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Commit a0e3a18f authored by Steven Rostedt (VMware)'s avatar Steven Rostedt (VMware)
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ring-buffer: Bring back context level recursive checks 



Commit 1a149d7d ("ring-buffer: Rewrite trace_recursive_(un)lock() to be
simpler") replaced the context level recursion checks with a simple counter.
This would prevent the ring buffer code from recursively calling itself more
than the max number of contexts that exist (Normal, softirq, irq, nmi). But
this change caused a lockup in a specific case, which was during suspend and
resume using a global clock. Adding a stack dump to see where this occurred,
the issue was in the trace global clock itself:

  trace_buffer_lock_reserve+0x1c/0x50
  __trace_graph_entry+0x2d/0x90
  trace_graph_entry+0xe8/0x200
  prepare_ftrace_return+0x69/0xc0
  ftrace_graph_caller+0x78/0xa8
  queued_spin_lock_slowpath+0x5/0x1d0
  trace_clock_global+0xb0/0xc0
  ring_buffer_lock_reserve+0xf9/0x390

The function graph tracer traced queued_spin_lock_slowpath that was called
by trace_clock_global. This pointed out that the trace_clock_global() is not
reentrant, as it takes a spin lock. It depended on the ring buffer recursive
lock from letting that happen.

By removing the context detection and adding just a max number of allowable
recursions, it allowed the trace_clock_global() to be entered again and try
to retake the spinlock it already held, causing a deadlock.

Fixes: 1a149d7d ("ring-buffer: Rewrite trace_recursive_(un)lock() to be simpler")
Reported-by: default avatarDavid Weinehall <david.weinehall@gmail.com>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent 4397f045

kernel/trace/ring_buffer.c

+45 −17
Original line number Original line Diff line number Diff line
@@ -2534,29 +2534,59 @@ rb_wakeups(struct ring_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer) 
 * The lock and unlock are done within a preempt disable section.

 * The lock and unlock are done within a preempt disable section.

 * The current_context per_cpu variable can only be modified

 * The current_context per_cpu variable can only be modified

 * by the current task between lock and unlock. But it can

 * by the current task between lock and unlock. But it can

 * be modified more than once via an interrupt. There are four

 * be modified more than once via an interrupt. To pass this

 * different contexts that we need to consider.

 * information from the lock to the unlock without having to

 * access the 'in_interrupt()' functions again (which do show

 * a bit of overhead in something as critical as function tracing,

 * we use a bitmask trick.

 *

 *

 *  Normal context.

 *  bit 0 =  NMI context

 *  SoftIRQ context

 *  bit 1 =  IRQ context

 *  IRQ context

 *  bit 2 =  SoftIRQ context

 *  NMI context

 *  bit 3 =  normal context.

 *

 *

 * If for some reason the ring buffer starts to recurse, we

 * This works because this is the order of contexts that can

 * only allow that to happen at most 4 times (one for each

 * preempt other contexts. A SoftIRQ never preempts an IRQ

 * context). If it happens 5 times, then we consider this a

 * context.

 * recusive loop and do not let it go further.

 *

 * When the context is determined, the corresponding bit is

 * checked and set (if it was set, then a recursion of that context

 * happened).

 *

 * On unlock, we need to clear this bit. To do so, just subtract

 * 1 from the current_context and AND it to itself.

 *

 * (binary)

 *  101 - 1 = 100

 *  101 & 100 = 100 (clearing bit zero)

 *

 *  1010 - 1 = 1001

 *  1010 & 1001 = 1000 (clearing bit 1)

 *

 * The least significant bit can be cleared this way, and it

 * just so happens that it is the same bit corresponding to

 * the current context.

 */

 */





static __always_inline int

static __always_inline int

trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)

trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)

{

{

	if (cpu_buffer->current_context >= 4)

	unsigned int val = cpu_buffer->current_context;

	unsigned long pc = preempt_count();

	int bit;



	if (!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))

		bit = RB_CTX_NORMAL;

	else

		bit = pc & NMI_MASK ? RB_CTX_NMI :

			pc & HARDIRQ_MASK ? RB_CTX_IRQ :

			pc & SOFTIRQ_OFFSET ? 2 : RB_CTX_SOFTIRQ;



	if (unlikely(val & (1 << bit)))

		return 1;

		return 1;





	cpu_buffer->current_context++;

	val |= (1 << bit);

	/* Interrupts must see this update */

	cpu_buffer->current_context = val;

	barrier();





	return 0;

	return 0;

}

}
@@ -2564,9 +2594,7 @@ trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer) 
static __always_inline void

static __always_inline void

trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)

trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)

{

{

	/* Don't let the dec leak out */

	cpu_buffer->current_context &= cpu_buffer->current_context - 1;

	barrier();

	cpu_buffer->current_context--;

}

}





/**

/**