lkml.org 
[lkml]   [2018]   [Jan]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH 1/2] ring-buffer: Bring back context level recursive checks
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Commit 1a149d7d3f45 ("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: 1a149d7d3f45 ("ring-buffer: Rewrite trace_recursive_(un)lock() to be simpler")
Reported-by: David Weinehall <david.weinehall@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 62 +++++++++++++++++++++++++++++++++-------------
1 file changed, 45 insertions(+), 17 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9ab18995ff1e..0cddf60186da 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -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 current_context per_cpu variable can only be modified
* by the current task between lock and unlock. But it can
- * be modified more than once via an interrupt. There are four
- * different contexts that we need to consider.
+ * be modified more than once via an interrupt. To pass this
+ * 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.
- * SoftIRQ context
- * IRQ context
- * NMI context
+ * bit 0 = NMI context
+ * bit 1 = IRQ context
+ * bit 2 = SoftIRQ context
+ * bit 3 = normal context.
*
- * If for some reason the ring buffer starts to recurse, we
- * only allow that to happen at most 4 times (one for each
- * context). If it happens 5 times, then we consider this a
- * recusive loop and do not let it go further.
+ * This works because this is the order of contexts that can
+ * preempt other contexts. A SoftIRQ never preempts an IRQ
+ * context.
+ *
+ * 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
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;

- cpu_buffer->current_context++;
- /* Interrupts must see this update */
- barrier();
+ val |= (1 << bit);
+ cpu_buffer->current_context = val;

return 0;
}
@@ -2564,9 +2594,7 @@ trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
static __always_inline void
trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)
{
- /* Don't let the dec leak out */
- barrier();
- cpu_buffer->current_context--;
+ cpu_buffer->current_context &= cpu_buffer->current_context - 1;
}

/**
--
2.13.2

\
 
 \ /
  Last update: 2018-01-16 03:01    [W:0.032 / U:5.840 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site