Re: [9fans] lock diagnostics on Fossil server

[erik quanstrom <quanstro@quanstro.net>]

On Mon Oct 25 22:03:53 EDT 2010, cinap_lenrek@gmx.de wrote:

> hm... wouldnt it just crash if mh->mount is nil?
>

perhaps you are reading the diff backwards?  it used to
crash when mh->mount was nil.  leading to a lock loop.
i added the test to see that mh->mount != nil after the
rlock on mh->lock is acquired. otherwise, there is a race
with unmount which can make mh->mount nil while we're running.

our newer faster processors with more cores were making
this event likely enough that a few receipies would crash
the machine within 5 minutes.

- erik

---

/n/sources/plan9//sys/src/9/port/chan.c:1012,1018 - chan.c:1012,1020
  				/*
  				 * mh->mount->to == c, so start at mh->mount->next
  				 */
+ 				f = nil;
  				rlock(&mh->lock);
+ 				if(mh->mount)
  				for(f = mh->mount->next; f; f = f->next)
  					if((wq = ewalk(f->to, nil, names+nhave, ntry)) != nil)
  						break;