On Thu, 2 Apr 2020 14:14:40 -0400 Steven Rostedt wrote: > On Thu, 2 Apr 2020 16:19:20 +0900 > Masami Hiramatsu wrote: > > > On Wed, 1 Apr 2020 11:04:01 -0400 > > Steven Rostedt wrote: > > > > > On Wed, 1 Apr 2020 10:21:12 -0400 > > > Steven Rostedt wrote: > > > > > > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > > > > index 6519b7afc499..7f1466253ca8 100644 > > > > --- a/kernel/trace/trace.c > > > > +++ b/kernel/trace/trace.c > > > > @@ -3487,6 +3487,14 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter, > > > > */ > > > > if (iter->ent && iter->ent != iter->temp) { > > > > if (!iter->temp || iter->temp_size < iter->ent_size) { > > > > + /* > > > > + * This function is only used to add markers between > > > > + * events that are far apart (see trace_print_lat_context()), > > > > + * but if this is called in an atomic context (like NMIs) > > > > + * we can't call kmalloc(), thus just return NULL. > > > > + */ > > > > + if (in_atomic() || irqs_disabled()) > > > > + return NULL; > > > > kfree(iter->temp); > > > > iter->temp = kmalloc(iter->ent_size, GFP_KERNEL); > > > > if (!iter->temp) > > > > > > Peter informed me on IRC not to use in_atomic() as it doesn't catch > > > spin_locks when CONFIG_PREEMPT is not defined. > > > > > > As the issue is just with ftrace_dump(), I'll have it use a static buffer > > > instead of 128 bytes. Which should be big enough for most events, and if > > > not, then it will just miss the markers. > > > > > > That sounds good, but the below patch seems to do different thing. > > Does it just makes trace_find_next_entry() always fail if it is > > called from ftrace_dump()? > > Bah! I send my emails on a different machine than I create the patches on. > Below was my first iteration, then I decided to at least try to print some, > changed it, but never copied the new version over, and ended up sending the > last one. > Ah, got it :) > Here's the actual patch! > > -- Steve > > From: "Steven Rostedt (VMware)" > Subject: [PATCH] tracing: Do not allocate buffer in trace_find_next_entry() in > atomic > > When dumping out the trace data in latency format, a check is made to peek > at the next event to compare its timestamp to the current one, and if the > delta is of a greater size, it will add a marker showing so. But to do this, > it needs to save the current event otherwise peeking at the next event will > remove the current event. To save the event, a temp buffer is used, and if > the event is bigger than the temp buffer, the temp buffer is freed and a > bigger buffer is allocated. > > This allocation is a problem when called in atomic context. The only way > this gets called via atomic context is via ftrace_dump(). Thus, use a static > buffer of 128 bytes (which covers most events), and if the event is bigger > than that, simply return NULL. The callers of trace_find_next_entry() need > to handle a NULL case, as that's what would happen if the allocation failed. > > Link: https://lore.kernel.org/r/20200326091256.GR11705@shao2-debian > > Fixes: ff895103a84ab ("tracing: Save off entry when peeking at next entry") > Reported-by: kernel test robot > Signed-off-by: Steven Rostedt (VMware) > --- > kernel/trace/trace.c | 20 +++++++++++++++++++- > 1 file changed, 19 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 6519b7afc499..4b7bbfe7f809 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -3472,6 +3472,9 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu, > return next; > } > > +#define STATIC_TEMP_BUF_SIZE 128 > +static char static_temp_buf[STATIC_TEMP_BUF_SIZE]; > + > /* Find the next real entry, without updating the iterator itself */ > struct trace_entry *trace_find_next_entry(struct trace_iterator *iter, > int *ent_cpu, u64 *ent_ts) > @@ -3480,13 +3483,26 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter, > int ent_size = iter->ent_size; > struct trace_entry *entry; > > + /* > + * If called from ftrace_dump(), then the iter->temp buffer > + * will be the static_temp_buf and not created from kmalloc. > + * If the entry size is greater than the buffer, we can > + * not save it. Just return NULL in that case. This is only > + * used to add markers when two consecutive events' time > + * stamps have a large delta. See trace_print_lat_context() > + */ > + if (iter->temp == static_temp_buf && > + STATIC_TEMP_BUF_SIZE < ent_size) > + return NULL; > + > /* > * The __find_next_entry() may call peek_next_entry(), which may > * call ring_buffer_peek() that may make the contents of iter->ent > * undefined. Need to copy iter->ent now. > */ > if (iter->ent && iter->ent != iter->temp) { > - if (!iter->temp || iter->temp_size < iter->ent_size) { > + if ((!iter->temp || iter->temp_size < iter->ent_size) && > + !WARN_ON_ONCE(iter->temp == static_temp_buf)) { This must not happen because ent_size == iter->ent_size. If it happens, it should return NULL without any trial of kfree() and kmalloc(), becuase it will cause illegal freeing memory and memory leak. (Note that the iter->temp never be freed in ftrace_dump() path) Anyway, this condition is completery same as above return code. > kfree(iter->temp); > iter->temp = kmalloc(iter->ent_size, GFP_KERNEL); > if (!iter->temp) > @@ -9203,6 +9219,8 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) > > /* Simulate the iterator */ > trace_init_global_iter(&iter); > + /* Can not use kmalloc for iter.temp */ > + iter.temp = static_temp_buf; > You may miss initializing temp_size here. iter.temp_size = STATIC_TEMP_BUF_SIZE; BTW, as I pointed, if the iter->temp is for avoiding the data overwritten by ringbuffer writer, would we need to use it for ftrace_dump() too? It seems that ftrace_dump() stops tracing. void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { [...] /* Only allow one dump user at a time. */ if (atomic_inc_return(&dump_running) != 1) { atomic_dec(&dump_running); return; } /* * Always turn off tracing when we dump. * We don't need to show trace output of what happens * between multiple crashes. * * If the user does a sysrq-z, then they can re-enable * tracing with echo 1 > tracing_on. */ tracing_off(); Thank you, > for_each_tracing_cpu(cpu) { > atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled); > -- > 2.20.1 > -- Masami Hiramatsu