analysis of snapshot-related system deadlock

[Don Lewis]

On 27 Sep, Don Lewis wrote:

> Probably the easiest fix would be to call runningbufwakeup() from
> ffs_copyonwrite() before grabbing "snaplk", and increase runningbufspace
> again before returning from ffs_copyonwrite().  The bufs waiting for
> "snaplk" aren't yet async writes currently running, to borrow from the
> comment on waitrunningbufspace().

Sleeping on "wdrain" is probably also undesirable if a thread holds
"snaplk" because of the potential impact on the activity of other
threads.  A possible fix would be for bufwrite() to skip the call to
waitrunningbufspace() if the thread holds "snaplk".  This might cause
problems with queuing an excessive amount of I/O, but I don't think so
because the outer bufwrite() will still call waitrunningbufspace().