[PATCH] rcutorture: remove unneeded check

[PATCH] rcutorture: remove unneeded check

[trix]

From: Tom Rix <trix@redhat.com>

clang static analysis reports this problem:

rcutorture.c:1999:2: warning: Called function pointer
  is null (null dereference)
        cur_ops->sync(); /* Later readers see above write. */
        ^~~~~~~~~~~~~~~

This is a false positive triggered by an earlier, later ignored
NULL check of sync() op.  By inspection of the rcu_torture_ops,
the sync() op is never uninitialized.  So this earlier check is
not needed.

Signed-off-by: Tom Rix <trix@redhat.com>
---
 kernel/rcu/rcutorture.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index beba9e7963c8..6efc03a1d623 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1989,7 +1989,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
 	unsigned long stopat;
 	static DEFINE_TORTURE_RANDOM(trs);
 
-	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+	if  (cur_ops->call && cur_ops->cb_barrier) {
 		init_rcu_head_on_stack(&fcs.rh);
 		selfpropcb = true;
 	}
-- 
2.18.1

Re: [PATCH] rcutorture: remove unneeded check

[Paul E. McKenney]

On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> From: Tom Rix <trix@redhat.com>
> 
> clang static analysis reports this problem:
> 
> rcutorture.c:1999:2: warning: Called function pointer
>   is null (null dereference)
>         cur_ops->sync(); /* Later readers see above write. */
>         ^~~~~~~~~~~~~~~
> 
> This is a false positive triggered by an earlier, later ignored
> NULL check of sync() op.  By inspection of the rcu_torture_ops,
> the sync() op is never uninitialized.  So this earlier check is
> not needed.

You lost me on this one.  This check is at the very beginning of
rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
where exactly is this check?

In any case, the check is needed because all three functions are invoked
if there is a self-propagating RCU callback that ensures that there is
always an RCU grace period outstanding.

Ah.  Is clang doing local analysis and assuming that because there was
a NULL check earlier, then the pointer might be NULL later?  That does
not seem to me to be a sound check.

So please let me know exactly what is causing clang to emit this
diagnostic.  It might or might not be worth fixing this, but either way
I need to understand the situation so as to be able to understand the
set of feasible fixes.

						Thanx, Paul

> Signed-off-by: Tom Rix <trix@redhat.com>
> ---
>  kernel/rcu/rcutorture.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7963c8..6efc03a1d623 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>  	unsigned long stopat;
>  	static DEFINE_TORTURE_RANDOM(trs);
>  
> -	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> +	if  (cur_ops->call && cur_ops->cb_barrier) {
>  		init_rcu_head_on_stack(&fcs.rh);
>  		selfpropcb = true;
>  	}
> -- 
> 2.18.1
> 

Re: [PATCH] rcutorture: remove unneeded check

[Tom Rix]

On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>> From: Tom Rix <trix@redhat.com>
>>
>> clang static analysis reports this problem:
>>
>> rcutorture.c:1999:2: warning: Called function pointer
>>   is null (null dereference)
>>         cur_ops->sync(); /* Later readers see above write. */
>>         ^~~~~~~~~~~~~~~
>>
>> This is a false positive triggered by an earlier, later ignored
>> NULL check of sync() op.  By inspection of the rcu_torture_ops,
>> the sync() op is never uninitialized.  So this earlier check is
>> not needed.
> You lost me on this one.  This check is at the very beginning of
> rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
> earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
> where exactly is this check?
>
> In any case, the check is needed because all three functions are invoked
> if there is a self-propagating RCU callback that ensures that there is
> always an RCU grace period outstanding.
>
> Ah.  Is clang doing local analysis and assuming that because there was
> a NULL check earlier, then the pointer might be NULL later?  That does
> not seem to me to be a sound check.
>
> So please let me know exactly what is causing clang to emit this
> diagnostic.  It might or might not be worth fixing this, but either way
> I need to understand the situation so as to be able to understand the
> set of feasible fixes.
>
> 						Thanx, Paul

In rcu_prog_nr() there is check for for sync.

if ( ... cur_op->sync ...

   do something

This flags in clang's static analyzer as 'could be null'

later in the function, in a reachable block it is used

cur_ops->sync()

I agree this is not a good check that's why i said is was a false positive.

