linux-kernel - Re: [PATCH] raid5: panic() on dma_wait_for_async

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Date:	Tue, 20 Nov 2012 14:13:43 +1100
From:	NeilBrown <neilb@...e.de>
To:	Dan Williams <djbw@...com>
Cc:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	Vinod Koul <vinod.koul@...el.com>,
	"Tomasz Figa" <t.figa@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>
Subject: Re: [PATCH] raid5: panic() on dma_wait_for_async_tx() error

On Mon, 19 Nov 2012 18:23:57 -0800 Dan Williams <djbw@...com> wrote:

> On Tue, 2012-11-20 at 09:18 +1100, NeilBrown wrote:
> > On Mon, 19 Nov 2012 05:22:25 +0000 Dan Williams <djbw@...com> wrote:
> > 
> > > 
> > > 
> > > On 11/18/12 5:06 PM, "NeilBrown" <neilb@...e.de> wrote:
> > > 
> > > >
> > > >Hi Dan,
> > > > could you comment on this please?  Would it make sense to arrange for
> > > >errors
> > > > to propagate up?  Or should we arrange to do a software-fallback in the
> > > >dma
> > > > engine is a problem?  What sort of things can cause error here anyway?
> > > 
> > > Propagating up is missing reliable "dma abort" operation.
> > > 
> > > In these cases the engine failed to complete due to hardware hang / driver
> > > bug, or has hit a memory error (uncorrectable even with software
> > > fallback).  This originally should have been using async_tx_quiesce()
> > > which also does the panic.
> > > 
> > > The engines that I have worked with have either lacked support for
> > > aborting, or were otherwise unable to recover from a hardware hang.
> > > However, for engines that do support error recovery they should be able to
> > > hide the failure from the upper layers.
> > >
> > 
> > So maybe I could:
> > 
> > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> > index ac09fa4..ffbf0ca 100644
> > --- a/drivers/md/raid5.c
> > +++ b/drivers/md/raid5.c
> > @@ -3268,7 +3268,7 @@ static void handle_stripe_expansion(struct r5conf *conf, struct stripe_head *sh)
> >  	/* done submitting copies, wait for them to complete */
> >  	if (tx) {
> >  		async_tx_ack(tx);
> > -		dma_wait_for_async_tx(tx);
> > +		async_tx_quiesce(&tx);
> >  	}
> >  }
> >  
> > 
> > 
> > and then the panic would be somebody else's problem?
> > 
> > I note that handle_stripe_expansion has:
> > 
> >  		async_tx_ack(tx);
> > 		dma_wait_for_async_tx(tx);
> > 
> > while async_tx_quiesce() has:
> > 
> > 		if (dma_wait_for_async_tx(*tx) == DMA_ERROR)
> > 			panic("DMA_ERROR waiting for transaction\n");
> > 		async_tx_ack(*tx);
> > 
> > 
> > i.e. the same two functions called in the reverse order.  Is the order
> > important?  Is handle_stripe_expansion wrong?   Should the patch I apply
> > actually be:
> > 
> > 
> > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> > index ac09fa4..e51d903 100644
> > --- a/drivers/md/raid5.c
> > +++ b/drivers/md/raid5.c
> > @@ -3266,10 +3266,7 @@ static void handle_stripe_expansion(struct r5conf *conf, struct stripe_head *sh)
> >  
> >  		}
> >  	/* done submitting copies, wait for them to complete */
> > -	if (tx) {
> > -		async_tx_ack(tx);
> > -		dma_wait_for_async_tx(tx);
> > -	}
> > +	async_tx_quiesce(&tx);
> >  }
> >  
> 
> Yes, this one, handles it like the other cases of needing to do a
> synchronous wait and does not care if tx is NULL.

Thanks.  Following is now in my for-next branch.

NeilBrown

From e25a8de38d6584ffd042dbef3a5a8eb518b8813b Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb@...e.de>
Date: Tue, 20 Nov 2012 14:11:15 +1100
Subject: [PATCH] md/raid5: use async_tx_quiesce() instead of open-coding it.

handle_stripe_expansion contains:

        if (tx) {
                async_tx_ack(tx);
                dma_wait_for_async_tx(tx);
        }

which is very similar to the body of async_tx_quiesce(),
except that the later handles an error from dma_wait_for_async_tx()
(admittedly by panicing, but that decision belongs in the dma
code, not the md code).

So just us async_tx_quiesce().

Acked-by: Dan Williams <djbw@...com>
Reported-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Signed-off-by: NeilBrown <neilb@...e.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index ac09fa4..e51d903 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3266,10 +3266,7 @@ static void handle_stripe_expansion(struct r5conf *conf, struct stripe_head *sh)
 
 		}
 	/* done submitting copies, wait for them to complete */
-	if (tx) {
-		async_tx_ack(tx);
-		dma_wait_for_async_tx(tx);
-	}
+	async_tx_quiesce(&tx);
 }
 
 /*

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