[PATCH] ide task_map_rq() preempt count imbalance

[Tejun Huh]

 Hello,

 2.5 kernel continued to fail on my test machine with a lot of
scheduling while atomic messages.  The offending function was
include/linux/ide.h:task_sectors() which calls task_map_rq() followed
by process_that_request_first(), taskfile_{input|output}_data() and
task_unmap_rq().

static inline void *task_map_rq(struct request *rq, unsigned long *flags)
{
	/*
	 * fs request
	 */
	if (rq->cbio)
		return rq_map_buffer(rq, flags);

	/*
	 * task request
	 */
	return rq->buffer + blk_rq_offset(rq);
}

static inline void task_unmap_rq(struct request *rq, char *buffer, unsigned long *flags)
{
	if (rq->cbio)
		rq_unmap_buffer(buffer, flags);
}

 rq_[un]map_buffer() eventually call into k[un]map_atomic() which
adjust preempt_count().  The problem is that rq->cbio is cleared by
process_that_request_first() before calling task_unmap_rq(), so the
preempt_count() isn't decremented properly.

 As it seems that rq->cbio test is to avoid the cost of calling into
k[un]map_atomic, I just removed the optimization and it worked for me.

 I'm not familiar with ide and block layer.  If I got something wrong,
please point out.

-- 
tejun

# This is a BitKeeper generated patch for the following project:
# Project Name: Linux kernel tree
# This patch format is intended for GNU patch command version 2.5 or higher.
# This patch includes the following deltas:
#	           ChangeSet	1.1412  -> 1.1413 
#	 include/linux/ide.h	1.72    -> 1.73   
#
# The following is the BitKeeper ChangeSet Log
# --------------------------------------------
# 03/09/03	tj@xxxxxxxxxxxxxx	1.1413
# - task_[un]map_rq() preempt count imbalance fixed.
# --------------------------------------------
#
diff -Nru a/include/linux/ide.h b/include/linux/ide.h
--- a/include/linux/ide.h	Wed Sep  3 14:50:20 2003
+++ b/include/linux/ide.h	Wed Sep  3 14:50:20 2003
@@ -853,22 +853,12 @@
 
 static inline void *task_map_rq(struct request *rq, unsigned long *flags)
 {
-	/*
-	 * fs request
-	 */
-	if (rq->cbio)
-		return rq_map_buffer(rq, flags);
-
-	/*
-	 * task request
-	 */
-	return rq->buffer + blk_rq_offset(rq);
+	return rq_map_buffer(rq, flags);
 }
 
 static inline void task_unmap_rq(struct request *rq, char *buffer, unsigned long *flags)
 {
-	if (rq->cbio)
-		rq_unmap_buffer(buffer, flags);
+	rq_unmap_buffer(buffer, flags);
 }
 
 #endif /* !CONFIG_IDE_TASKFILE_IO */
-
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