Re: [PATCH v12 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

[Jens Remus]

Hello Steve!

On 01.07.2025 02:53, Steven Rostedt wrote:
> Make unwind_deferred_request() NMI-safe so tracers in NMI context can
> call it and safely request a user space stacktrace when the task exits.

> diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h

> @@ -2,6 +2,9 @@
>  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>  
> +#include <asm/local64.h>
> +#include <asm/local.h>

This creates the following circular dependency, that breaks the build on
s390 as follows, whenever local64.h is included first, so that local64_t
is not yet defined when unwind_deferred_types.h gets included down the
line:


Circular dependency:

include/linux/unwind_deferred_types.h
arch/<arch>/include/generated/asm/local64.h
include/asm-generic/local64.h
include/linux/percpu.h
include/linux/sched.h
include/linux/unwind_deferred_types.h


Compile error on s390:

  CC      net/ipv4/proc.o
In file included from ./include/linux/sched.h:49,
                 from ./include/linux/percpu.h:12,
                 from ./include/asm-generic/local64.h:5,
                 from ./arch/s390/include/generated/asm/local64.h:1,
                 from ./include/linux/u64_stats_sync.h:71,
                 from ./include/net/snmp.h:47,
                 from ./include/net/netns/mib.h:5,
                 from ./include/net/net_namespace.h:17,
                 from net/ipv4/proc.c:31:
./include/linux/unwind_deferred_types.h:17:9: error: unknown type name ‘local64_t’; did you mean ‘local_t’?
   17 |         local64_t               timestamp;
      |         ^~~~~~~~~
      |         local_t


Reason it fails on s390:

net/ipv4/proc.c
+- include/net/net_namespace.h
   +- include/net/netns/mib.h
      +- include/net/snmp.h
         +- include/linux/u64_stats_sync.h
            +- arch/s390/include/generated/asm/local64.h   <-- ISSUE: local64.h gets included before unwind_deferred_types.h
               +- include/asm-generic/local64.h
                  +- include/linux/percpu.h
                     +- include/linux/sched.h
                        +- include/linux/unwind_deferred_types.h   <-- ERROR: local64_t not yet defined


Reason it does not fail on x86:

net/ipv4/proc.c
+- include/net/net_namespace.h
   +- include/linux/workqueue.h
   |  +- include/linux/timer.h
   |     +- include/linux/ktime.h
   |        +- include/linux/jiffies.h
   |           +- include/linux/time.h
   |               +- include/linux/time32.h
   |                  +- include/linux/timex.h
   |                     +- arch/x86/include/asm/timex.h
   |                        +- arch/x86/include/asm/tsc.h
   |                           +- arch/x86/include/asm/msr.h
   |                              +- include/linux/percpu.h
   |                                 +- include/linux/sched.h
   |                                    +- include/linux/unwind_deferred_types.h   <-- OK: unwind_deferred_types.h gets included before local64.h
   |                                       +- arch/x86/include/generated/asm/local64.h
   |                                          +- include/asm-generic/local64.h
   |                                           [ +- include/linux/percpu.h ]
   +- include/net/netns/mib.h
      +- include/net/snmp.h
         +- include/linux/u64_stats_sync.h
            +- arch/x86/include/generated/asm/local64.h   <-- OK: local64.h comes after unwind_deferred_types.h
             [ +- include/asm-generic/local64.h ]
                [ +- include/linux/percpu.h ]
                   [ +- include/linux/sched.h ]
                      [ +- include/linux/unwind_deferred_types.h ]


My colleague Heiko Carstens (on Cc) suggested the following potential fix
that breaks the circular dependency, provided local[64].h does not ever
need percpu.h.  At least I verified that defconfig x86 and s390 builds
still work.

diff --git a/include/asm-generic/local.h b/include/asm-generic/local.h
index 7f97018df66f..3e7ce6a9e18e 100644
--- a/include/asm-generic/local.h
+++ b/include/asm-generic/local.h
@@ -2,7 +2,6 @@
 #ifndef _ASM_GENERIC_LOCAL_H
 #define _ASM_GENERIC_LOCAL_H
 
-#include <linux/percpu.h>
 #include <linux/atomic.h>
 #include <asm/types.h>
 
diff --git a/include/asm-generic/local64.h b/include/asm-generic/local64.h
index 14963a7a6253..1f9af89916cb 100644
--- a/include/asm-generic/local64.h
+++ b/include/asm-generic/local64.h
@@ -2,7 +2,6 @@
 #ifndef _ASM_GENERIC_LOCAL64_H
 #define _ASM_GENERIC_LOCAL64_H
 
-#include <linux/percpu.h>
 #include <asm/types.h>

> +
>  struct unwind_cache {
>  	unsigned int		nr_entries;
>  	unsigned long		entries[];
> @@ -10,8 +13,8 @@ struct unwind_cache {
>  struct unwind_task_info {
>  	struct unwind_cache	*cache;
>  	struct callback_head	work;
> -	u64			timestamp;
> -	int			pending;
> +	local64_t		timestamp;
> +	local_t			pending;
>  };
>  
>  #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@xxxxxxxxxx

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