Re: vrele vs. syncer deadlock

[David Holland <dholland-tech%netbsd.org@localhost>]

On Thu, Dec 22, 2016 at 12:57:10PM +0100, J. Hannken-Illjes wrote:
 > > On 11 Dec 2016, at 21:01, David Holland <dholland-tech%netbsd.org@localhost> wrote:
 > > 
 > > On a low-memory machine Nick ran into the following deadlock:
 > > 
 > >  (a) rename -> vrele on child -> inactive -> truncate -> getblk ->
 > >      no memory in buffer pool -> wait for syncer
 > 
 > This prediction seems wrong.  It is not the syncers job to free
 > memory to buffer pool - it will just update and write buffers
 > ending up on one of the buffer queues.
 > 
 > To release memory to the buffer pool the pagedaemon calls buf_drain()
 > when it detects a low memory condition.

ok, but buf_drain doesn't write out buffers; only the syncer does
that. So if they're all dirty it won't do anything.

If anyone has a reliable way to reproduce this, adding a print in
buf_trim() when it can't get a buffer to drop might be useful.

(PR 51665 looks like another instance of this problem, but it might be
something different as it doesn't allow dropping into ddb.)

-- 
David A. Holland
dholland%netbsd.org@localhost