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

Hi, 

On Fri, 2021-06-04 at 15:29 -0300, Arnaldo Carvalho de Melo wrote:
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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.

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

yes

This function has quite a number of callers (direct and indirect) so the the
patch is becoming huge. 

One of these callers is thread__find_map, which returns an addr_location
(actually it's an output argument). This addr_location holds references to map,
maps and thread without getting any refcnt (actually in one function it gets it
on the thread and a comment tells to put it once done). If I'm not wrong, this
addr_location is never malloced (always a local variable) and, is should be
present in parts of the code where there should be a refcnt on the thread.
Therefore, maybe it does not get the refcnts since it assumes that thread (upon
which depends maps and as a consequence map) is always refcnted in its context.
However, I think that it should get all refcnts anyways for clarity and to
prevent possible misuses (if I understood correctly, Ian is of the same
opinion).

My solution would be to add the refcnt grabbing for map, maps and thread in
thread__find_map, releasing them in addr_location__put, and then making sure all
callers call it when no longer in use.

Following the same reasoning, I added refcnt grabbing also to mem_info,
branch_info (map was already refcnted, I added it also to maps for coherency),
map_symbol (as in branch_info, I added it to maps), and in other places in which
I saw a pointer was passed without refcounting.

Most changes are quite trivial, however, the changelog is huge:
48 files changed, 472 insertions(+), 157 deletions(-)
Most of them are just returns converted to goto for calling the __put functions.

Doing so, I managed to remove memory leaks caused by refcounting also in perf-
report (I wanted to try also perf top but I encountered another memory-related
issue). However, the changelog is huge and testing all of it is challenging
(especially since I can test missing puts only with ASan's LeakSanitizer and its
reports are usually full of leaks, which I am trying to fix along the way, I
will send some patches in the following days). How would you go about it? Do you
have any suggestions?

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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 :-)

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 - 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.

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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.

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 - 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

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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.

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 - maps__empty: should probably take a reader lock.

Indeed.

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 - 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.

This issue is easier to solve than expected since the map is returned as **mapp,
so it's just a matter of making sure that the caller always passes it and then
puts the refcnt.

Thanks,
Riccardo

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

- Arnaldo