However when looking closer at how cur_ops is set, it is never uninitialized.

So the check is not needed.

This is not a fix, the code works fine.  It is a small optimization.

Tom

>
>> Signed-off-by: Tom Rix <trix@redhat.com>
>> ---
>>  kernel/rcu/rcutorture.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>> index beba9e7963c8..6efc03a1d623 100644
>> --- a/kernel/rcu/rcutorture.c
>> +++ b/kernel/rcu/rcutorture.c
>> @@ -1989,7 +1989,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>>  	unsigned long stopat;
>>  	static DEFINE_TORTURE_RANDOM(trs);
>>  
>> -	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
>> +	if  (cur_ops->call && cur_ops->cb_barrier) {
>>  		init_rcu_head_on_stack(&fcs.rh);
>>  		selfpropcb = true;
>>  	}
>> -- 
>> 2.18.1
>>

Re: [PATCH] rcutorture: remove unneeded check

[Paul E. McKenney]

On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
> 
> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> > On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> >> From: Tom Rix <trix@redhat.com>
> >>
> >> clang static analysis reports this problem:
> >>
> >> rcutorture.c:1999:2: warning: Called function pointer
> >>   is null (null dereference)
> >>         cur_ops->sync(); /* Later readers see above write. */
> >>         ^~~~~~~~~~~~~~~
> >>
> >> This is a false positive triggered by an earlier, later ignored
> >> NULL check of sync() op.  By inspection of the rcu_torture_ops,
> >> the sync() op is never uninitialized.  So this earlier check is
> >> not needed.
> > You lost me on this one.  This check is at the very beginning of
> > rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
> > earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
> > where exactly is this check?
> >
> > In any case, the check is needed because all three functions are invoked
> > if there is a self-propagating RCU callback that ensures that there is
> > always an RCU grace period outstanding.
> >
> > Ah.  Is clang doing local analysis and assuming that because there was
> > a NULL check earlier, then the pointer might be NULL later?  That does
> > not seem to me to be a sound check.
> >
> > So please let me know exactly what is causing clang to emit this
> > diagnostic.  It might or might not be worth fixing this, but either way
> > I need to understand the situation so as to be able to understand the
> > set of feasible fixes.
> >
> > 						Thanx, Paul
> 
> In rcu_prog_nr() there is check for for sync.
> 
> if ( ... cur_op->sync ...
> 
>    do something
> 
> This flags in clang's static analyzer as 'could be null'
> 
> later in the function, in a reachable block it is used
> 
> cur_ops->sync()
> 
> I agree this is not a good check that's why i said is was a false positive.
> 
> However when looking closer at how cur_ops is set, it is never uninitialized.
> 
> So the check is not needed.

OK, got it, and thank you for the explanation.

> This is not a fix, the code works fine.  It is a small optimization.

Well, there really is a bug.  Yes, right now all ->sync pointers are
non-NULL, but they have not been in the past and might not be in the
future.  So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
not be called or it should return immediately without doing anything.

My current thought is something like the (untested) patch below, of
course with your Reported-by.

Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index beba9e7..44749be 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
 	unsigned long stopat;
 	static DEFINE_TORTURE_RANDOM(trs);
 
-	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+	if (!cur_ops->sync) 
+		return; // Cannot do need_resched() forward progress testing without ->sync.
+	if (cur_ops->call && cur_ops->cb_barrier) {
 		init_rcu_head_on_stack(&fcs.rh);
 		selfpropcb = true;
 	}
@@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
 
 	if (!fwd_progress)
 		return 0; /* Not requested, so don't do it. */
-	if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
-	    cur_ops == &rcu_busted_ops) {
+	if ((!cur_ops->sync && !cur_ops->call) ||
+	    !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
 		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
 		return 0;
 	}

Re: [PATCH] rcutorture: remove unneeded check

[Tom Rix]

