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Date:   Mon, 2 Jan 2023 12:33:35 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Thomas Richter <tmricht@linux.ibm.com>,
        Namhyung Kim <namhyung@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        svens@linux.ibm.com, gor@linux.ibm.com, sumanthk@linux.ibm.com,
        hca@linux.ibm.com
Subject: Re: [PATCH] perf lock: Fix core dump in command perf lock contention
Message-ID: <Y7L5T2IHdovfLgWp@kernel.org>
References: <20221230102627.2410847-1-tmricht@linux.ibm.com>
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Em Fri, Dec 30, 2022 at 11:26:27AM +0100, Thomas Richter escreveu:
> The test case perf lock contention dumps core on s390. Run the following
> commands:
>  # ./perf lock record -- ./perf bench sched messaging
>  # Running 'sched/messaging' benchmark:
>  # 20 sender and receiver processes per group
>  # 10 groups == 400 processes run
> 
>      Total time: 2.799 [sec]
>  [ perf record: Woken up 1 times to write data ]
>  [ perf record: Captured and wrote 0.073 MB perf.data (100 samples) ]
>  #
>  # ./perf lock contention
>  Segmentation fault (core dumped)
>  #
> 
> The function call stack is lengthy, here are the top 5 functions:
>  # gdb ./perf core.24048
>  GNU gdb (GDB) Fedora Linux 12.1-6.fc37
>  Copyright (C) 2022 Free Software Foundation, Inc.
>  Core was generated by `./perf lock contention'.
>  Program terminated with signal SIGSEGV, Segmentation fault.
>  #0  0x00000000011dd25c in machine__is_lock_function (machine=0x3029e28,
>          addr=1789230) at util/machine.c:3356
>          3356 machine->sched.text_end = kmap->unmap_ip(kmap, sym->start);
> 
>  (gdb) where
>   #0  0x00000000011dd25c in machine__is_lock_function (machine=0x3029e28,\
>          addr=1789230) at util/machine.c:3356
>   #1  0x000000000109f244 in callchain_id (evsel=0x30313e0,\
>          sample=0x3ffea4f77d0) at builtin-lock.c:957
>   #2  0x000000000109e094 in get_key_by_aggr_mode (key=0x3ffea4f7290,\
>          addr=27758136, evsel=0x30313e0, sample=0x3ffea4f77d0) \
>          at builtin-lock.c:586
>   #3  0x000000000109f4d0 in report_lock_contention_begin_event \
>          (evsel=0x30313e0, sample=0x3ffea4f77d0)
>          at builtin-lock.c:1004
>   #4  0x00000000010a00ae in evsel__process_contention_begin \
>          (evsel=0x30313e0, sample=0x3ffea4f77d0)
>          at builtin-lock.c:1254
>   #5  0x00000000010a0e14 in process_sample_event (tool=0x3ffea4f8480, \
>          event=0x3ff85601ef8, sample=0x3ffea4f77d0,
>          evsel=0x30313e0, machine=0x3029e28) at builtin-lock.c:1464
>           sample=0x3ffea4f77d0, evsel=0x30313e0, machine=0x3029e28) \
> 	 at util/session.c:1523
>   .....
> 
> The issue is in function machine__is_lock_function() in file
> ./util/machine.c lines 3355:
>    /* should not fail from here */
>    sym = machine__find_kernel_symbol_by_name(machine, "__sched_text_end",
> 		                             &kmap);
>    machine->sched.text_end = kmap->unmap_ip(kmap, sym->start)
> 
> On s390 the symbol __sched_text_end is *NOT* in the symbol list and the
> resulting pointer sym is set to NULL. The sym->start is then a NULL pointer
> access and generates the core dump.
> 
> The reason why __sched_text_end is not in the symbol list on s390 is
> simple:
> When the symbol list is created at perf start up with function calls
>   dso__load
>   +--> dso__load_vmlinux_path
>        +--> dso__load_vmlinux
>             +--> dso__load_sym
> 	         +--> dso__load_sym_internal (reads kernel symbols)
> 		 +--> symbols__fixup_end
> 		 +--> symbols__fixup_duplicate
> 
> The issue is in function symbols__fixup_duplicate(). It deletes all
> symbols with have the same address. On s390
>  # nm -g  ~/linux/vmlinux| fgrep c68390
>  0000000000c68390 T __cpuidle_text_start
>  0000000000c68390 T __sched_text_end
>  #
> two symbols have identical addresses and __sched_text_end is considered
> duplicate (in ascending sort order) and removed from the symbol list.
> Therefore it is missing and an invalid pointer reference occurs.
> The code checks for symbol __sched_text_start and when it exists assumes
> symbol __sched_text_end is also in the symbol table. However this is
> not the case on s390.
> 
> Same situation exists for symbol __lock_text_start:
> 0000000000c68770 T __cpuidle_text_end
> 0000000000c68770 T __lock_text_start
> This symbol is also removed from the symbol table but used in function
> machine__is_lock_function().
> 
> To fix this and keep duplicate symbols in the symbol table, set
> symbol_conf.allow_aliases to true. This prevents the removal of duplicate
> symbols in function symbols__fixup_duplicate().
> 
> Output After:
>  # ./perf lock contention
>  contended total wait  max wait  avg wait    type   caller
> 
>         48   124.39 ms 123.99 ms   2.59 ms rwsem:W unlink_anon_vmas+0x24a
>         47    83.68 ms  83.26 ms   1.78 ms rwsem:W free_pgtables+0x132
>          5    41.22 us  10.55 us   8.24 us rwsem:W free_pgtables+0x140
>          4    40.12 us  20.55 us  10.03 us rwsem:W copy_process+0x1ac8
>  #
> 
> Fixes: cc2367eebb0c ("machine: Adopt is_lock_function() from builtin-lock.c")

Humm, is that really the cset that introduces the problem? It just moves
things around, the cset that introduced the is_lock_function() function,
that assumed that __sched_text_end was always available was:

commit 0d2997f750d1de394231bc22768dab94a5b5db2f
Author: Namhyung Kim <namhyung@kernel.org>
Date:   Wed Jun 15 09:32:22 2022 -0700

    perf lock: Look up callchain for the contended locks

---

Right? Namhyung? Can you spot any problem in enabling duplicates as a
fix?

- Arnaldo

> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
> ---
>  tools/perf/builtin-lock.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
> index 718b82bfcdff..506c2fe42d52 100644
> --- a/tools/perf/builtin-lock.c
> +++ b/tools/perf/builtin-lock.c
> @@ -1670,6 +1670,7 @@ static int __cmd_report(bool display_info)
>  
>  	/* for lock function check */
>  	symbol_conf.sort_by_name = true;
> +	symbol_conf.allow_aliases = true;
>  	symbol__init(&session->header.env);
>  
>  	if (!data.is_pipe) {
> @@ -1757,6 +1758,7 @@ static int __cmd_contention(int argc, const char **argv)
>  
>  	/* for lock function check */
>  	symbol_conf.sort_by_name = true;
> +	symbol_conf.allow_aliases = true;
>  	symbol__init(&session->header.env);
>  
>  	if (use_bpf) {
> -- 
> 2.38.1

-- 

- Arnaldo