[PATCH RT] preempt-rt: implement rt_downgrade_write

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>
Subject: preempt-rt: implement rt_downgrade_write

The current code of rt_downgrade_write simply does a BUG(). There
are places in the kernel that uses this code, and will crash a runnning
preempt-rt kernel.

The rt_downgrade_write converts a rwsem held for write into a rwsem
held for read without ever releasing the semaphore. In -rt, the rwsems
are simply a mutex. There is nothing different between a rwsem held
for write, and one held for read. The difference is that one held for
read can nest.

This patch changes the BUG_ON() to simply BUG if the caller is not
the owner of the semaphore.

This patch comes from my rt-git repo, and has been tested there.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/rt.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6-rt/kernel/rt.c
===================================================================
--- linux-2.6-rt.orig/kernel/rt.c	2009-04-15 10:02:37.000000000 -0400
+++ linux-2.6-rt/kernel/rt.c	2009-04-15 11:28:13.000000000 -0400
@@ -316,7 +316,7 @@ EXPORT_SYMBOL(rt_up_read);
  */
 void  rt_downgrade_write(struct rw_semaphore *rwsem)
 {
-	BUG();
+	BUG_ON(rt_mutex_real_owner(&rwsem->lock) != current);
 }
 EXPORT_SYMBOL(rt_downgrade_write);
 

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