Re: [PATCH] perf ksymbol: fix memory leak: decrease refcount of map and dso

Em Fri, Jun 04, 2021 at 05:16:39PM +0200, Riccardo Mancini escreveu:

Hi,

On Fri, 2021-06-04 at 10:22 -0300, Arnaldo Carvalho de Melo wrote:

quoted

Em Thu, Jun 03, 2021 at 09:26:40PM -0700, Ian Rogers escreveu:

quoted

On Wed, Jun 2, 2021 at 4:15 PM Riccardo Mancini [off-list ref] wrote:

quoted

+++ b/tools/perf/util/machine.c

@@ -776,6 +776,7 @@ static int machine__process_ksymbol_register(struct

machine *machine,
                if (dso) {
                        dso->kernel = DSO_SPACE__KERNEL;
                        map = map__new2(0, dso);
+                       dso__put(dso);

quoted

Will this cause 2 puts if the map allocation fails? Perhaps this
should be "if (map) dso__put(dso);".

I think its just a matter of removing the put in the error path, i.e.
the patch becomes what is at the end of this message.

I.e. if map__new2() fails, we want to drop the dso reference, and if it
works, we already have a reference to it, obtained in map__new2().

Agree.
I'm sorry for this stupid oversight.
Should we make it a series including the fix to the issue you pointed out below,
or should I send you a v2 and fix the other issue in a subsequent patch?

Please send a v2 patch, and then consider starting a new series with the
issues below.
 

quoted

But looking at this code now I realize that maps__find() should grab a
refcount for the map it returns, because in this
machine__process_ksymbol_register() function we use reference that 'map'
after the if block, i.e. we use it if it came from maps__find() or if we
created it machine__process_ksymbol_register, so there is a possible
race where other thread removes it from the list and map__put()s it
ending up in map__delete() while we still use it in
machine__process_ksymbol_register(), right?

Agree. It should be placed before up_read to avoid races, right?

Yes, we have to grab a refcount while we are sure its not going away,
then return that as the lookup result, whoever receives that refcounted
entry should use it and then drop the refcount.

Then we would need to see where it's called and add the appropriate map__put.

yes
 

In addition, having a look at other possible concurrency issues in map.c:

Its good to have new eyes looking at this, exactly at a time we're
discussing further parallelizing perf :-)

 - maps__for_each_entry should always be called with either read or write lock,
am I right? It looks like this is not done in certain parts of the code. If such

Right.

lock is taken, then grabbing the refcount on the looping variable is not needed
unless we need to return it, right?

Right, returning an entry needs to take a refcount.

 - maps__first and map__next do not grab a refcount and neither a lock. If
they're used through a lock-protected loop, it's not a problem, but maybe it's

yes

worth making explicit that they are not to be used directly (through either a
comment or adding some underscores in their names).

yes, __ in front means, in kernel style, that it does less than the non
__ prefixed, same name, function.

 - maps__empty: should probably take a reader lock.

Indeed.

 - maps__find_symbol: the returned symbol is not protected (the caller does not
receive a refcount to neither map or dso, so if dso is deleted, his reference to
the symbol gets invalidated). Depending on how it's being used it might not be a
problem, but in the general scenario I think it's not thread-safe.

Yes, that function is also problematic.

Thanks for looking into this, please consider sending patches for these
issues,

- Arnaldo