Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Mathieu Desnoyers via lttng-dev]

On 2023-03-17 13:02, Ondřej Surý wrote:

On 17. 3. 2023, at 14:44, Mathieu Desnoyers  
wrote:

I would indeed like to remove all the custom atomics assembly code from liburcu 
now that there are good atomics support in the major compilers (gcc and clang).


Here's very preliminary implementation:

https://gitlab.isc.org/isc-projects/userspace-rcu/-/merge_requests/2

I just did something wrong somewhere along the path and it doesn't compile now,
but it did for me locally.

I am submitting this now as it's 18:00 Friday evening and my kids are starting 
to
be angry at me :).

This will need some more work - I think some of the cmm_ macros might be dropped
now, and somebody who does that more often than I should take a look at the 
memory
orderings.


A few comments:

cmm_barrier() should rather be __atomic_signal_fence().

Also I notice this macro pattern (coding style):

#define uatomic_set(addr, v) __atomic_store_n((addr), (v), __ATOMIC_RELEASE)

The extra parentheses for parameters are not needed, because the comma is pretty
much the last operator in terms of priority. The following would be preferred
specifically because those are separated by comma:

#define uatomic_set(addr, v) __atomic_store_n(addr, v, __ATOMIC_RELEASE)

Our memory barrier semantic are similar to the Linux kernel, where the following
imply ACQ_REL because they return something: cmpxchg, add_return, sub_return, 
xchg.

The rest (add, sub, and, or, inc, dec) are __ATOMIC_RELAXED. Note that
cmm_smp_mb__before/after_uatomic_*() need to be implemented as
__atomic_thread_fence(__ATOMIC_ACQ_REL).

There are some architectures where we will want to keep a specialized version
of those add, sub, and, or, inc, dec operations which include the ACQ_REL 
semantic,
e.g. x86, where this is implied by the LOCK prefix. For those the 
cmm_smp_mb__before/after_uatomic_*()
will be no-ops.

The CMM_STORE_SHARED is not meant to have a RELEASE semantic. It is meant to
update variables that don't need the release ordering. The ATOMIC_CONSUME was
not the intent at the CMM_LOAD_SHARED level neither.

(this is just from looking around at the patches, it would be better if we can 
have the
patches posted to the mailing list for further discussion)

Thanks!

Mathieu




Ondrej
--
Ondřej Surý (He/Him)
ond...@sury.org



--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Ondřej Surý via lttng-dev]

> On 17. 3. 2023, at 14:44, Mathieu Desnoyers  
> wrote:
> 
> I would indeed like to remove all the custom atomics assembly code from 
> liburcu now that there are good atomics support in the major compilers (gcc 
> and clang). 

Here's very preliminary implementation:

https://gitlab.isc.org/isc-projects/userspace-rcu/-/merge_requests/2

I just did something wrong somewhere along the path and it doesn't compile now,
but it did for me locally.

I am submitting this now as it's 18:00 Friday evening and my kids are starting 
to
be angry at me :).

This will need some more work - I think some of the cmm_ macros might be dropped
now, and somebody who does that more often than I should take a look at the 
memory
orderings.

Ondrej
--
Ondřej Surý (He/Him)
ond...@sury.org

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Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Duncan Sands via lttng-dev]

Hi Ondřej,


I have two questions:

1. I've tried to help TSAN by replacing the custom atomics with __atomic gcc
   primitives - that seems to work pretty well.  Note that using C11 stdatomics
   is frankly not possible here because it would require wrapping everything 
into
   _Atomic().


I'm using the attached patch which does much the same.

Ciao, Duncan.diff --git a/External/UserspaceRCU/userspace-rcu/include/urcu/system.h b/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
index faae390554..82898d1dbe 100644
--- a/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
+++ b/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
@@ -26,34 +26,45 @@
  * Identify a shared load. A cmm_smp_rmc() or cmm_smp_mc() should come
  * before the load.
  */
-#define _CMM_LOAD_SHARED(p)	   CMM_ACCESS_ONCE(p)
+#define _CMM_LOAD_SHARED(p)	\
+	__extension__		\
+	({			\
+		__typeof__(p) v;\
+		__atomic_load(, , __ATOMIC_RELAXED);	\
+		v;		\
+	})
 
 /*
  * Load a data from shared memory, doing a cache flush if required.
  */
-#define CMM_LOAD_SHARED(p)			\
-	__extension__			\
-	({\
-		cmm_smp_rmc();		\
-		_CMM_LOAD_SHARED(p);	\
+#define CMM_LOAD_SHARED(p)	\
+	__extension__		\
+	({			\
+		__typeof__(p) v;\
+		__atomic_load(, , __ATOMIC_ACQUIRE);	\
+		v;		\
 	})
 
