[Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Jiunn Chang]

The UP.rst file calls for locks acquired within RCU callback functions
to use _irq variants (spin_lock_irqsave() or similar), which does work,
but can be overkill.  This commit therefore instead calls for _bh variants
(spin_lock_bh() or similar), while noting that _irq does work.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
---
 Documentation/RCU/UP.rst | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst
index 67715a47ae89..e26dda27430c 100644
--- a/Documentation/RCU/UP.rst
+++ b/Documentation/RCU/UP.rst
@@ -113,12 +113,13 @@ Answer to Quick Quiz #1:
 Answer to Quick Quiz #2:
 	What locking restriction must RCU callbacks respect?
 
-	Any lock that is acquired within an RCU callback must be
-	acquired elsewhere using an _irq variant of the spinlock
-	primitive.  For example, if "mylock" is acquired by an
-	RCU callback, then a process-context acquisition of this
-	lock must use something like spin_lock_irqsave() to
-	acquire the lock.
+	Any lock that is acquired within an RCU callback must be acquired
+	elsewhere using an _bh variant of the spinlock primitive.
+	For example, if "mylock" is acquired by an RCU callback, then
+	a process-context acquisition of this lock must use something
+	like spin_lock_bh() to acquire the lock.  Please note that
+	it is also OK to use _irq variants of spinlocks, for example,
+	spin_lock_irqsave().
 
 	If the process-context code were to simply use spin_lock(),
 	then, since RCU callbacks can be invoked from softirq context,
-- 
2.22.0

Re: [Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Shuah Khan]

On 6/27/19 3:01 PM, Jiunn Chang wrote:
> The UP.rst file calls for locks acquired within RCU callback functions
> to use _irq variants (spin_lock_irqsave() or similar), which does work,
> but can be overkill.  This commit therefore instead calls for _bh variants
> (spin_lock_bh() or similar), while noting that _irq does work.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Should this by Suggested-by?

> Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
> ---
>   Documentation/RCU/UP.rst | 13 +++++++------
>   1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst
> index 67715a47ae89..e26dda27430c 100644
> --- a/Documentation/RCU/UP.rst
> +++ b/Documentation/RCU/UP.rst
> @@ -113,12 +113,13 @@ Answer to Quick Quiz #1:
>   Answer to Quick Quiz #2:
>   	What locking restriction must RCU callbacks respect?
>   
> -	Any lock that is acquired within an RCU callback must be
> -	acquired elsewhere using an _irq variant of the spinlock
> -	primitive.  For example, if "mylock" is acquired by an
> -	RCU callback, then a process-context acquisition of this
> -	lock must use something like spin_lock_irqsave() to
> -	acquire the lock.
> +	Any lock that is acquired within an RCU callback must be acquired
> +	elsewhere using an _bh variant of the spinlock primitive.
> +	For example, if "mylock" is acquired by an RCU callback, then
> +	a process-context acquisition of this lock must use something
> +	like spin_lock_bh() to acquire the lock.  Please note that
> +	it is also OK to use _irq variants of spinlocks, for example,
> +	spin_lock_irqsave().
>   
>   	If the process-context code were to simply use spin_lock(),
>   	then, since RCU callbacks can be invoked from softirq context,
> 

thanks,
-- Shuah

Re: [Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Paul E. McKenney]

On Thu, Jun 27, 2019 at 04:01:35PM -0600, Shuah Khan wrote:
> On 6/27/19 3:01 PM, Jiunn Chang wrote:
> >The UP.rst file calls for locks acquired within RCU callback functions
> >to use _irq variants (spin_lock_irqsave() or similar), which does work,
> >but can be overkill.  This commit therefore instead calls for _bh variants
> >(spin_lock_bh() or similar), while noting that _irq does work.
> >
> >Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> 
> Should this by Suggested-by?

I wrote it and Jiunn converted my change to .rst, so I believe that
this is correct as is.