On 10/9/20 4:50 PM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
>>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>>>> From: Tom Rix <trix@redhat.com>
>>>>
>>>> clang static analysis reports this problem:
>>>>
>>>> rcutorture.c:1999:2: warning: Called function pointer
>>>>   is null (null dereference)
>>>>         cur_ops->sync(); /* Later readers see above write. */
>>>>         ^~~~~~~~~~~~~~~
>>>>
>>>> This is a false positive triggered by an earlier, later ignored
>>>> NULL check of sync() op.  By inspection of the rcu_torture_ops,
>>>> the sync() op is never uninitialized.  So this earlier check is
>>>> not needed.
>>> You lost me on this one.  This check is at the very beginning of
>>> rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
>>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
>>> where exactly is this check?
>>>
>>> In any case, the check is needed because all three functions are invoked
>>> if there is a self-propagating RCU callback that ensures that there is
>>> always an RCU grace period outstanding.
>>>
>>> Ah.  Is clang doing local analysis and assuming that because there was
>>> a NULL check earlier, then the pointer might be NULL later?  That does
>>> not seem to me to be a sound check.
>>>
>>> So please let me know exactly what is causing clang to emit this
>>> diagnostic.  It might or might not be worth fixing this, but either way
>>> I need to understand the situation so as to be able to understand the
>>> set of feasible fixes.
>>>
>>> 						Thanx, Paul
>> In rcu_prog_nr() there is check for for sync.
>>
>> if ( ... cur_op->sync ...
>>
>>    do something
>>
>> This flags in clang's static analyzer as 'could be null'
>>
>> later in the function, in a reachable block it is used
>>
>> cur_ops->sync()
>>
>> I agree this is not a good check that's why i said is was a false positive.
>>
>> However when looking closer at how cur_ops is set, it is never uninitialized.
>>
>> So the check is not needed.
> OK, got it, and thank you for the explanation.
>
>> This is not a fix, the code works fine.  It is a small optimization.
> Well, there really is a bug.  Yes, right now all ->sync pointers are
> non-NULL, but they have not been in the past and might not be in the
> future.  So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
> not be called or it should return immediately without doing anything.
>
> My current thought is something like the (untested) patch below, of
> course with your Reported-by.
>
> Thoughts?

Yes that would be fine.

In in review these other cases need to be been take care of.

Thanks,

Reported-by: Tom Rix <trix@redhat.com>

> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7..44749be 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>  	unsigned long stopat;
>  	static DEFINE_TORTURE_RANDOM(trs);
>  
> -	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> +	if (!cur_ops->sync) 
> +		return; // Cannot do need_resched() forward progress testing without ->sync.
> +	if (cur_ops->call && cur_ops->cb_barrier) {
>  		init_rcu_head_on_stack(&fcs.rh);
>  		selfpropcb = true;
>  	}
> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>  
>  	if (!fwd_progress)
>  		return 0; /* Not requested, so don't do it. */
> -	if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
> -	    cur_ops == &rcu_busted_ops) {
> +	if ((!cur_ops->sync && !cur_ops->call) ||
> +	    !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
>  		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
>  		return 0;
>  	}
>

Re: [PATCH] rcutorture: remove unneeded check

[Paul E. McKenney]

On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
> 
> On 10/9/20 4:50 PM, Paul E. McKenney wrote:
> > On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
> >> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> >>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> >>>> From: Tom Rix <trix@redhat.com>
> >>>>
> >>>> clang static analysis reports this problem:
> >>>>
> >>>> rcutorture.c:1999:2: warning: Called function pointer
> >>>>   is null (null dereference)
> >>>>         cur_ops->sync(); /* Later readers see above write. */
> >>>>         ^~~~~~~~~~~~~~~
> >>>>
> >>>> This is a false positive triggered by an earlier, later ignored
> >>>> NULL check of sync() op.  By inspection of the rcu_torture_ops,
> >>>> the sync() op is never uninitialized.  So this earlier check is
> >>>> not needed.
> >>> You lost me on this one.  This check is at the very beginning of
> >>> rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
> >>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
> >>> where exactly is this check?
> >>>
> >>> In any case, the check is needed because all three functions are invoked
> >>> if there is a self-propagating RCU callback that ensures that there is
> >>> always an RCU grace period outstanding.
> >>>
> >>> Ah.  Is clang doing local analysis and assuming that because there was
> >>> a NULL check earlier, then the pointer might be NULL later?  That does
> >>> not seem to me to be a sound check.
> >>>
> >>> So please let me know exactly what is causing clang to emit this
> >>> diagnostic.  It might or might not be worth fixing this, but either way
> >>> I need to understand the situation so as to be able to understand the
> >>> set of feasible fixes.
> >>>
> >>> 						Thanx, Paul
> >> In rcu_prog_nr() there is check for for sync.
> >>
> >> if ( ... cur_op->sync ...
> >>
> >>    do something
> >>
> >> This flags in clang's static analyzer as 'could be null'
> >>
> >> later in the function, in a reachable block it is used
> >>
> >> cur_ops->sync()
> >>
> >> I agree this is not a good check that's why i said is was a false positive.
> >>
> >> However when looking closer at how cur_ops is set, it is never uninitialized.
> >>
> >> So the check is not needed.
> > OK, got it, and thank you for the explanation.
> >
> >> This is not a fix, the code works fine.  It is a small optimization.
> > Well, there really is a bug.  Yes, right now all ->sync pointers are
> > non-NULL, but they have not been in the past and might not be in the
> > future.  So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
> > not be called or it should return immediately without doing anything.
> >
> > My current thought is something like the (untested) patch below, of
> > course with your Reported-by.
> >
> > Thoughts?
> 
> Yes that would be fine.
> 
> In in review these other cases need to be been take care of.