 /*
  * Identify a shared store. A cmm_smp_wmc() or cmm_smp_mc() should
  * follow the store.
  */
-#define _CMM_STORE_SHARED(x, v)	__extension__ ({ CMM_ACCESS_ONCE(x) = (v); })
+#define _CMM_STORE_SHARED(x, v)	\
+	__extension__		\
+	({			\
+		__typeof__(x) w = v;\
+		__atomic_store(, , __ATOMIC_RELAXED);	\
+	})
 
 /*
  * Store v into x, where x is located in shared memory. Performs the
  * required cache flush after writing. Returns v.
  */
-#define CMM_STORE_SHARED(x, v)		\
-	__extension__			\
-	({\
-		__typeof__(x) _v = _CMM_STORE_SHARED(x, v);		\
-		cmm_smp_wmc();		\
-		_v = _v;	/* Work around clang "unused result" */	\
+#define CMM_STORE_SHARED(x, v)	\
+	__extension__		\
+	({			\
+		__typeof__(x) w = v;\
+		__atomic_store(, , __ATOMIC_RELEASE);	\
 	})
 
 #endif /* _URCU_SYSTEM_H */
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Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Mathieu Desnoyers via lttng-dev]

On 2023-03-17 11:50, Ondřej Surý wrote:

On 17. 3. 2023, at 14:44, Mathieu Desnoyers  
wrote:

Sure, can you please submit the patch as a separate email with subject/commit 
message/signed-off-by tag ?



https://gitlab.isc.org/isc-projects/userspace-rcu/-/merge_requests/1.patch

Would this work for you?

Or do you need to have the patch attached?


Having the patch attached (e.g. using git send-email) would be better, 
but I don't mind downloading the file for this time. Merged into liburcu 
master branch, thanks!


Mathieu



Ondrej
--
Ondřej Surý (He/Him)
ond...@sury.org



--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Ondřej Surý via lttng-dev]

> On 17. 3. 2023, at 14:44, Mathieu Desnoyers  
> wrote:
> 
> Sure, can you please submit the patch as a separate email with subject/commit 
> message/signed-off-by tag ?


https://gitlab.isc.org/isc-projects/userspace-rcu/-/merge_requests/1.patch

Would this work for you?

Or do you need to have the patch attached?

Ondrej
--
Ondřej Surý (He/Him)
ond...@sury.org

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Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Mathieu Desnoyers via lttng-dev]

On 2023-03-14 08:26, Ondřej Surý via lttng-dev wrote:

Hey,

we use ThreadSanitizer in BIND 9 CI quite extensively and with userspace-rcu
it's lit like an American house on Saturnalia ;).


Haha, I have no doubt about it. Userspace RCU is all about concurrent 
accesses, and so far possesses no TSAN annotations.




I have two questions:

1. I've tried to help TSAN by replacing the custom atomics with __atomic gcc
   primitives - that seems to work pretty well.  Note that using C11 stdatomics
   is frankly not possible here because it would require wrapping everything 
into
   _Atomic().


Agreed. gcc __atomic seems to be the way to go.



   Do you want me to contribute this back? And how should I plug this into the
   existing structure?  This touches:

include/urcu/static/pointer.h
include/urcu/system.h
include/urcu/uatomic.h


I would indeed like to remove all the custom atomics assembly code from 
liburcu now that there are good atomics support in the major compilers 
(gcc and clang). I am also tempted to bump the base compiler version 
requirements (for both gcc and clang) to something less ancient than 
what we have currently for the next liburcu releases if it helps us rely 
on non-buggy atomics implementations. The currently supported compilers 
are stated in the README.md file:


"Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 
or better.


The C compiler used needs to support at least C99. The C++ compiler used
needs to support at least C++11.

The GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
supported, with the following exceptions:

  - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
accesses to offsets in a TLS structure on 32-bit x86. These 
versions are

therefore not compatible with `liburcu` on x86 32-bit
(i386, i486, i586, i686).
The problem has been reported to the GCC community:

  - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
See 
  - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic 
builtins

support. For ARM this was introduced with GCC 4.4:
.
  - Linux aarch64 depends on GCC 5.1 or better because prior versions
perform unsafe access to deallocated stack.

Clang version 3.0 (based on LLVM 3.0) is supported."

For gcc, I wonder if gcc-4.8 has appropriate support for __atomic on all 
supported architectures supported by liburcu ?


I also wonder what would be a good conservative baseline version for clang.

As we introduce a newer compiler baseline version, I would be tempted to 
add a compiler version detection in include/urcu/compiler.h and #warn 
whenever the compiler is too old. This is similar to what we do for the
compiler disallow list with URCU_GCC_VERSION, but enforced with a 
warning rather than a #error. The last thing I want is to end up wasting 
people's time due to compiling with a buggy old compiler, so I favor a 
"fail early" approach.





2. I know there's KTSAN, so it must work somehow, but was there any success
   on using ThreadSanitizer on projects using Userspace-RCU?  It mostly seems
   to highlight the CDS parts of the code.


Not AFAIK.



