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ring-buffer: Have rb_iter_head_event() handle concurrent writer

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Have the ring_buffer_iter structure have a place to store an event, such
that it can not be overwritten by a writer, and load it in such a way via
rb_iter_head_event() that it will return NULL and reset the iter to the
start of the current page if a writer updated the page.

Link: http://lkml.kernel.org/r/20200317213416.306959216@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware) 2020-03-17 17:32:27 -04:00
parent 28e3fc56a4
commit 785888c544
1 changed files with 76 additions and 32 deletions
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108
kernel/trace/ring_buffer.c
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@ -503,11 +503,13 @@ struct trace_buffer {
struct ring_buffer_iter {
struct ring_buffer_per_cpu *cpu_buffer;
unsigned long head;
unsigned long next_event;
struct buffer_page *head_page;
struct buffer_page *cache_reader_page;
unsigned long cache_read;
u64 read_stamp;
u64 page_stamp;
struct ring_buffer_event *event;
};
/**
@ -1914,17 +1916,61 @@ rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->reader_page->read);
}
static __always_inline struct ring_buffer_event *
rb_iter_head_event(struct ring_buffer_iter *iter)
{
return __rb_page_index(iter->head_page, iter->head);
}
static __always_inline unsigned rb_page_commit(struct buffer_page *bpage)
{
return local_read(&bpage->page->commit);
}
static struct ring_buffer_event *
rb_iter_head_event(struct ring_buffer_iter *iter)
{
struct ring_buffer_event *event;
struct buffer_page *iter_head_page = iter->head_page;
unsigned long commit;
unsigned length;
/*
* When the writer goes across pages, it issues a cmpxchg which
* is a mb(), which will synchronize with the rmb here.
* (see rb_tail_page_update() and __rb_reserve_next())
*/
commit = rb_page_commit(iter_head_page);
smp_rmb();
event = __rb_page_index(iter_head_page, iter->head);
length = rb_event_length(event);
/*
* READ_ONCE() doesn't work on functions and we don't want the
* compiler doing any crazy optimizations with length.
*/
barrier();
if ((iter->head + length) > commit || length > BUF_MAX_DATA_SIZE)
/* Writer corrupted the read? */
goto reset;
memcpy(iter->event, event, length);
/*
* If the page stamp is still the same after this rmb() then the
* event was safely copied without the writer entering the page.
*/
smp_rmb();
/* Make sure the page didn't change since we read this */
if (iter->page_stamp != iter_head_page->page->time_stamp ||
commit > rb_page_commit(iter_head_page))
goto reset;
iter->next_event = iter->head + length;
return iter->event;
reset:
/* Reset to the beginning */
iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
iter->head = 0;
iter->next_event = 0;
return NULL;
}
/* Size is determined by what has been committed */
static __always_inline unsigned rb_page_size(struct buffer_page *bpage)
{
@ -1962,6 +2008,7 @@ static void rb_inc_iter(struct ring_buffer_iter *iter)
iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
iter->head = 0;
iter->next_event = 0;
}
/*
@ -3548,6 +3595,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
/* Iterator usage is expected to have record disabled */
iter->head_page = cpu_buffer->reader_page;
iter->head = cpu_buffer->reader_page->read;
iter->next_event = iter->head;
iter->cache_reader_page = iter->head_page;
iter->cache_read = cpu_buffer->read;
@ -3625,7 +3673,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
return 0;
/* Still racy, as it may return a false positive, but that's OK */
return ((iter->head_page == commit_page && iter->head == commit) ||
return ((iter->head_page == commit_page && iter->head >= commit) ||
(iter->head_page == reader && commit_page == head_page &&
head_page->read == commit &&
iter->head == rb_page_commit(cpu_buffer->reader_page)));
@ -3853,15 +3901,22 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
static void rb_advance_iter(struct ring_buffer_iter *iter)
{
struct ring_buffer_per_cpu *cpu_buffer;
struct ring_buffer_event *event;
unsigned length;
cpu_buffer = iter->cpu_buffer;
/* If head == next_event then we need to jump to the next event */
if (iter->head == iter->next_event) {
/* If the event gets overwritten again, there's nothing to do */
if (rb_iter_head_event(iter) == NULL)
return;
}
iter->head = iter->next_event;
/*
* Check if we are at the end of the buffer.
*/
if (iter->head >= rb_page_size(iter->head_page)) {
if (iter->next_event >= rb_page_size(iter->head_page)) {
/* discarded commits can make the page empty */
if (iter->head_page == cpu_buffer->commit_page)
return;
@ -3869,27 +3924,7 @@ static void rb_advance_iter(struct ring_buffer_iter *iter)
return;
}
event = rb_iter_head_event(iter);
length = rb_event_length(event);
/*
* This should not be called to advance the header if we are
* at the tail of the buffer.
*/
if (RB_WARN_ON(cpu_buffer,
(iter->head_page == cpu_buffer->commit_page) &&
(iter->head + length > rb_commit_index(cpu_buffer))))
return;
rb_update_iter_read_stamp(iter, event);
iter->head += length;
/* check for end of page padding */
if ((iter->head >= rb_page_size(iter->head_page)) &&
(iter->head_page != cpu_buffer->commit_page))
rb_inc_iter(iter);
rb_update_iter_read_stamp(iter, iter->event);
}
static int rb_lost_events(struct ring_buffer_per_cpu *cpu_buffer)
@ -4017,6 +4052,8 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
}
event = rb_iter_head_event(iter);
if (!event)
goto again;
switch (event->type_len) {
case RINGBUF_TYPE_PADDING:
@ -4233,10 +4270,16 @@ ring_buffer_read_prepare(struct trace_buffer *buffer, int cpu, gfp_t flags)
if (!cpumask_test_cpu(cpu, buffer->cpumask))
return NULL;
iter = kmalloc(sizeof(*iter), flags);
iter = kzalloc(sizeof(*iter), flags);
if (!iter)
return NULL;
iter->event = kmalloc(BUF_MAX_DATA_SIZE, flags);
if (!iter->event) {
kfree(iter);
return NULL;
}
cpu_buffer = buffer->buffers[cpu];
iter->cpu_buffer = cpu_buffer;
@ -4317,6 +4360,7 @@ ring_buffer_read_finish(struct ring_buffer_iter *iter)
atomic_dec(&cpu_buffer->record_disabled);
atomic_dec(&cpu_buffer->buffer->resize_disabled);
kfree(iter->event);
kfree(iter);
}
EXPORT_SYMBOL_GPL(ring_buffer_read_finish);