk:
*/
case DISKOP_SPARE_INACTIVE:
- close_sync(conf);
+ if (conf->start_future > 0) {
+ MD_BUG();
+ err = -EBUSY;
+ break;
+ }
sdisk = conf->mirrors + spare_disk;
sdisk->operational = 0;
sdisk->write_only = 0;
@@ -964,7 +985,11 @@
* property)
*/
case DISKOP_SPARE_ACTIVE:
- close_sync(conf);
+ if (conf->start_future > 0) {
+ MD_BUG();
+ err = -EBUSY;
+ break;
+ }
sdisk = conf->mirrors + spare_disk;
fdisk = conf->mirrors + failed_disk;

@@ -1328,23 +1353,25 @@
int bsize;
int disk;
int block_nr;
+ int buffs;

+ if (!sector_nr) {
+ /* we want enough buffers to hold twice the window of 128*/
+ buffs = 128 *2 / (PAGE_SIZE>>9);
+ buffs = raid1_grow_buffers(conf, buffs);
+ if (buffs < 2)
+ goto nomem;
+ conf->window = buffs*(PAGE_SIZE>>9)/2;
+ }
spin_lock_irq(&conf->segment_lock);
if (!sector_nr) {
/* initialize ...*/
- int buffs;
conf->start_active = 0;
conf->start_ready = 0;
conf->start_pending = 0;
conf->start_future = 0;
conf->phase = 0;
- /* we want enough buffers to hold twice the window of 128*/
- buffs = 128 *2 / (PAGE_SIZE>>9);
- buffs = raid1_grow_buffers(conf, buffs);
- if (buffs < 2)
- goto nomem;

- conf->window = buffs*(PAGE_SIZE>>9)/2;
conf->cnt_future += conf->cnt_done+conf->cnt_pending;
conf->cnt_done = conf->cnt_pending = 0;
if (conf->cnt_ready || conf->cnt_active)
@@ -1429,7 +1456,6 @@

nomem:
raid1_shrink_buffers(conf);
- spin_unlock_irq(&conf->segment_lock);
return -ENOMEM;
}

-
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