Messages in this thread | | | Date | Fri, 20 Jul 2018 11:42:47 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: [PATCHv3 4/4] perf tools: Fix struct comm_str removal crash |
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Em Fri, Jul 20, 2018 at 12:17:40PM +0200, Jiri Olsa escreveu: > On Fri, Jul 20, 2018 at 10:20:55AM +0900, Namhyung Kim wrote: > > Hi Arnaldo, > > > > On Thu, Jul 19, 2018 at 03:31:14PM -0300, Arnaldo Carvalho de Melo wrote: > > > Em Thu, Jul 19, 2018 at 03:28:43PM -0300, Arnaldo Carvalho de Melo escreveu: > > > > Em Thu, Jul 19, 2018 at 04:33:45PM +0200, Jiri Olsa escreveu: > > > > > +++ b/tools/perf/util/comm.c > > > > > @@ -18,11 +18,9 @@ struct comm_str { > > > > > static struct rb_root comm_str_root; > > > > > static struct rw_semaphore comm_str_lock = {.lock = PTHREAD_RWLOCK_INITIALIZER,}; > > > > > > > > > > -static struct comm_str *comm_str__get(struct comm_str *cs) > > > > > +static bool comm_str__get(struct comm_str *cs) > > > > > { > > > > > - if (cs) > > > > > - refcount_inc(&cs->refcnt); > > > > > - return cs; > > > > > + return cs ? refcount_inc_not_zero(&cs->refcnt) : false; > > > > > } > > > > > > > > I don't like changing the semantics of a __get() operation this way, I > > > > think it should stay like all the others, i.e. return the object with > > > > the desired refcount or return NULL if that is not possible. > > > > > > > > Otherwise we'll have to switch gears when debugging refcounts in various > > > > objects, that start having slightly different semantics for reference > > > > counting. > > > > > > > > We should try to find a fix that maintains the semantics of refcounting. > > > > > > After looking at the code, this refcount_inc_not_zero returns bool comes > > > from the kernel, trying to see how this is used with __get() operations > > > there, if at all. > > > > Something like this? > > > > static struct comm_str *comm_str__get(struct comm_str *cs) > > { > > if (cs && refcount_inc_not_zero(&cs->refcnt)) > > return cs; > > return NULL; > > } > > > > > > Other than that I don't have better idea, so > > > > Acked-by: Namhyung Kim <namhyung@kernel.org> > > > > Thanks, > > Namhyung > > righ, we can change comm_str__get like that, attached v3
Thanks, glad it was so easy. :-)
- Arnaldo
> thanks, > jirka > > > --- > 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. > > Acked-by: Namhyung Kim <namhyung@kernel.org> > Link: http://lkml.kernel.org/n/tip-vrizt6sw1lu1ybsrl9l0wwln@git.kernel.org > Signed-off-by: Jiri Olsa <jolsa@kernel.org> > --- > 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; > -- > 2.17.1
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