I am having a difficult time interpreting this sentence, but will
optimistically assume that it means that you are good with this approach.
If my optimism is unwarranted, please let me know so I can fix whatever
might be broken.

> Reported-by: Tom Rix <trix@redhat.com>

How does the commit below look?

							Thanx, Paul

------------------------------------------------------------------------

commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Fri Oct 9 19:51:55 2020 -0700

    rcutorture: Don't do need_resched() testing if ->sync is NULL
    
    If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
    attempt to call through it.  This commit therefore flags cases where
    neither need_resched() nor call_rcu() forward-progress testing
    can be performed due to NULL function pointers, and also causes
    rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
    is NULL.
    
    Reported-by: Tom Rix <trix@redhat.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index beba9e7..44749be 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
 	unsigned long stopat;
 	static DEFINE_TORTURE_RANDOM(trs);
 
-	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+	if (!cur_ops->sync) 
+		return; // Cannot do need_resched() forward progress testing without ->sync.
+	if (cur_ops->call && cur_ops->cb_barrier) {
 		init_rcu_head_on_stack(&fcs.rh);
 		selfpropcb = true;
 	}
@@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
 
 	if (!fwd_progress)
 		return 0; /* Not requested, so don't do it. */
-	if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
-	    cur_ops == &rcu_busted_ops) {
+	if ((!cur_ops->sync && !cur_ops->call) ||
+	    !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
 		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
 		return 0;
 	}

Re: [PATCH] rcutorture: remove unneeded check

[Tom Rix]

On 10/9/20 7:57 PM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
>> On 10/9/20 4:50 PM, Paul E. McKenney wrote:
>>> On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>>>> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
>>>>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>>>>>> From: Tom Rix <trix@redhat.com>
>>>>>>
>>>>>> clang static analysis reports this problem:
>>>>>>
>>>>>> rcutorture.c:1999:2: warning: Called function pointer
>>>>>>   is null (null dereference)
>>>>>>         cur_ops->sync(); /* Later readers see above write. */
>>>>>>         ^~~~~~~~~~~~~~~
>>>>>>
>>>>>> This is a false positive triggered by an earlier, later ignored
>>>>>> NULL check of sync() op.  By inspection of the rcu_torture_ops,
>>>>>> the sync() op is never uninitialized.  So this earlier check is
>>>>>> not needed.
>>>>> You lost me on this one.  This check is at the very beginning of
>>>>> rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
>>>>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
>>>>> where exactly is this check?
>>>>>
>>>>> In any case, the check is needed because all three functions are invoked
>>>>> if there is a self-propagating RCU callback that ensures that there is
>>>>> always an RCU grace period outstanding.
>>>>>
>>>>> Ah.  Is clang doing local analysis and assuming that because there was
>>>>> a NULL check earlier, then the pointer might be NULL later?  That does
>>>>> not seem to me to be a sound check.
>>>>>
>>>>> So please let me know exactly what is causing clang to emit this
>>>>> diagnostic.  It might or might not be worth fixing this, but either way
>>>>> I need to understand the situation so as to be able to understand the
>>>>> set of feasible fixes.
>>>>>
>>>>> 						Thanx, Paul
>>>> In rcu_prog_nr() there is check for for sync.
>>>>
>>>> if ( ... cur_op->sync ...
>>>>
>>>>    do something
>>>>
>>>> This flags in clang's static analyzer as 'could be null'
>>>>
>>>> later in the function, in a reachable block it is used
>>>>
>>>> cur_ops->sync()
>>>>
>>>> I agree this is not a good check that's why i said is was a false positive.
>>>>
>>>> However when looking closer at how cur_ops is set, it is never uninitialized.
>>>>
>>>> So the check is not needed.
>>> OK, got it, and thank you for the explanation.
>>>
>>>> This is not a fix, the code works fine.  It is a small optimization.
>>> Well, there really is a bug.  Yes, right now all ->sync pointers are
>>> non-NULL, but they have not been in the past and might not be in the
>>> future.  So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
>>> not be called or it should return immediately without doing anything.
>>>
>>> My current thought is something like the (untested) patch below, of
>>> course with your Reported-by.
>>>
>>> Thoughts?
>> Yes that would be fine.
>>
>> In in review these other cases need to be been take care of.
> I am having a difficult time interpreting this sentence, but will
> optimistically assume that it means that you are good with this approach.
> If my optimism is unwarranted, please let me know so I can fix whatever
> might be broken.
>
>> Reported-by: Tom Rix <trix@redhat.com>
> How does the commit below look?

