From 289a5a25c5d0949e4d9b9ca4f334b6af5a83e090 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (Red Hat)" Date: Thu, 28 May 2015 13:14:51 -0400 Subject: [PATCH] ring-buffer: Give NMIs a chance to lock the reader_lock Currently, if an NMI does a dump of a ring buffer, it disables all ring buffers from ever doing any writes again. This is because it wont take the locks for the cpu_buffer and this can cause corruption if it preempted a read, or a read happens on another CPU for the current cpu buffer. This is a bit overkill. First, it should at least try to take the lock, and if it fails then disable it. Also, there's no need to disable all ring buffers, even those that are unrelated to what is being read. Only disable the per cpu ring buffer that is being read if it can not get the lock for it. Signed-off-by: Steven Rostedt --- kernel/trace/ring_buffer.c | 72 ++++++++++++++++++++------------------ 1 file changed, 37 insertions(+), 35 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 6d6ebcea3463..e9420fdc7409 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3859,19 +3859,36 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts) } EXPORT_SYMBOL_GPL(ring_buffer_iter_peek); -static inline int rb_ok_to_lock(void) +static inline bool rb_reader_lock(struct ring_buffer_per_cpu *cpu_buffer) { + if (likely(!in_nmi())) { + raw_spin_lock(&cpu_buffer->reader_lock); + return true; + } + /* * If an NMI die dumps out the content of the ring buffer - * do not grab locks. We also permanently disable the ring - * buffer too. A one time deal is all you get from reading - * the ring buffer from an NMI. + * trylock must be used to prevent a deadlock if the NMI + * preempted a task that holds the ring buffer locks. If + * we get the lock then all is fine, if not, then continue + * to do the read, but this can corrupt the ring buffer, + * so it must be permanently disabled from future writes. + * Reading from NMI is a oneshot deal. */ - if (likely(!in_nmi())) - return 1; + if (raw_spin_trylock(&cpu_buffer->reader_lock)) + return true; - tracing_off_permanent(); - return 0; + /* Continue without locking, but disable the ring buffer */ + atomic_inc(&cpu_buffer->record_disabled); + return false; +} + +static inline void +rb_reader_unlock(struct ring_buffer_per_cpu *cpu_buffer, bool locked) +{ + if (likely(locked)) + raw_spin_unlock(&cpu_buffer->reader_lock); + return; } /** @@ -3891,21 +3908,18 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts, struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu]; struct ring_buffer_event *event; unsigned long flags; - int dolock; + bool dolock; if (!cpumask_test_cpu(cpu, buffer->cpumask)) return NULL; - dolock = rb_ok_to_lock(); again: local_irq_save(flags); - if (dolock) - raw_spin_lock(&cpu_buffer->reader_lock); + dolock = rb_reader_lock(cpu_buffer); event = rb_buffer_peek(cpu_buffer, ts, lost_events); if (event && event->type_len == RINGBUF_TYPE_PADDING) rb_advance_reader(cpu_buffer); - if (dolock) - raw_spin_unlock(&cpu_buffer->reader_lock); + rb_reader_unlock(cpu_buffer, dolock); local_irq_restore(flags); if (event && event->type_len == RINGBUF_TYPE_PADDING) @@ -3958,9 +3972,7 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts, struct ring_buffer_per_cpu *cpu_buffer; struct ring_buffer_event *event = NULL; unsigned long flags; - int dolock; - - dolock = rb_ok_to_lock(); + bool dolock; again: /* might be called in atomic */ @@ -3971,8 +3983,7 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts, cpu_buffer = buffer->buffers[cpu]; local_irq_save(flags); - if (dolock) - raw_spin_lock(&cpu_buffer->reader_lock); + dolock = rb_reader_lock(cpu_buffer); event = rb_buffer_peek(cpu_buffer, ts, lost_events); if (event) { @@ -3980,8 +3991,7 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts, rb_advance_reader(cpu_buffer); } - if (dolock) - raw_spin_unlock(&cpu_buffer->reader_lock); + rb_reader_unlock(cpu_buffer, dolock); local_irq_restore(flags); out: @@ -4262,21 +4272,17 @@ int ring_buffer_empty(struct ring_buffer *buffer) { struct ring_buffer_per_cpu *cpu_buffer; unsigned long flags; - int dolock; + bool dolock; int cpu; int ret; - dolock = rb_ok_to_lock(); - /* yes this is racy, but if you don't like the race, lock the buffer */ for_each_buffer_cpu(buffer, cpu) { cpu_buffer = buffer->buffers[cpu]; local_irq_save(flags); - if (dolock) - raw_spin_lock(&cpu_buffer->reader_lock); + dolock = rb_reader_lock(cpu_buffer); ret = rb_per_cpu_empty(cpu_buffer); - if (dolock) - raw_spin_unlock(&cpu_buffer->reader_lock); + rb_reader_unlock(cpu_buffer, dolock); local_irq_restore(flags); if (!ret) @@ -4296,21 +4302,17 @@ int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu) { struct ring_buffer_per_cpu *cpu_buffer; unsigned long flags; - int dolock; + bool dolock; int ret; if (!cpumask_test_cpu(cpu, buffer->cpumask)) return 1; - dolock = rb_ok_to_lock(); - cpu_buffer = buffer->buffers[cpu]; local_irq_save(flags); - if (dolock) - raw_spin_lock(&cpu_buffer->reader_lock); + dolock = rb_reader_lock(cpu_buffer); ret = rb_per_cpu_empty(cpu_buffer); - if (dolock) - raw_spin_unlock(&cpu_buffer->reader_lock); + rb_reader_unlock(cpu_buffer, dolock); local_irq_restore(flags); return ret; -- 2.34.1