From: Zheng Yejian zhengyejian1@huawei.com
[ Upstream commit 2d093282b0d4357373497f65db6a05eb0c28b7c8 ]
When pages are removed in rb_remove_pages(), 'cpu_buffer->read' is set to 0 in order to make sure any read iterators reset themselves. However, this will mess 'entries' stating, see following steps:
# cd /sys/kernel/tracing/ # 1. Enlarge ring buffer prepare for later reducing: # echo 20 > per_cpu/cpu0/buffer_size_kb # 2. Write a log into ring buffer of cpu0: # taskset -c 0 echo "hello1" > trace_marker # 3. Read the log: # cat per_cpu/cpu0/trace_pipe <...>-332 [000] ..... 62.406844: tracing_mark_write: hello1 # 4. Stop reading and see the stats, now 0 entries, and 1 event readed: # cat per_cpu/cpu0/stats entries: 0 [...] read events: 1 # 5. Reduce the ring buffer # echo 7 > per_cpu/cpu0/buffer_size_kb # 6. Now entries became unexpected 1 because actually no entries!!! # cat per_cpu/cpu0/stats entries: 1 [...] read events: 0
To fix it, introduce 'page_removed' field to count total removed pages since last reset, then use it to let read iterators reset themselves instead of changing the 'read' pointer.
Link: https://lore.kernel.org/linux-trace-kernel/20230724054040.3489499-1-zhengyej...
Cc: mhiramat@kernel.org Cc: vnagarnaik@google.com Fixes: 83f40318dab0 ("ring-buffer: Make removal of ring buffer pages atomic") Signed-off-by: Zheng Yejian zhengyejian1@huawei.com Signed-off-by: Steven Rostedt (Google) rostedt@goodmis.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/trace/ring_buffer.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 593e446f6c487..3b8c53264441e 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -526,6 +526,8 @@ struct ring_buffer_per_cpu { rb_time_t write_stamp; rb_time_t before_stamp; u64 read_stamp; + /* pages removed since last reset */ + unsigned long pages_removed; /* ring buffer pages to update, > 0 to add, < 0 to remove */ long nr_pages_to_update; struct list_head new_pages; /* new pages to add */ @@ -561,6 +563,7 @@ struct ring_buffer_iter { struct buffer_page *head_page; struct buffer_page *cache_reader_page; unsigned long cache_read; + unsigned long cache_pages_removed; u64 read_stamp; u64 page_stamp; struct ring_buffer_event *event; @@ -1833,6 +1836,8 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) to_remove = rb_list_head(to_remove)->next; head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD; } + /* Read iterators need to reset themselves when some pages removed */ + cpu_buffer->pages_removed += nr_removed;
next_page = rb_list_head(to_remove)->next;
@@ -1854,12 +1859,6 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) cpu_buffer->head_page = list_entry(next_page, struct buffer_page, list);
- /* - * change read pointer to make sure any read iterators reset - * themselves - */ - cpu_buffer->read = 0; - /* pages are removed, resume tracing and then free the pages */ atomic_dec(&cpu_buffer->record_disabled); raw_spin_unlock_irq(&cpu_buffer->reader_lock); @@ -4105,6 +4104,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
iter->cache_reader_page = iter->head_page; iter->cache_read = cpu_buffer->read; + iter->cache_pages_removed = cpu_buffer->pages_removed;
if (iter->head) { iter->read_stamp = cpu_buffer->read_stamp; @@ -4558,12 +4558,13 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts) buffer = cpu_buffer->buffer;
/* - * Check if someone performed a consuming read to - * the buffer. A consuming read invalidates the iterator - * and we need to reset the iterator in this case. + * Check if someone performed a consuming read to the buffer + * or removed some pages from the buffer. In these cases, + * iterator was invalidated and we need to reset it. */ if (unlikely(iter->cache_read != cpu_buffer->read || - iter->cache_reader_page != cpu_buffer->reader_page)) + iter->cache_reader_page != cpu_buffer->reader_page || + iter->cache_pages_removed != cpu_buffer->pages_removed)) rb_iter_reset(iter);
again: @@ -5005,6 +5006,7 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer) cpu_buffer->last_overrun = 0;
rb_head_page_activate(cpu_buffer); + cpu_buffer->pages_removed = 0; }
/* Must have disabled the cpu buffer then done a synchronize_rcu */