ring-buffer: Bring back context level recursive checks
Commit1a149d7d3f
("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:1a149d7d3f
("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>
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@ -2534,29 +2534,59 @@ rb_wakeups(struct ring_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
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* The lock and unlock are done within a preempt disable section.
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* The current_context per_cpu variable can only be modified
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* by the current task between lock and unlock. But it can
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* be modified more than once via an interrupt. There are four
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* different contexts that we need to consider.
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* be modified more than once via an interrupt. To pass this
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* information from the lock to the unlock without having to
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* access the 'in_interrupt()' functions again (which do show
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* a bit of overhead in something as critical as function tracing,
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* we use a bitmask trick.
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*
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* Normal context.
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* SoftIRQ context
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* IRQ context
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* NMI context
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* bit 0 = NMI context
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* bit 1 = IRQ context
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* bit 2 = SoftIRQ context
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* bit 3 = normal context.
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*
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* If for some reason the ring buffer starts to recurse, we
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* only allow that to happen at most 4 times (one for each
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* context). If it happens 5 times, then we consider this a
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* recusive loop and do not let it go further.
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* This works because this is the order of contexts that can
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* preempt other contexts. A SoftIRQ never preempts an IRQ
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* context.
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*
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* When the context is determined, the corresponding bit is
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* checked and set (if it was set, then a recursion of that context
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* happened).
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*
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* On unlock, we need to clear this bit. To do so, just subtract
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* 1 from the current_context and AND it to itself.
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*
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* (binary)
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* 101 - 1 = 100
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* 101 & 100 = 100 (clearing bit zero)
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*
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* 1010 - 1 = 1001
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* 1010 & 1001 = 1000 (clearing bit 1)
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*
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* The least significant bit can be cleared this way, and it
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* just so happens that it is the same bit corresponding to
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* the current context.
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*/
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static __always_inline int
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trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
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{
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if (cpu_buffer->current_context >= 4)
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unsigned int val = cpu_buffer->current_context;
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unsigned long pc = preempt_count();
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int bit;
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if (!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
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bit = RB_CTX_NORMAL;
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else
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bit = pc & NMI_MASK ? RB_CTX_NMI :
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pc & HARDIRQ_MASK ? RB_CTX_IRQ :
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pc & SOFTIRQ_OFFSET ? 2 : RB_CTX_SOFTIRQ;
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if (unlikely(val & (1 << bit)))
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return 1;
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cpu_buffer->current_context++;
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/* Interrupts must see this update */
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barrier();
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val |= (1 << bit);
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cpu_buffer->current_context = val;
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return 0;
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}
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@ -2564,9 +2594,7 @@ trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
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static __always_inline void
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trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)
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{
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/* Don't let the dec leak out */
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barrier();
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cpu_buffer->current_context--;
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cpu_buffer->current_context &= cpu_buffer->current_context - 1;
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}
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/**
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