| Message ID | 20230324125037.2719020-1-zhengyejian1@huawei.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | ring-buffer: Fix race while reader and writer are on the same page | expand |
On Fri, 24 Mar 2023 20:50:37 +0800 Zheng Yejian <zhengyejian1@huawei.com> wrote: > Fixes: 77ae365eca89 ("ring-buffer: make lockless") > Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com> > --- > kernel/trace/ring_buffer.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index c6f47b6cfd5f..79fd5e10ee05 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -2942,6 +2942,13 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer, > event->array[0] = length; > } else > event->type_len = DIV_ROUND_UP(length, RB_ALIGNMENT); > + > + /* > + * The 'event' may be reserved from the page which is reading > + * by reader, make sure 'event' is completely updated before > + * reader_page->page->commit being set. > + */ > + smp_wmb(); This isn't the place to put this. We only care before the commit is updated, not at *ever* update to the event (this can be called several times before a commit). If we need to add a smp_wmb() it's best to be in rb_set_commit_to_write() > } > > static unsigned rb_calculate_event_length(unsigned length) > @@ -4684,7 +4691,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) > > /* > * Make sure we see any padding after the write update > - * (see rb_reset_tail()) > + * (see rb_reset_tail()). > + * > + * In addition, writer may be writing on the reader page > + * if the page has not been fully filled, so the read barrier > + * is also needed to make sure we see the completely updated > + * event that reserved by writer (see rb_update_event()). > */ > smp_rmb(); > I think we want this instead: diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 2d5c3caff32d..22d05cd04a3a 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3092,6 +3092,10 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer) if (RB_WARN_ON(cpu_buffer, rb_is_reader_page(cpu_buffer->tail_page))) return; + /* + * No need for a memory barrier here, as the update + * of the tail_page did it for this page. + */ local_set(&cpu_buffer->commit_page->page->commit, rb_page_write(cpu_buffer->commit_page)); rb_inc_page(&cpu_buffer->commit_page); @@ -3101,6 +3105,8 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer) while (rb_commit_index(cpu_buffer) != rb_page_write(cpu_buffer->commit_page)) { + /* Make sure the readers see the content of what is committed. */ + smp_wmb(); local_set(&cpu_buffer->commit_page->page->commit, rb_page_write(cpu_buffer->commit_page)); RB_WARN_ON(cpu_buffer, @@ -4676,7 +4682,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) /* * Make sure we see any padding after the write update - * (see rb_reset_tail()) + * (see rb_reset_tail()). + * + * In addition, a writer may be writing on the reader page + * if the page has not been fully filled, so the read barrier + * is also needed to make sure we see the completely updated + * event reserved by the writer (see rb_tail_page_update()). */ smp_rmb();
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index c6f47b6cfd5f..79fd5e10ee05 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -2942,6 +2942,13 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer, event->array[0] = length; } else event->type_len = DIV_ROUND_UP(length, RB_ALIGNMENT); + + /* + * The 'event' may be reserved from the page which is reading + * by reader, make sure 'event' is completely updated before + * reader_page->page->commit being set. + */ + smp_wmb(); } static unsigned rb_calculate_event_length(unsigned length) @@ -4684,7 +4691,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) /* * Make sure we see any padding after the write update - * (see rb_reset_tail()) + * (see rb_reset_tail()). + * + * In addition, writer may be writing on the reader page + * if the page has not been fully filled, so the read barrier + * is also needed to make sure we see the completely updated + * event that reserved by writer (see rb_update_event()). */ smp_rmb();
When user reads file 'trace_pipe', kernel keeps printing following logs that warn at "cpu_buffer->reader_page->read > rb_page_size(reader)". It just looks like there's an infinite loop in tracing_read_pipe(). This problem occurs several times on arm64 when testing v5.10 and below. Call trace: rb_get_reader_page+0x248/0x1300 rb_buffer_peek+0x34/0x160 ring_buffer_peek+0xbc/0x224 peek_next_entry+0x98/0xbc __find_next_entry+0xc4/0x1c0 trace_find_next_entry_inc+0x30/0x94 tracing_read_pipe+0x198/0x304 vfs_read+0xb4/0x1e0 ksys_read+0x74/0x100 __arm64_sys_read+0x24/0x30 el0_svc_common.constprop.0+0x7c/0x1bc do_el0_svc+0x2c/0x94 el0_svc+0x20/0x30 el0_sync_handler+0xb0/0xb4 el0_sync+0x160/0x180 Then I dump the vmcore and look into the problematic per_cpu ring_buffer, I found that tail_page/commit_page/reader_page are on the same page while reader_page->read is obviously abnormal: tail_page == commit_page == reader_page == { .write = 0x100d20, .read = 0x8f9f4805, // Far greater than 0xd20, obviously abnormal!!! .entries = 0x10004c, .real_end = 0x0, .page = { .time_stamp = 0x857257416af0, .commit = 0xd20, // This page hasn't been full filled. // .data[0...0xd20] seems normal } } The root cause is most likely the race that reader and writer are on the same page while reader saw an event that not fully committed by writer. To fix this, add memory barriers to make sure that reader can see every completely committed event. Since commit a0fcaaed0c46 ("ring-buffer: Fix race between reset page and reading page") has added the read barrier in rb_get_reader_page(), here we just need to add the write barrier. Fixes: 77ae365eca89 ("ring-buffer: make lockless") Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com> --- kernel/trace/ring_buffer.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)