Looks fine.

Thanks

Tom

>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Fri Oct 9 19:51:55 2020 -0700
>
>     rcutorture: Don't do need_resched() testing if ->sync is NULL
>     
>     If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
>     attempt to call through it.  This commit therefore flags cases where
>     neither need_resched() nor call_rcu() forward-progress testing
>     can be performed due to NULL function pointers, and also causes
>     rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
>     is NULL.
>     
>     Reported-by: Tom Rix <trix@redhat.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7..44749be 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>  	unsigned long stopat;
>  	static DEFINE_TORTURE_RANDOM(trs);
>  
> -	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> +	if (!cur_ops->sync) 
> +		return; // Cannot do need_resched() forward progress testing without ->sync.
> +	if (cur_ops->call && cur_ops->cb_barrier) {
>  		init_rcu_head_on_stack(&fcs.rh);
>  		selfpropcb = true;
>  	}
> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>  
>  	if (!fwd_progress)
>  		return 0; /* Not requested, so don't do it. */
> -	if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
> -	    cur_ops == &rcu_busted_ops) {
> +	if ((!cur_ops->sync && !cur_ops->call) ||
> +	    !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
>  		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
>  		return 0;
>  	}
>

Re: [PATCH] rcutorture: remove unneeded check

[Paul E. McKenney]

On Fri, Oct 09, 2020 at 07:57:43PM -0700, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
> > 
> > On 10/9/20 4:50 PM, Paul E. McKenney wrote:
> > > On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
> > >> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> > >>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> > >>>> From: Tom Rix <trix@redhat.com>
> > >>>>
> > >>>> clang static analysis reports this problem:
> > >>>>
> > >>>> rcutorture.c:1999:2: warning: Called function pointer
> > >>>>   is null (null dereference)
> > >>>>         cur_ops->sync(); /* Later readers see above write. */
> > >>>>         ^~~~~~~~~~~~~~~
> > >>>>
> > >>>> This is a false positive triggered by an earlier, later ignored
> > >>>> NULL check of sync() op.  By inspection of the rcu_torture_ops,
> > >>>> the sync() op is never uninitialized.  So this earlier check is
> > >>>> not needed.
> > >>> You lost me on this one.  This check is at the very beginning of
> > >>> rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
> > >>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
> > >>> where exactly is this check?
> > >>>
> > >>> In any case, the check is needed because all three functions are invoked
> > >>> if there is a self-propagating RCU callback that ensures that there is
> > >>> always an RCU grace period outstanding.
> > >>>
> > >>> Ah.  Is clang doing local analysis and assuming that because there was
> > >>> a NULL check earlier, then the pointer might be NULL later?  That does
> > >>> not seem to me to be a sound check.

