From: Jiri Olsa jolsa@redhat.com
[ Upstream commit 46b3722cc7765582354488da633aafffcb138458 ]
We occasionaly hit following assert failure in 'perf top', when processing the /proc info in multiple threads.
perf: ...include/linux/refcount.h:109: refcount_inc: Assertion `!(!refcount_inc_not_zero(r))' failed.
The gdb backtrace looks like this:
[Switching to Thread 0x7ffff11ba700 (LWP 13749)] 0x00007ffff50839fb in raise () from /lib64/libc.so.6 (gdb) #0 0x00007ffff50839fb in raise () from /lib64/libc.so.6 #1 0x00007ffff5085800 in abort () from /lib64/libc.so.6 #2 0x00007ffff507c0da in __assert_fail_base () from /lib64/libc.so.6 #3 0x00007ffff507c152 in __assert_fail () from /lib64/libc.so.6 #4 0x0000000000535373 in refcount_inc (r=0x7fffdc009be0) at ...include/linux/refcount.h:109 #5 0x00000000005354f1 in comm_str__get (cs=0x7fffdc009bc0) at util/comm.c:24 #6 0x00000000005356bd in __comm_str__findnew (str=0x7fffd000b260 ":2", root=0xbed5c0 <comm_str_root>) at util/comm.c:72 #7 0x000000000053579e in comm_str__findnew (str=0x7fffd000b260 ":2", root=0xbed5c0 <comm_str_root>) at util/comm.c:95 #8 0x000000000053582e in comm__new (str=0x7fffd000b260 ":2", timestamp=0, exec=false) at util/comm.c:111 #9 0x00000000005363bc in thread__new (pid=2, tid=2) at util/thread.c:57 #10 0x0000000000523da0 in ____machine__findnew_thread (machine=0xbfde38, threads=0xbfdf28, pid=2, tid=2, create=true) at util/machine.c:457 #11 0x0000000000523eb4 in __machine__findnew_thread (machine=0xbfde38, ...
The failing assertion is this one:
REFCOUNT_WARN(!refcount_inc_not_zero(r), ...
The problem is that we keep global comm_str_root list, which is accessed by multiple threads during the 'perf top' startup and following 2 paths can race:
thread 1: ... thread__new comm__new comm_str__findnew down_write(&comm_str_lock); __comm_str__findnew comm_str__get
thread 2: ... comm__override or comm__free comm_str__put refcount_dec_and_test down_write(&comm_str_lock); rb_erase(&cs->rb_node, &comm_str_root);
Because thread 2 first decrements the refcnt and only after then it removes the struct comm_str from the list, the thread 1 can find this object on the list with refcnt equls to 0 and hit the assert.
This patch fixes the thread 1 __comm_str__findnew path, by ignoring objects that already dropped the refcnt to 0. For the rest of the objects we take the refcnt before comparing its name and release it afterwards with comm_str__put, which can also release the object completely.
Signed-off-by: Jiri Olsa jolsa@kernel.org Acked-by: Namhyung Kim namhyung@kernel.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Andi Kleen ak@linux.intel.com Cc: David Ahern dsahern@gmail.com Cc: Kan Liang kan.liang@linux.intel.com Cc: Lukasz Odzioba lukasz.odzioba@intel.com Cc: Peter Zijlstra peterz@infradead.org Cc: Wang Nan wangnan0@huawei.com Cc: kernel-team@lge.com Link: http://lkml.kernel.org/r/20180720101740.GA27176@krava Signed-off-by: Arnaldo Carvalho de Melo acme@redhat.com Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- tools/perf/util/comm.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/tools/perf/util/comm.c b/tools/perf/util/comm.c index 8808570f8e9c..327e01cb1aa2 100644 --- a/tools/perf/util/comm.c +++ b/tools/perf/util/comm.c @@ -18,9 +18,10 @@ static struct rb_root comm_str_root;
static struct comm_str *comm_str__get(struct comm_str *cs) { - if (cs) - refcount_inc(&cs->refcnt); - return cs; + if (cs && refcount_inc_not_zero(&cs->refcnt)) + return cs; + + return NULL; }
static void comm_str__put(struct comm_str *cs) @@ -62,9 +63,14 @@ static struct comm_str *comm_str__findnew(const char *str, struct rb_root *root) parent = *p; iter = rb_entry(parent, struct comm_str, rb_node);
+ /* + * If we race with comm_str__put, iter->refcnt is 0 + * and it will be removed within comm_str__put call + * shortly, ignore it in this search. + */ cmp = strcmp(str, iter->str); - if (!cmp) - return comm_str__get(iter); + if (!cmp && comm_str__get(iter)) + return iter;
if (cmp < 0) p = &(*p)->rb_left;