I can help TSAN to understand some of the code or suppress some of the warnings,
but I do want to prevent the code to be full of stuff like this:

static void
destroy_adbname_rcu_head(struct rcu_head *rcu_head) {
 dns_adbname_t *adbname = caa_container_of(rcu_head, dns_adbname_t,
   rcu_head);

#ifdef __SANITIZE_THREAD__
 SPINLOCK(>lock);
 SPINUNLOCK(>lock);
#endif

 destroy_adbname(adbname);
}


Indeed, we'd want to improve the liburcu header files and implementation 
by adding the appropriate annotation there.




I am absolutely sure that the adbname can be destroyed here (because of the
reference counting), but TSAN had a problem with it. Doing the "fake" barrier
with a spinlock here made it stop consider this to be a data race.

I also had to disable the auto_resize of cds_lfht when running under TSAN.

I am also worried that by hiding some code from TSAN, we might miss a legitimate
error.

All I found using Google was this notice from 2014:
https://www.mail-archive.com/valgrind-users@lists.sourceforge.net/msg05121.html

and perhaps this:
https://github.com/google/sanitizers/issues/1415

(Perhaps, I should look into annotating urcu code with TSAN annotations?)


Yes, I suspect we'll want to add TSAN annotation to liburcu code, and 
perhaps Helgrind and DRD annotations as well while we are at it. Those 
tools are very valuable development tools, which makes it worthwhile to 
add the relevant annotations to help them figure out liburcu intricacies.






3. As an extra bonus, this is going to be needed with clang-17 as noreturn is 
now
reserved word:


Sure, can 

[lttng-dev] userspace-rcu and ThreadSanitizer

[Ondřej Surý via lttng-dev]

Hey,

we use ThreadSanitizer in BIND 9 CI quite extensively and with userspace-rcu
it's lit like an American house on Saturnalia ;).

I have two questions:

1. I've tried to help TSAN by replacing the custom atomics with __atomic gcc
  primitives - that seems to work pretty well.  Note that using C11 stdatomics
  is frankly not possible here because it would require wrapping everything into
  _Atomic().

  Do you want me to contribute this back? And how should I plug this into the
  existing structure?  This touches:

include/urcu/static/pointer.h
include/urcu/system.h
include/urcu/uatomic.h


2. I know there's KTSAN, so it must work somehow, but was there any success
  on using ThreadSanitizer on projects using Userspace-RCU?  It mostly seems
  to highlight the CDS parts of the code.

I can help TSAN to understand some of the code or suppress some of the warnings,
but I do want to prevent the code to be full of stuff like this:

static void
destroy_adbname_rcu_head(struct rcu_head *rcu_head) {
dns_adbname_t *adbname = caa_container_of(rcu_head, dns_adbname_t,
  rcu_head);

#ifdef __SANITIZE_THREAD__
SPINLOCK(>lock);
SPINUNLOCK(>lock);
#endif

destroy_adbname(adbname);
}

I am absolutely sure that the adbname can be destroyed here (because of the
reference counting), but TSAN had a problem with it. Doing the "fake" barrier
with a spinlock here made it stop consider this to be a data race.

I also had to disable the auto_resize of cds_lfht when running under TSAN.

I am also worried that by hiding some code from TSAN, we might miss a legitimate
error.

All I found using Google was this notice from 2014:
https://www.mail-archive.com/valgrind-users@lists.sourceforge.net/msg05121.html

and perhaps this:
https://github.com/google/sanitizers/issues/1415

(Perhaps, I should look into annotating urcu code with TSAN annotations?)



3. As an extra bonus, this is going to be needed with clang-17 as noreturn is 
now
reserved word:

diff --git a/include/urcu/uatomic/generic.h b/include/urcu/uatomic/generic.h
index 89d1cfa..c3762b0 100644
--- a/include/urcu/uatomic/generic.h
+++ b/include/urcu/uatomic/generic.h
@@ -38,7 +38,7 @@ extern "C" {
 #endif

 #if !defined __OPTIMIZE__  || defined UATOMIC_NO_LINK_ERROR
-static inline __attribute__((always_inline, noreturn))
+static inline __attribute__((always_inline, __noreturn__))
 void _uatomic_link_error(void)
 {
 #ifdef ILLEGAL_INSTR
diff --git a/src/urcu-call-rcu-impl.h b/src/urcu-call-rcu-impl.h
index 187727e..cc76f53 100644
--- a/src/urcu-call-rcu-impl.h
+++ b/src/urcu-call-rcu-impl.h
@@ -1055,7 +1055,7 @@ void urcu_register_rculfhash_atfork(struct urcu_atfork 
*atfork)
  * This unregistration function is deprecated, meant only for internal
  * use by rculfhash.
  */
-__attribute__((noreturn))
+__attribute__((__noreturn__))
 void urcu_unregister_rculfhash_atfork(struct urcu_atfork *atfork 
__attribute__((unused)))
 {
urcu_die(EPERM);


Ondrej
--
Ondřej Surý (He/Him)
ond...@sury.org

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