Messages in this thread Patch in this message | | | Date | Tue, 17 Jul 2018 10:49:40 +0900 | From | Namhyung Kim <> | Subject | Re: [PATCH 1/4] perf tools: Fix struct comm_str removal crash |
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Hi Jiri,
On Mon, Jul 16, 2018 at 12:29:34PM +0200, Jiri Olsa wrote: > On Sun, Jul 15, 2018 at 10:08:27PM +0900, Namhyung Kim wrote: > > SNIP > > > > 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 2 path, by removing the struct comm_str > > > FIRST from the list and only AFTER calling comm_str__put on it. This > > > way the thread 1 finds only valid objects on the list. > > > > I'm not sure we can unconditionally remove the comm_str from the tree. > > It should be removed only if refcount is going to zero IMHO. > > Otherwise it could end up having multiple comm_str entry for a same > > name. > > right, but it wouldn't crash ;-) > > how about attached change, that actualy deals with the refcnt > race I'm running the tests now, seems ok so far
I think we can keep if the refcount is back to non-zero. What about this? (not tested..)
static struct comm_str *comm_str__get(cs) { if (cs) refcount_inc_no_warn(&cs->refcnt); // should be added return cs; }
static void comm_str__put(cs) { if (cs && refcount_dec_and_test(&cs->refcnt)) { down_write(&comm_str_lock); /* might race with comm_str__findnew() */ if (!refcount_read(&cs->refcnt)) { rb_erase(&cs->rb_node, &comm_str_root); zfree(&cs->str); free(cs); } up_write(&comm_str_lock); } }
Thanks, Namhyung
> > > --- > diff --git a/tools/perf/util/comm.c b/tools/perf/util/comm.c > index 7798a2cc8a86..592b03548021 100644 > --- a/tools/perf/util/comm.c > +++ b/tools/perf/util/comm.c > @@ -18,22 +18,27 @@ 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; > } > > -static void comm_str__put(struct comm_str *cs) > +static int comm_str__put(struct comm_str *cs, bool lock) > { > - if (cs && refcount_dec_and_test(&cs->refcnt)) { > + if (!cs || !refcount_dec_and_test(&cs->refcnt)) > + return 0; > + > + if (lock) > down_write(&comm_str_lock); > - rb_erase(&cs->rb_node, &comm_str_root); > + > + rb_erase(&cs->rb_node, &comm_str_root); > + > + if (lock) > up_write(&comm_str_lock); > - zfree(&cs->str); > - free(cs); > - } > + > + zfree(&cs->str); > + free(cs); > + return 1; > } > > static struct comm_str *comm_str__alloc(const char *str) > @@ -67,9 +72,22 @@ struct comm_str *__comm_str__findnew(const char *str, struct rb_root *root) > parent = *p; > iter = rb_entry(parent, struct comm_str, rb_node); > > - cmp = strcmp(str, iter->str); > - if (!cmp) > - return comm_str__get(iter); > + /* > + * If we race with comm_str__put, iter->refcnt == 0 > + * and it will be removed within comm_str__put > + * thread shortly, ignore it in this search. > + */ > + if (comm_str__get(iter)) { > + cmp = strcmp(str, iter->str); > + if (!cmp) > + return iter; > + /* > + * If we actualy had to remove the item, restart > + * the search to have the clean tree search. > + */ > + if (comm_str__put(iter, false)) > + return __comm_str__findnew(str, root); > + } > > if (cmp < 0) > p = &(*p)->rb_left; > @@ -125,7 +143,7 @@ int comm__override(struct comm *comm, const char *str, u64 timestamp, bool exec) > if (!new) > return -ENOMEM; > > - comm_str__put(old); > + comm_str__put(old, true); > comm->comm_str = new; > comm->start = timestamp; > if (exec) > @@ -136,7 +154,7 @@ int comm__override(struct comm *comm, const char *str, u64 timestamp, bool exec) > > void comm__free(struct comm *comm) > { > - comm_str__put(comm->comm_str); > + comm_str__put(comm->comm_str, true); > free(comm); > } >
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