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ring-buffer: Fix recursion protection transitions between interrupt context

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The recursion protection of the ring buffer depends on preempt_count() to be
correct. But it is possible that the ring buffer gets called after an
interrupt comes in but before it updates the preempt_count(). This will
trigger a false positive in the recursion code.

Use the same trick from the ftrace function callback recursion code which
uses a "transition" bit that gets set, to allow for a single recursion for
to handle transitions between contexts.

Cc: stable@vger.kernel.org
Fixes: 567cd4da54 ("ring-buffer: User context bit recursion checking")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware) 2020-11-02 15:31:27 -05:00
parent 906695e593
commit b02414c8f0
1 changed files with 46 additions and 12 deletions
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58
kernel/trace/ring_buffer.c
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@ -438,14 +438,16 @@ enum {
}; };
/* /*
* Used for which event context the event is in. * Used for which event context the event is in.
* NMI = 0 * TRANSITION = 0
* IRQ = 1 * NMI = 1
* SOFTIRQ = 2 * IRQ = 2
* NORMAL = 3 * SOFTIRQ = 3
* NORMAL = 4
* *
* See trace_recursive_lock() comment below for more details. * See trace_recursive_lock() comment below for more details.
*/ */
enum { enum {
RB_CTX_TRANSITION,
RB_CTX_NMI, RB_CTX_NMI,
RB_CTX_IRQ, RB_CTX_IRQ,
RB_CTX_SOFTIRQ, RB_CTX_SOFTIRQ,
@ -3014,10 +3016,10 @@ rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
* a bit of overhead in something as critical as function tracing, * a bit of overhead in something as critical as function tracing,
* we use a bitmask trick. * we use a bitmask trick.
* *
* bit 0 = NMI context * bit 1 = NMI context
* bit 1 = IRQ context * bit 2 = IRQ context
* bit 2 = SoftIRQ context * bit 3 = SoftIRQ context
* bit 3 = normal context. * bit 4 = normal context.
* *
* This works because this is the order of contexts that can * This works because this is the order of contexts that can
* preempt other contexts. A SoftIRQ never preempts an IRQ * preempt other contexts. A SoftIRQ never preempts an IRQ
@ -3040,6 +3042,30 @@ rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
* The least significant bit can be cleared this way, and it * The least significant bit can be cleared this way, and it
* just so happens that it is the same bit corresponding to * just so happens that it is the same bit corresponding to
* the current context. * the current context.
*
* Now the TRANSITION bit breaks the above slightly. The TRANSITION bit
* is set when a recursion is detected at the current context, and if
* the TRANSITION bit is already set, it will fail the recursion.
* This is needed because there's a lag between the changing of
* interrupt context and updating the preempt count. In this case,
* a false positive will be found. To handle this, one extra recursion
* is allowed, and this is done by the TRANSITION bit. If the TRANSITION
* bit is already set, then it is considered a recursion and the function
* ends. Otherwise, the TRANSITION bit is set, and that bit is returned.
*
* On the trace_recursive_unlock(), the TRANSITION bit will be the first
* to be cleared. Even if it wasn't the context that set it. That is,
* if an interrupt comes in while NORMAL bit is set and the ring buffer
* is called before preempt_count() is updated, since the check will
* be on the NORMAL bit, the TRANSITION bit will then be set. If an
* NMI then comes in, it will set the NMI bit, but when the NMI code
* does the trace_recursive_unlock() it will clear the TRANSTION bit
* and leave the NMI bit set. But this is fine, because the interrupt
* code that set the TRANSITION bit will then clear the NMI bit when it
* calls trace_recursive_unlock(). If another NMI comes in, it will
* set the TRANSITION bit and continue.
*
* Note: The TRANSITION bit only handles a single transition between context.
*/ */
static __always_inline int static __always_inline int
@ -3055,8 +3081,16 @@ trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
bit = pc & NMI_MASK ? RB_CTX_NMI : bit = pc & NMI_MASK ? RB_CTX_NMI :
pc & HARDIRQ_MASK ? RB_CTX_IRQ : RB_CTX_SOFTIRQ; pc & HARDIRQ_MASK ? RB_CTX_IRQ : RB_CTX_SOFTIRQ;
if (unlikely(val & (1 << (bit + cpu_buffer->nest)))) if (unlikely(val & (1 << (bit + cpu_buffer->nest)))) {
return 1; /*
* It is possible that this was called by transitioning
* between interrupt context, and preempt_count() has not
* been updated yet. In this case, use the TRANSITION bit.
*/
bit = RB_CTX_TRANSITION;
if (val & (1 << (bit + cpu_buffer->nest)))
return 1;
}
val |= (1 << (bit + cpu_buffer->nest)); val |= (1 << (bit + cpu_buffer->nest));
cpu_buffer->current_context = val; cpu_buffer->current_context = val;
@ -3071,8 +3105,8 @@ trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->current_context - (1 << cpu_buffer->nest); cpu_buffer->current_context - (1 << cpu_buffer->nest);
} }
/* The recursive locking above uses 4 bits */ /* The recursive locking above uses 5 bits */
#define NESTED_BITS 4 #define NESTED_BITS 5
/** /**
* ring_buffer_nest_start - Allow to trace while nested * ring_buffer_nest_start - Allow to trace while nested