Re: [PATCH 001 of 6] md: Fix an occasional deadlock in raid5

[Andrew Morton]

On Tue, 15 Jan 2008 21:01:17 -0800 (PST) dean gaudet <dean@xxxxxxxxxx> wrote:

> On Mon, 14 Jan 2008, NeilBrown wrote:
> 
> > 
> > raid5's 'make_request' function calls generic_make_request on
> > underlying devices and if we run out of stripe heads, it could end up
> > waiting for one of those requests to complete.
> > This is bad as recursive calls to generic_make_request go on a queue
> > and are not even attempted until make_request completes.
> > 
> > So: don't make any generic_make_request calls in raid5 make_request
> > until all waiting has been done.  We do this by simply setting
> > STRIPE_HANDLE instead of calling handle_stripe().
> > 
> > If we need more stripe_heads, raid5d will get called to process the
> > pending stripe_heads which will call generic_make_request from a
> > different thread where no deadlock will happen.
> > 
> > 
> > This change by itself causes a performance hit.  So add a change so
> > that raid5_activate_delayed is only called at unplug time, never in
> > raid5.  This seems to bring back the performance numbers.  Calling it
> > in raid5d was sometimes too soon...
> > 
> > Cc: "Dan Williams" <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: Neil Brown <neilb@xxxxxxx>
> 
> probably doesn't matter, but for the record:
> 
> Tested-by: dean gaudet <dean@xxxxxxxxxx>
> 
> this time i tested with internal and external bitmaps and it survived 8h 
> and 14h resp. under the parallel tar workload i used to reproduce the 
> hang.
> 
> btw this should probably be a candidate for 2.6.22 and .23 stable.
> 

hm, Neil said

  The first fixes a bug which could make it a candidate for 24-final. 
  However it is a deadlock that seems to occur very rarely, and has been in
  mainline since 2.6.22.  So letting it into one more release shouldn't be
  a big problem.  While the fix is fairly simple, it could have some
  unexpected consequences, so I'd rather go for the next cycle.

food fight!
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