In this case, the diagnostic was clearly pointing out a latent bug, so
my bad.  So more of a code-review comment than a diagnostic.  Therefore,
if clang really wants to be in the code-review space, I suggest that it
more clearly explain its code-review feedback.  ;-)

							Thanx, Paul

> > >>> So please let me know exactly what is causing clang to emit this
> > >>> diagnostic.  It might or might not be worth fixing this, but either way
> > >>> I need to understand the situation so as to be able to understand the
> > >>> set of feasible fixes.
> > >>>
> > >>> 						Thanx, Paul
> > >> In rcu_prog_nr() there is check for for sync.
> > >>
> > >> if ( ... cur_op->sync ...
> > >>
> > >>    do something
> > >>
> > >> This flags in clang's static analyzer as 'could be null'
> > >>
> > >> later in the function, in a reachable block it is used
> > >>
> > >> cur_ops->sync()
> > >>
> > >> I agree this is not a good check that's why i said is was a false positive.
> > >>
> > >> However when looking closer at how cur_ops is set, it is never uninitialized.
> > >>
> > >> So the check is not needed.
> > > OK, got it, and thank you for the explanation.
> > >
> > >> This is not a fix, the code works fine.  It is a small optimization.
> > > Well, there really is a bug.  Yes, right now all ->sync pointers are
> > > non-NULL, but they have not been in the past and might not be in the
> > > future.  So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
> > > not be called or it should return immediately without doing anything.
> > >
> > > My current thought is something like the (untested) patch below, of
> > > course with your Reported-by.
> > >
> > > Thoughts?
> > 
> > Yes that would be fine.
> > 
> > In in review these other cases need to be been take care of.
> 
> I am having a difficult time interpreting this sentence, but will
> optimistically assume that it means that you are good with this approach.
> If my optimism is unwarranted, please let me know so I can fix whatever
> might be broken.
> 
> > Reported-by: Tom Rix <trix@redhat.com>
> 
> How does the commit below look?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Fri Oct 9 19:51:55 2020 -0700
> 
>     rcutorture: Don't do need_resched() testing if ->sync is NULL
>     
>     If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
>     attempt to call through it.  This commit therefore flags cases where
>     neither need_resched() nor call_rcu() forward-progress testing
>     can be performed due to NULL function pointers, and also causes
>     rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
>     is NULL.
>     
>     Reported-by: Tom Rix <trix@redhat.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7..44749be 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>  	unsigned long stopat;
>  	static DEFINE_TORTURE_RANDOM(trs);
>  
> -	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> +	if (!cur_ops->sync) 
> +		return; // Cannot do need_resched() forward progress testing without ->sync.
> +	if (cur_ops->call && cur_ops->cb_barrier) {
>  		init_rcu_head_on_stack(&fcs.rh);
>  		selfpropcb = true;
>  	}
> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>  
>  	if (!fwd_progress)
>  		return 0; /* Not requested, so don't do it. */
> -	if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
> -	    cur_ops == &rcu_busted_ops) {
> +	if ((!cur_ops->sync && !cur_ops->call) ||
> +	    !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
>  		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
>  		return 0;
>  	}

Re: [PATCH] rcutorture: remove unneeded check

[Tom Rix]

On 10/10/20 10:57 AM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 07:57:43PM -0700, Paul E. McKenney wrote:
>> On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
>>> On 10/9/20 4:50 PM, Paul E. McKenney wrote:
>>>> On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>>>>> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
>>>>>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>>>>>>> From: Tom Rix <trix@redhat.com>
>>>>>>>
>>>>>>> clang static analysis reports this problem:
>>>>>>>
>>>>>>> rcutorture.c:1999:2: warning: Called function pointer
>>>>>>>   is null (null dereference)
>>>>>>>         cur_ops->sync(); /* Later readers see above write. */
>>>>>>>         ^~~~~~~~~~~~~~~
>>>>>>>
>>>>>>> This is a false positive triggered by an earlier, later ignored
>>>>>>> NULL check of sync() op.  By inspection of the rcu_torture_ops,
>>>>>>> the sync() op is never uninitialized.  So this earlier check is
>>>>>>> not needed.
>>>>>> You lost me on this one.  This check is at the very beginning of
>>>>>> rcu_torture_fwd_prog_nr().  Or are you saying that clang is seeing an
>>>>>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers?  If so,
>>>>>> where exactly is this check?
>>>>>>
>>>>>> In any case, the check is needed because all three functions are invoked
>>>>>> if there is a self-propagating RCU callback that ensures that there is
>>>>>> always an RCU grace period outstanding.
>>>>>>
>>>>>> Ah.  Is clang doing local analysis and assuming that because there was
>>>>>> a NULL check earlier, then the pointer might be NULL later?  That does
>>>>>> not seem to me to be a sound check.
> In this case, the diagnostic was clearly pointing out a latent bug, so
> my bad.  So more of a code-review comment than a diagnostic.  Therefore,
> if clang really wants to be in the code-review space, I suggest that it
> more clearly explain its code-review feedback.  ;-)

