netdev - Re: [PATCH RFC] spinlock: split out debugging check from spin_lock

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Message-ID: <20130411191409.GA9790@hmsreliant.think-freely.org>
Date:	Thu, 11 Apr 2013 15:14:09 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Bart Van Assche <bvanassche@....org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] spinlock: split out debugging check from
 spin_lock_mutex

On Thu, Apr 11, 2013 at 07:31:07PM +0200, Bart Van Assche wrote:
> On 04/11/13 17:18, Neil Horman wrote:
> >Bart, this patch should fix your problem.  Could you please test it and confirm?
> 
> The fails to build on my setup after having applied that patch:
> 
> kernel/mutex.c: In function ‘__mutex_trylock_slowpath’:
> kernel/mutex.c:446:2: error: implicit declaration of function
> ‘spin_unlock_mutex_raw’ [-Werror=implicit-function-declaration]
> 
> Bart.
> 


Sorry, turned out to be a stupid typo on my part.  This is a fixed version, I've
verified that it builds with and without CONFIG_DEBUG_MUTEXS enabled


commit 2f78b4659f119302849f2b753cc1fe5ee87125bc
Author: Neil Horman <nhorman@...driver.com>
Date:   Thu Apr 11 11:05:12 2013 -0400

    spinlock: split out debugging check from spin_lock_mutex
    
    Bart, this patch should fix your problem.  Could you please test it and confirm?
    
    Bart Van Assche recently reported a warning to me:
    
    <IRQ>  [<ffffffff8103d79f>] warn_slowpath_common+0x7f/0xc0
     [<ffffffff8103d7fa>] warn_slowpath_null+0x1a/0x20
     [<ffffffff814761dd>] mutex_trylock+0x16d/0x180
     [<ffffffff813968c9>] netpoll_poll_dev+0x49/0xc30
     [<ffffffff8136a2d2>] ? __alloc_skb+0x82/0x2a0
     [<ffffffff81397715>] netpoll_send_skb_on_dev+0x265/0x410
     [<ffffffff81397c5a>] netpoll_send_udp+0x28a/0x3a0
     [<ffffffffa0541843>] ? write_msg+0x53/0x110 [netconsole]
     [<ffffffffa05418bf>] write_msg+0xcf/0x110 [netconsole]
     [<ffffffff8103eba1>] call_console_drivers.constprop.17+0xa1/0x1c0
     [<ffffffff8103fb76>] console_unlock+0x2d6/0x450
     [<ffffffff8104011e>] vprintk_emit+0x1ee/0x510
     [<ffffffff8146f9f6>] printk+0x4d/0x4f
     [<ffffffffa0004f1d>] scsi_print_command+0x7d/0xe0 [scsi_mod]
    
    This resulted from my commit ca99ca14c which introduced a mutex_trylock
    operation in a path that could execute in interrupt context.  When mutex
    debugging is enabled, the above warns the user when we are in fact exectuting in
    interrupt context.
    
    I think this is a false positive however.  The check is intended to catch users
    who might be issuing sleeping calls in irq context, but the use of mutex_trylock
    here is guaranteed not to sleep.
    
    We could fix this by replacing the DEBUG_LOCK_WARN_ON check in spin_lock_mutex
    with a __might_sleep call in the appropriate parent mutex operations, but for
    the sake of effiency (which It seems is why the check was put in the spin lock
    code only when debug is enabled), lets split the spin_lock_mutex call into two
    components, where the outer component does the debug checking.  Then
    mutex_trylock can just call the inner part as its callable from irq context
    safely.
    
    Signed-off-by: Neil Horman <nhorman@...driver.com>
    Reported-by: Bart Van Assche <bvanassche@....org>
    CC: Bart Van Assche <bvanassche@....org>
    CC: David Miller <davem@...emloft.net>
    CC: netdev@...r.kernel.org

diff --git a/kernel/mutex-debug.h b/kernel/mutex-debug.h
index 0799fd3..aac0858 100644
--- a/kernel/mutex-debug.h
+++ b/kernel/mutex-debug.h
@@ -37,13 +37,19 @@ static inline void mutex_clear_owner(struct mutex *lock)
 	lock->owner = NULL;
 }
 
+#define spin_lock_mutex_raw(lock, flags)		\
+	do {						\
+							\
+		local_irq_save(flags);			\
+		arch_spin_lock(&(lock)->rlock.raw_lock);\
+	} while (0)
+
 #define spin_lock_mutex(lock, flags)			\
 	do {						\
 		struct mutex *l = container_of(lock, struct mutex, wait_lock); \
 							\
 		DEBUG_LOCKS_WARN_ON(in_interrupt());	\
-		local_irq_save(flags);			\
-		arch_spin_lock(&(lock)->rlock.raw_lock);\
+		spin_lock_mutex_raw(lock, flags);	\
 		DEBUG_LOCKS_WARN_ON(l->magic != l);	\
 	} while (0)
 
diff --git a/kernel/mutex.c b/kernel/mutex.c
index 52f2301..a62a5f8 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -431,7 +431,7 @@ static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
 	unsigned long flags;
 	int prev;
 
-	spin_lock_mutex(&lock->wait_lock, flags);
+	spin_lock_mutex_raw(&lock->wait_lock, flags);
 
 	prev = atomic_xchg(&lock->count, -1);
 	if (likely(prev == 1)) {
diff --git a/kernel/mutex.h b/kernel/mutex.h
index 4115fbf..af6ffb4 100644
--- a/kernel/mutex.h
+++ b/kernel/mutex.h
@@ -11,6 +11,8 @@
 
 #define spin_lock_mutex(lock, flags) \
 		do { spin_lock(lock); (void)(flags); } while (0)
+#define spin_lock_mutex_raw(lock, flags) spin_lock_mutex((lock), (flags))
+
 #define spin_unlock_mutex(lock, flags) \
 		do { spin_unlock(lock); (void)(flags); } while (0)
 #define mutex_remove_waiter(lock, waiter, ti) \
--
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