Commit f1a83e65 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

lockdep: Correctly annotate hardirq context in irq_exit() 



There was a reported deadlock on -rt which lockdep didn't report.

It turns out that in irq_exit() we tell lockdep that the hardirq
context ends and then do all kinds of locking afterwards.

To fix it, move trace_hardirq_exit() to the very end of irq_exit(), this
ensures all locking in tick_irq_exit() and rcu_irq_exit() are properly
recorded as happening from hardirq context.

This however leads to the 'fun' little problem of running softirqs
while in hardirq context. To cure this make the softirq code a little
more complex (in the CONFIG_TRACE_IRQFLAGS case).

Due to stack swizzling arch dependent trickery we cannot pass an
argument to __do_softirq() to tell it if it was done from hardirq
context or not; so use a side-band argument.

When we do __do_softirq() from hardirq context, 'atomically' flip to
softirq context and back, so that no locking goes without being in
either hard- or soft-irq context.

I didn't find any new problems in mainline using this patch, but it
did show the -rt problem.

Reported-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-dgwc5cdksbn0jk09vbmcc9sa@git.kernel.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 9dd12201

kernel/softirq.c

+45 −9
Original line number Diff line number Diff line
@@ -213,14 +213,52 @@ EXPORT_SYMBOL(local_bh_enable_ip); 
#define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)

#define MAX_SOFTIRQ_RESTART 10



#ifdef CONFIG_TRACE_IRQFLAGS

/*

 * Convoluted means of passing __do_softirq() a message through the various

 * architecture execute_on_stack() bits.

 *

 * When we run softirqs from irq_exit() and thus on the hardirq stack we need

 * to keep the lockdep irq context tracking as tight as possible in order to

 * not miss-qualify lock contexts and miss possible deadlocks.

 */

static DEFINE_PER_CPU(int, softirq_from_hardirq);



static inline void lockdep_softirq_from_hardirq(void)

{

	this_cpu_write(softirq_from_hardirq, 1);

}



static inline void lockdep_softirq_start(void)

{

	if (this_cpu_read(softirq_from_hardirq))

		trace_hardirq_exit();

	lockdep_softirq_enter();

}



static inline void lockdep_softirq_end(void)

{

	lockdep_softirq_exit();

	if (this_cpu_read(softirq_from_hardirq)) {

		this_cpu_write(softirq_from_hardirq, 0);

		trace_hardirq_enter();

	}

}



#else

static inline void lockdep_softirq_from_hardirq(void) { }

static inline void lockdep_softirq_start(void) { }

static inline void lockdep_softirq_end(void) { }

#endif



asmlinkage void __do_softirq(void)

{

	struct softirq_action *h;

	__u32 pending;

	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;

	int cpu;

	unsigned long old_flags = current->flags;

	int max_restart = MAX_SOFTIRQ_RESTART;

	struct softirq_action *h;

	__u32 pending;

	int cpu;



	/*

	 * Mask out PF_MEMALLOC s current task context is borrowed for the
@@ -233,7 +271,7 @@ asmlinkage void __do_softirq(void) 
	account_irq_enter_time(current);



	__local_bh_disable(_RET_IP_, SOFTIRQ_OFFSET);

	lockdep_softirq_enter();

	lockdep_softirq_start();



	cpu = smp_processor_id();

restart:
@@ -280,16 +318,13 @@ restart: 
		wakeup_softirqd();

	}



	lockdep_softirq_exit();



	lockdep_softirq_end();

	account_irq_exit_time(current);

	__local_bh_enable(SOFTIRQ_OFFSET);

	WARN_ON_ONCE(in_interrupt());

	tsk_restore_flags(current, old_flags, PF_MEMALLOC);

}







asmlinkage void do_softirq(void)

{

	__u32 pending;
@@ -332,6 +367,7 @@ void irq_enter(void) 
static inline void invoke_softirq(void)

{

	if (!force_irqthreads) {

		lockdep_softirq_from_hardirq();

#ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK

		/*

		 * We can safely execute softirq on the current stack if
@@ -377,13 +413,13 @@ void irq_exit(void) 
#endif



	account_irq_exit_time(current);

	trace_hardirq_exit();

	preempt_count_sub(HARDIRQ_OFFSET);

	if (!in_interrupt() && local_softirq_pending())

		invoke_softirq();



	tick_irq_exit();

	rcu_irq_exit();

	trace_hardirq_exit(); /* must be last! */

}



/*

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