On Fri, 2011-02-18 at 16:02 -0800, James Bottomley wrote:
> On Wed, 2011-02-16 at 19:06 -0800, Dan Williams wrote:
> > During initial scan libsas drivers start their phys and notify libsas
> > with PORTE_BYTES_DMAED events as port links are established. This
> > notification in turn causes libsas to post DISCE_DISCOVER_DOMAIN events
> > to the queue. Calling scsi_flush_work() at the end of scan_finished
> > guarantees that all preceding PORTE_BYTES_DMAED events have been
> > registered in the queue, but it does not guarantee that the resulting
> > DISCE_DISCOVER_DOMAIN events have been processed because
> > flush_workqueue() explicitly avoids live-locking with incoming work.
> >
> > Introduce sas_flush_discovery() to guarantee that all initial discovery
> > events have completed. It is called after the driver determines all
> > initial PORTE_BYTES_DMAED events have had a chance to enter the queue.
> > This does not cover BCNs that are generated during expander bring up,
> > only the initial sas_discover_domain() event.
>
> I think this is a workaround for an old bug in workqueue flushing (the
> flush doesn't clean work it causes) ... I thought that's been fixed for
> ages (well, months at least) ... have you verified that this is still a
> problem?
>
Hmm... I saw this initially on 2.6.36.
Latest git still has the "livelock" comment [1], and I was the able to
capture the following trace with two disks connected on a 2.6.38-rc5
build. The second "sas_discover_domain" completion occurs after the
"first flush done".
# tracer: nop
#
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
<...>-5 [007] 93.849947: sas_porte_bytes_dmaed: sas_porte_bytes_dmaed: done
<...>-5 [007] 94.444643: sas_discover_domain: sas_discover_domain: complete
<...>-5 [007] 94.451993: sas_porte_bytes_dmaed: sas_porte_bytes_dmaed: done
<...>-1792 [006] 94.452011: isci_host_scan_finished: isci_host_scan_finished: first flush done
<...>-5 [007] 94.773256: sas_discover_domain: sas_discover_domain: complete
<...>-1792 [006] 94.773270: isci_host_scan_finished: isci_host_scan_finished: second flush done
--
Dan
[1]: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=kernel/workqueue.c;h=11869faa;hb=HEAD#l2201
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