Ok.

I have another a change in other area queued up.

I'll improve it's and future comments.

Tom

> 							Thanx, Paul
>
>>>>>> So please let me know exactly what is causing clang to emit this
>>>>>> diagnostic.  It might or might not be worth fixing this, but either way
>>>>>> I need to understand the situation so as to be able to understand the
>>>>>> set of feasible fixes.
>>>>>>
>>>>>> 						Thanx, Paul
>>>>> In rcu_prog_nr() there is check for for sync.
>>>>>
>>>>> if ( ... cur_op->sync ...
>>>>>
>>>>>    do something
>>>>>
>>>>> This flags in clang's static analyzer as 'could be null'
>>>>>
>>>>> later in the function, in a reachable block it is used
>>>>>
>>>>> cur_ops->sync()
>>>>>
>>>>> I agree this is not a good check that's why i said is was a false positive.
>>>>>
>>>>> However when looking closer at how cur_ops is set, it is never uninitialized.
>>>>>
>>>>> So the check is not needed.
>>>> OK, got it, and thank you for the explanation.
>>>>
>>>>> This is not a fix, the code works fine.  It is a small optimization.
>>>> Well, there really is a bug.  Yes, right now all ->sync pointers are
>>>> non-NULL, but they have not been in the past and might not be in the
>>>> future.  So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
>>>> not be called or it should return immediately without doing anything.
>>>>
>>>> My current thought is something like the (untested) patch below, of
>>>> course with your Reported-by.
>>>>
>>>> Thoughts?
>>> Yes that would be fine.
>>>
>>> In in review these other cases need to be been take care of.
>> I am having a difficult time interpreting this sentence, but will
>> optimistically assume that it means that you are good with this approach.
>> If my optimism is unwarranted, please let me know so I can fix whatever
>> might be broken.
>>
>>> Reported-by: Tom Rix <trix@redhat.com>
>> How does the commit below look?
>>
>> 							Thanx, Paul
>>
>> ------------------------------------------------------------------------
>>
>> commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
>> Author: Paul E. McKenney <paulmck@kernel.org>
>> Date:   Fri Oct 9 19:51:55 2020 -0700
>>
>>     rcutorture: Don't do need_resched() testing if ->sync is NULL
>>     
>>     If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
>>     attempt to call through it.  This commit therefore flags cases where
>>     neither need_resched() nor call_rcu() forward-progress testing
>>     can be performed due to NULL function pointers, and also causes
>>     rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
>>     is NULL.
>>     
>>     Reported-by: Tom Rix <trix@redhat.com>
>>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>
>> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>> index beba9e7..44749be 100644
>> --- a/kernel/rcu/rcutorture.c
>> +++ b/kernel/rcu/rcutorture.c
>> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>>  	unsigned long stopat;
>>  	static DEFINE_TORTURE_RANDOM(trs);
>>  
>> -	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
>> +	if (!cur_ops->sync) 
>> +		return; // Cannot do need_resched() forward progress testing without ->sync.
>> +	if (cur_ops->call && cur_ops->cb_barrier) {
>>  		init_rcu_head_on_stack(&fcs.rh);
>>  		selfpropcb = true;
>>  	}
>> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>>  
>>  	if (!fwd_progress)
>>  		return 0; /* Not requested, so don't do it. */
>> -	if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
>> -	    cur_ops == &rcu_busted_ops) {
>> +	if ((!cur_ops->sync && !cur_ops->call) ||
>> +	    !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
>>  		VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
>>  		return 0;
>>  	}