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Message-ID: <20240805055922.6277-1-Tze-nan.Wu@mediatek.com>
Date: Mon, 5 Aug 2024 13:59:22 +0800
From: Tze-nan Wu <Tze-nan.Wu@...iatek.com>
To: Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
<mhiramat@...nel.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
<linux-kernel@...r.kernel.org>, <linux-trace-kernel@...r.kernel.org>,
<linux-mediatek@...ts.infradead.org>, <bobule.chang@...iatek.com>,
<eric-yc.wu@...iatek.com>, <wsd_upstream@...iatek.com>, Tze-nan Wu
<Tze-nan.Wu@...iatek.com>, Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
Subject: [PATCH RESEND] tracing: Fix overflow in get_free_elt()
"tracing_map->next_elt" in get_free_elt() is at risk of overflowing.
Once it overflows, new elements can still be inserted into the tracing_map
even though the maximum number of elements (`max_elts`) has been reached.
Continuing to insert elements after the overflow could result in the
tracing_map containing "tracing_map->max_size" elements, leaving no empty
entries.
If any attempt is made to insert an element into a full tracing_map using
`__tracing_map_insert()`, it will cause an infinite loop with preemption
disabled, leading to a CPU hang problem.
Fix this by preventing any further increments to "tracing_map->next_elt"
once it reaches "tracing_map->max_elt".
Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@...iatek.com>
---
We have encountered this issue internally after enabling the
throttle_rss_stat feature provided by Perfetto in background for more than
two days, during which `rss_stat` tracepoint was invoked over 2^32 times.
After tracing_map->next_elt overflow, new elements can continue to be
inserted to the tracing_map belong to `rss_stat`.
Then the CPU could hang inside the while dead loop in function
`__tracing_map_insert()` by calling it after the tracing_map left no empty
entry.
Call trace during hang:
__tracing_map_insert()
tracing_map_insert()
event_hist_trigger()
event_triggers_call()
__event_trigger_test_discard()
trace_event_buffer_commit()
trace_event_raw_event_rss_stat()
__traceiter_rss_stat()
trace_rss_stat()
mm_trace_rss_stat()
inc_mm_counter()
do_swap_page()
throttle_rss_stat is literally a synthetic event triggered by `rss_stat`
with condition:
1. $echo "rss_stat_throttled unsigned int mm_id unsigned int curr int
member long size" >> /sys/kernel/tracing/synthetic_events
2. $echo
'hist:keys=mm_id,member:bucket=size/0x80000:onchange($bucket).rss_stat_
throttled(mm_id,curr,member,size)' >
/sys/kernel/tracing/events/kmem/rss_stat/trigger
The hang issue could also be reproduced easily by calling a customize
trace event `myevent(u64 mycount)` for more than 2^32+(map_size-max_elts)
times during its histogram enabled with the key set to variable "mycount".
While we call `myevent` with different argument "mycount" everytime.
BTW, I added Cheng-jui to Co-developed because we have a lot of discussions
during debugging this.
---
kernel/trace/tracing_map.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/trace/tracing_map.c b/kernel/trace/tracing_map.c
index a4dcf0f24352..3a56e7c8aa4f 100644
--- a/kernel/trace/tracing_map.c
+++ b/kernel/trace/tracing_map.c
@@ -454,7 +454,7 @@ static struct tracing_map_elt *get_free_elt(struct tracing_map *map)
struct tracing_map_elt *elt = NULL;
int idx;
- idx = atomic_inc_return(&map->next_elt);
+ idx = atomic_fetch_add_unless(&map->next_elt, 1, map->max_elts);
if (idx < map->max_elts) {
elt = *(TRACING_MAP_ELT(map->elts, idx));
if (map->ops && map->ops->elt_init)
@@ -699,7 +699,7 @@ void tracing_map_clear(struct tracing_map *map)
{
unsigned int i;
- atomic_set(&map->next_elt, -1);
+ atomic_set(&map->next_elt, 0);
atomic64_set(&map->hits, 0);
atomic64_set(&map->drops, 0);
@@ -783,7 +783,7 @@ struct tracing_map *tracing_map_create(unsigned int map_bits,
map->map_bits = map_bits;
map->max_elts = (1 << map_bits);
- atomic_set(&map->next_elt, -1);
+ atomic_set(&map->next_elt, 0);
map->map_size = (1 << (map_bits + 1));
map->ops = ops;
--
2.18.0
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