From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnaldo Carvalho de Melo Subject: [PATCH 24/27] perf tools: Fix struct comm_str removal crash Date: Wed, 25 Jul 2018 14:59:58 -0300 Message-ID: <20180725180001.15108-25-acme@kernel.org> References: <20180725180001.15108-1-acme@kernel.org> Return-path: In-Reply-To: <20180725180001.15108-1-acme@kernel.org> Sender: linux-kernel-owner@vger.kernel.org To: Ingo Molnar Cc: Clark Williams , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Jiri Olsa , Jiri Olsa , Alexander Shishkin , Andi Kleen , David Ahern , Kan Liang , Lukasz Odzioba , Peter Zijlstra , Wang Nan , kernel-team@lge.com, Arnaldo Carvalho de Melo List-Id: linux-perf-users.vger.kernel.org From: Jiri Olsa 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 ) at util/comm.c:72 #7 0x000000000053579e in comm_str__findnew (str=0x7fffd000b260 ":2", root=0xbed5c0 ) 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 Acked-by: Namhyung Kim Cc: Alexander Shishkin Cc: Andi Kleen Cc: David Ahern Cc: Kan Liang Cc: Lukasz Odzioba Cc: Peter Zijlstra Cc: Wang Nan Cc: kernel-team@lge.com Link: http://lkml.kernel.org/r/20180720101740.GA27176@krava Signed-off-by: Arnaldo Carvalho de Melo --- 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 7798a2cc8a86..31279a7bd919 100644 --- a/tools/perf/util/comm.c +++ b/tools/perf/util/comm.c @@ -20,9 +20,10 @@ static struct rw_semaphore comm_str_lock = {.lock = PTHREAD_RWLOCK_INITIALIZER,} 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) @@ -67,9 +68,14 @@ 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; -- 2.14.4