Re: [9fans] lock diagnostics on Fossil server

[erik quanstrom]

On Mon Oct 25 22:03:53 EDT 2010, cinap_len...@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;