race conditions for destroying and opening a dev

[Kostik Belousov]

On Thu, Sep 16, 2010 at 12:00:29PM -0700, Matthew Jacob wrote:
> 
> Has anyone seen this scenario before? I am seeing it in RELENG_7, but 
> the code in question exists through to head.
> 
> Thread 1:
> 
> (kgdb) where
> #0  sched_switch (td=0xffffff003a04ea80, newtd=0xffffff00210b4000, 
> flags=Variable "flags" is not available.
> ) at ../../../kern/sched_ule.c:1944
> #1  0xffffffff803b6091 in mi_switch (flags=1, newtd=0x0) at 
> ../../../kern/kern_synch.c:450
> #2  0xffffffff80402399 in sleepq_switch (wchan=0xffffff8413b50b60) at 
> ../../../kern/subr_sleepqueue.c:497
> #3  0xffffffff80402e8c in sleepq_timedwait (wchan=0xffffff8413b50b60) at 
> ../../../kern/subr_sleepqueue.c:615
> #4  0xffffffff803b682d in _sleep (ident=0xffffff8413b50b60, 
> lock=0xffffffff80b0ee00, priority=76, wmesg=0xffffffff806583bb "devdrn", 
> timo=100) at ../../../kern/kern_synch.c:228
> #5  0xffffffff8037640c in destroy_devl (dev=0xffffff003aaf0000) at 
> ../../../kern/kern_conf.c:874
> #6  0xffffffff80376759 in destroy_dev (dev=0xffffff003aaf0000) at 
> ../../../kern/kern_conf.c:916
> #7  0xffffffff8034c939 in g_dev_orphan (cp=0xffffff003a544800) at 
> ../../../geom/geom_dev.c:438
> #8  0xffffffff803506a0 in g_run_events () at ../../../geom/geom_event.c:164
> #9  0xffffffff80351f1c in g_event_procbody () at 
> ../../../geom/geom_kern.c:141
> #10 0xffffffff8038a73a in fork_exit (callout=0xffffffff80351eb0 
> <g_event_procbody at ../../../geom/geom_kern.c:132>, arg=0x0, 
> frame=0xffffff8413b50c80) at ../../../kern/kern_fork.c:829
> #11 0xffffffff805a747e in fork_trampoline () at 
> ../../../amd64/amd64/exception.S:564
> #12 0x0000000000000000 in ?? ()
> 
> This thread is waiting on the threadcount to go away- i.e., the last 
> close of the device to occur ("da16" in this case).
> 
> Thread 2:
> 
> (kgdb) where
> #0  sched_switch (td=0xffffff009bb4ca80, newtd=0xffffff003af43380, 
> flags=Variable "flags" is not available.
> ) at ../../../kern/sched_ule.c:1944
> #1  0xffffffff803b6091 in mi_switch (flags=1, newtd=0x0) at 
> ../../../kern/kern_synch.c:450
> #2  0xffffffff80402399 in sleepq_switch (wchan=0xffffffff80b0e040) at 
> ../../../kern/subr_sleepqueue.c:497
> #3  0xffffffff80402f84 in sleepq_wait (wchan=0xffffffff80b0e040) at 
> ../../../kern/subr_sleepqueue.c:580
> #4  0xffffffff803b5385 in _sx_xlock_hard (sx=0xffffffff80b0e040, 
> tid=18446742976810240640, opts=Variable "opts" is not available.
> ) at ../../../kern/kern_sx.c:562
> #5  0xffffffff803b5731 in _sx_xlock (sx=0xffffffff80b0e040, opts=0, 
> file=0xffffffff80652d27 "../../../geom/geom_dev.c", line=196) at sx.h:154
> #6  0xffffffff8034d1bc in g_dev_open (dev=0xffffff003aaf0000, flags=1, 
> fmt=Variable "fmt" is not available.
> ) at ../../../geom/geom_dev.c:196
> #7  0xffffffff80333741 in devfs_open (ap=0xffffff841dea88b0) at 
> ../../../fs/devfs/devfs_vnops.c:902
> #8  0xffffffff80601daf in VOP_OPEN_APV (vop=0xffffffff8089fb80, 
> a=0xffffff841dea88b0) at vnode_if.c:371
> #9  0xffffffff80467246 in vn_open_cred (ndp=0xffffff841dea8a00, 
> flagp=0xffffff841dea894c, cmode=Variable "cmode" is not available.
> ) at vnode_if.h:199
> #10 0xffffffff80463770 in kern_open (td=0xffffff009bb4ca80, 
> path=0x5114a0 <Address 0x5114a0 out of bounds>, pathseg=Variable 
> "pathseg" is not available.
> ) at ../../../kern/vfs_syscalls.c:1054
> #11 0xffffffff805c599e in syscall (frame=0xffffff841dea8c80) at 
> ../../../amd64/amd64/trap.c:911
> #12 0xffffffff805a723b in Xfast_syscall () at 
> ../../../amd64/amd64/exception.S:349
> #13 0x00000008009a219c in ?? ()
> 
> This thread was opening the device, bumped the refcount, but then wedged 
> on the geom topology lock .....
> 
> the refcount field is protected under devmtx....
> 
> Anyone seen this?
> 
> I'm half inclined to either add in CDP_SCHED_DTR when one calls 
> destroy_dev, or make dev_refthread look at CDP_ACTIVE, leaning more 
> toward the latter.
> 
> Any thoughts on this?

And who owns the topology lock ? Is it thread 1 ?

Destroy_devl() clears si_devsw for departing cdev, and *refthread()
checks si_devsw against NULL as an indicator of device destruction in
progress.

I think that this situation is what destroy_dev_sched(9) was created
for.
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