linux-kernel - Re: [PATCH/RFC 2.6.21 3/5] ehca: completion queue: remove use of do

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Message-Id: <200701121623.13687.hnguyen@linux.vnet.ibm.com>
Date:	Fri, 12 Jan 2007 16:23:13 +0100
From:	Hoang-Nam Nguyen <hnguyen@...ux.vnet.ibm.com>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Nathan Lynch <ntl@...ox.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	openfabrics-ewg@...nib.org, openib-general@...nib.org,
	raisch@...ibm.com
Subject: Re: [PATCH/RFC 2.6.21 3/5] ehca: completion queue: remove use of do_mmap()

Hi Roland!
>  > >  	spin_lock_irqsave(&ehca_cq_idr_lock, flags);
>  > >  	while (my_cq->nr_callbacks)
>  > >  		yield();
> 
>  > Isn't that code outright buggy?  Calling into the scheduler with a
>  > spinlock held and local interrupts disabled...
> 
> Yes, absolutely -- if nr_callbacks is ever nonzero then this will
> obviously crash instantly.
As Christoph R. mentioned in another thread I'm sending you a patch
to fix this bug. Thanks to all for this hint!
Purpose of the while loop is to wait until all completion entries
have been processed by a running completion handler. First then
the function continue with destroying completion queue. Thus, we do
unlock and lock around yield(), ie yield() is now called from a normal
process context without active lock. Hope that this pattern is ok.
In addition of yield issue this patch also fixes an unproper use of
spin_unlock() in ehca_irq.c.
Thanks
Nam


Signed-off-by Hoang-Nam Nguyen <hnguyen@...ibm.com>
---


 ehca_cq.c  |    5 ++++-
 ehca_irq.c |    4 +++-
 2 files changed, 7 insertions(+), 2 deletions(-)


diff -Nurp infiniband_orig/drivers/infiniband/hw/ehca/ehca_cq.c infiniband_work/drivers/infiniband/hw/ehca/ehca_cq.c
--- infiniband_orig/drivers/infiniband/hw/ehca/ehca_cq.c	2007-01-11 19:54:06.000000000 +0100
+++ infiniband_work/drivers/infiniband/hw/ehca/ehca_cq.c	2007-01-12 15:27:50.000000000 +0100
@@ -330,8 +330,11 @@ int ehca_destroy_cq(struct ib_cq *cq)
 	}
 
 	spin_lock_irqsave(&ehca_cq_idr_lock, flags);
-	while (my_cq->nr_callbacks)
+	while (my_cq->nr_callbacks) {
+		spin_unlock_irqrestore(&ehca_cq_idr_lock, flags);
 		yield();
+		spin_lock_irqsave(&ehca_cq_idr_lock, flags);
+	}
 
 	idr_remove(&ehca_cq_idr, my_cq->token);
 	spin_unlock_irqrestore(&ehca_cq_idr_lock, flags);
diff -Nurp infiniband_orig/drivers/infiniband/hw/ehca/ehca_irq.c infiniband_work/drivers/infiniband/hw/ehca/ehca_irq.c
--- infiniband_orig/drivers/infiniband/hw/ehca/ehca_irq.c	2007-01-11 19:53:33.000000000 +0100
+++ infiniband_work/drivers/infiniband/hw/ehca/ehca_irq.c	2007-01-12 15:27:50.000000000 +0100
@@ -440,7 +440,9 @@ void ehca_tasklet_eq(unsigned long data)
 					cq = idr_find(&ehca_cq_idr, token);
 
 					if (cq == NULL) {
-						spin_unlock(&ehca_cq_idr_lock);
+						spin_unlock_irqrestore(
+							&ehca_cq_idr_lock,
+							flags);
 						break;
 					}
 

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