From mboxrd@z Thu Jan  1 00:00:00 1970
From: ecashin@coraid.com
Subject: "unexpected unlock" when unlocking, conditional, lock in loop
Date: Sat, 6 Oct 2012 12:47:56 -0700
Message-ID: <1349552876.20963@cat.he.net>
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Cc: ecashin@coraid.com

Hi.  I have a function that enters with a lock held and does
an unlock inside a loop.

Sparse 0.4.4 is fine with this function until I introduce a conditional
between the unlock and the next lock.  In the minimal example below,
changing the "#if 1" to "#if 0" makes sparse generate the warning
below:

cd ~/git/linux && PATH=/opt/bin:$PATH make drivers/block/aoe/aoe.ko C=1
make[1]: Nothing to be done for `all'.
make[1]: Nothing to be done for `relocs'.
  CHK     include/linux/version.h
  CHK     include/generated/utsrelease.h
  CALL    scripts/checksyscalls.sh
  CHECK   drivers/block/aoe/demo.c
/build/ecashin/git/linux/arch/x86/include/asm/spinlock.h:81:9: warning: context imbalance in 'demofn' - unexpected unlock
  CC [M]  drivers/block/aoe/demo.o
  LD [M]  drivers/block/aoe/aoe.o
  MODPOST 1 modules
  CC      drivers/block/aoe/aoe.mod.o
  LD [M]  drivers/block/aoe/aoe.ko

I'm using 3.6.0-rc7 kernel sources.

Granted, I'm unusually tired today, but I can't think of a way that
conditionally printing a warning has changed the locking, so I
could use some help in determining whether this is a sparse bug
that might be fixed, one that I have to work around, or some
confusion of mine.

/* demo.c */
#include <linux/netdevice.h>

static spinlock_t lk;
static struct sk_buff_head q;
int demofn(void);

/* enters and returns with lk held */
int demofn(void)
{
	struct sk_buff *skb;

	while ((skb = skb_dequeue(&q))) {
		spin_unlock_irq(&lk);
#if 1
		dev_queue_xmit(skb);
#else
		if (dev_queue_xmit(skb) == NET_XMIT_DROP && net_ratelimit())
			pr_warn("informative warning\n");
#endif
		spin_lock_irq(&lk);
	}
	return 0;
}

-- 
  Ed Cashin
  ecashin@coraid.com	http://www.coraid.com/
  ecashin@noserose.net	http://noserose.net/e/