Re: [PATCH 1/4] perf tool cgroup: Initialize cgroup refcnt with refcount_set

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Jul 18, 2017 at 06:18:36PM -0700, David Carrillo-Cisneros escreveu:
> Atomic reference counters were replaced by refcount_t in
>   commit 79c5fe6db8c7 ("perf/core: Fix error handling in perf_event_alloc()")
> 
> In util/cgroup.c atomic_inc was replaced by refcount_inc, but the latter
> is not mean to initiliaze refcounts with zero value. Add a path
> to initialize cgrp->refcnt == 0 using refcount_set.
> 
> Before this patch:
> 
>   $ perf stat -e cycles -C 0 -G /
>   perf_before: /usr/local/.../tools/include/linux/refcount.h:108: refcount_inc: Assertion `!(!refcount_inc_not_zero(r))' failed.
>   Aborted (core dumped)
> 
> After this patch:
> 
>   $ perf stat -e cycles -C 0 -G /
>   Performance counter stats for 'CPU(s) 0':
>      17,516,664      cycles                    /

Ok, so this one was also reported by Brendan and I came up with an
alternative patch, that follows the usual sequence of steps, see below.

- Arnaldo

commit b13ca9843c4ea07c5a1dbf0295986b5dfeb6ef6f
Author: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Date:   Tue Jul 18 20:20:19 2017 -0300

    perf cgroup: Fix refcount usage
    
    When converting from atomic_t to refcount_t we didn't follow the usual
    step of initializing it to one before taking any new reference, which
    trips over checking if taking a reference for a freed refcount_t, fix
    it.
    
    Brendan's report:
    
     ---
    It's 4.12-rc7, with node v4.4.1. I'm building 4.13-rc1 now, as I hit
    what I think is another unrelated perf bug and I'm starting to wonder
    what else is broken on that version:
    
    (root) /mnt/src/linux-4.12-rc7/tools/perf # ./perf record -F 99 -a -e
    cpu-clock --cgroup=docker/f9e9d5df065b14646e8a11edc837a13877fd90c171137b2ba3feb67a0201cb65
    -g
    perf: /mnt/src/linux-4.12-rc7/tools/include/linux/refcount.h:108:
    refcount_inc: Assertion `!(!refcount_inc_not_zero(r))' failed.
    Aborted
    
    that used to work...
     ---
    
    Testing it:
    
    Before:
    
      # perf stat -e cycles -C 0 --cgroup /
      perf: /home/acme/git/linux/tools/include/linux/refcount.h:108: refcount_inc: Assertion `!(!refcount_inc_not_zero(r))' failed.
      Aborted (core dumped)
      #
    
    After:
    
      # perf stat -e cycles -C 0 --cgroup /
    ^C
      Performance counter stats for 'CPU(s) 0':
    
           132,081,393      cycles                    /
    
           2.492942763 seconds time elapsed
    
      #
    
    Reported-by: Brendan Gregg <brendan.d.gregg@gmail.com>
    Acked-by: Elena Reshetova <elena.reshetova@intel.com>
    Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: David Ahern <dsahern@gmail.com>
    Cc: David Carrillo-Cisneros <davidcc@google.com>
    Cc: Kees Kook <keescook@chromium.org>
    Cc: Krister Johansen <kjlx@templeofstupid.com>
    Cc: Paul Turner <pjt@google.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: Sudeep Holla <Sudeep.Holla@arm.com>
    Cc: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
    Cc: Wang Nan <wangnan0@huawei.com>
    Fixes: 79c5fe6db8c7 ("perf cgroup: Convert cgroup_sel.refcnt from atomic_t to refcount_t")
    Link: http://lkml.kernel.org/n/tip-l7ovfblq14ip2i08m1g0fkhv@git.kernel.org
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/util/cgroup.c b/tools/perf/util/cgroup.c
index 03347748f3fa..0e77bc9e5f3c 100644
--- a/tools/perf/util/cgroup.c
+++ b/tools/perf/util/cgroup.c
@@ -98,8 +98,10 @@ static int add_cgroup(struct perf_evlist *evlist, char *str)
 		cgrp = counter->cgrp;
 		if (!cgrp)
 			continue;
-		if (!strcmp(cgrp->name, str))
+		if (!strcmp(cgrp->name, str)) {
+			refcount_inc(&cgrp->refcnt);
 			break;
+		}
 
 		cgrp = NULL;
 	}
@@ -110,6 +112,7 @@ static int add_cgroup(struct perf_evlist *evlist, char *str)
 			return -1;
 
 		cgrp->name = str;
+		refcount_set(&cgrp->refcnt, 1);
 
 		cgrp->fd = open_cgroup(str);
 		if (cgrp->fd == -1) {
@@ -128,12 +131,11 @@ static int add_cgroup(struct perf_evlist *evlist, char *str)
 			goto found;
 		n++;
 	}
-	if (refcount_read(&cgrp->refcnt) == 0)
+	if (refcount_dec_and_test(&cgrp->refcnt))
 		free(cgrp);
 
 	return -1;
 found:
-	refcount_inc(&cgrp->refcnt);
 	counter->cgrp = cgrp;
 	return 0;
 }