							Thanx, Paul

> >Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
> >---
> >  Documentation/RCU/UP.rst | 13 +++++++------
> >  1 file changed, 7 insertions(+), 6 deletions(-)
> >
> >diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst
> >index 67715a47ae89..e26dda27430c 100644
> >--- a/Documentation/RCU/UP.rst
> >+++ b/Documentation/RCU/UP.rst
> >@@ -113,12 +113,13 @@ Answer to Quick Quiz #1:
> >  Answer to Quick Quiz #2:
> >  	What locking restriction must RCU callbacks respect?
> >-	Any lock that is acquired within an RCU callback must be
> >-	acquired elsewhere using an _irq variant of the spinlock
> >-	primitive.  For example, if "mylock" is acquired by an
> >-	RCU callback, then a process-context acquisition of this
> >-	lock must use something like spin_lock_irqsave() to
> >-	acquire the lock.
> >+	Any lock that is acquired within an RCU callback must be acquired
> >+	elsewhere using an _bh variant of the spinlock primitive.
> >+	For example, if "mylock" is acquired by an RCU callback, then
> >+	a process-context acquisition of this lock must use something
> >+	like spin_lock_bh() to acquire the lock.  Please note that
> >+	it is also OK to use _irq variants of spinlocks, for example,
> >+	spin_lock_irqsave().
> >  	If the process-context code were to simply use spin_lock(),
> >  	then, since RCU callbacks can be invoked from softirq context,
> >
> 
> thanks,
> -- Shuah
> 

Re: [Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Shuah Khan]

On 6/27/19 4:10 PM, Paul E. McKenney wrote:
> On Thu, Jun 27, 2019 at 04:01:35PM -0600, Shuah Khan wrote:
>> On 6/27/19 3:01 PM, Jiunn Chang wrote:
>>> The UP.rst file calls for locks acquired within RCU callback functions
>>> to use _irq variants (spin_lock_irqsave() or similar), which does work,
>>> but can be overkill.  This commit therefore instead calls for _bh variants
>>> (spin_lock_bh() or similar), while noting that _irq does work.
>>>
>>> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
>>
>> Should this by Suggested-by?
> 
> I wrote it and Jiunn converted my change to .rst, so I believe that
> this is correct as is.
> 

Great.

thanks,
-- Shuah

Re: [Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Steven Rostedt]

On Thu, 27 Jun 2019 15:10:45 -0700
"Paul E. McKenney" <paulmck@linux.ibm.com> wrote:

> On Thu, Jun 27, 2019 at 04:01:35PM -0600, Shuah Khan wrote:
> > On 6/27/19 3:01 PM, Jiunn Chang wrote:  
> > >The UP.rst file calls for locks acquired within RCU callback functions
> > >to use _irq variants (spin_lock_irqsave() or similar), which does work,
> > >but can be overkill.  This commit therefore instead calls for _bh variants
> > >(spin_lock_bh() or similar), while noting that _irq does work.
> > >
> > >Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>  
> > 
> > Should this by Suggested-by?  
> 
> I wrote it and Jiunn converted my change to .rst, so I believe that
> this is correct as is.

Note, you did send Jiunn an explicit Signed-off-by when you wrote it,
correct? As Signed-off-by is equivalent to a signature.

-- Steve

Re: [Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Paul E. McKenney]

On Thu, Jun 27, 2019 at 06:29:38PM -0400, Steven Rostedt wrote:
> On Thu, 27 Jun 2019 15:10:45 -0700
> "Paul E. McKenney" <paulmck@linux.ibm.com> wrote:
> 
> > On Thu, Jun 27, 2019 at 04:01:35PM -0600, Shuah Khan wrote:
> > > On 6/27/19 3:01 PM, Jiunn Chang wrote:  
> > > >The UP.rst file calls for locks acquired within RCU callback functions
> > > >to use _irq variants (spin_lock_irqsave() or similar), which does work,
> > > >but can be overkill.  This commit therefore instead calls for _bh variants
> > > >(spin_lock_bh() or similar), while noting that _irq does work.
> > > >
> > > >Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>  
> > > 
> > > Should this by Suggested-by?  
> > 
> > I wrote it and Jiunn converted my change to .rst, so I believe that
> > this is correct as is.
> 
> Note, you did send Jiunn an explicit Signed-off-by when you wrote it,
> correct? As Signed-off-by is equivalent to a signature.

Indeed I did, but I now see that it was via private email.  Here it is
again for public consumption, and Jiunn's patch is based on this one,
just translated to .rst.  I once again verified that the Jiunn's version
is word-for-word identical to mine, so Jiunn's patch should be good.  ;-)

							Thanx, Paul

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

commit a293734a310b463a0dba68409943a4e6065cd39d
Author: Paul E. McKenney <paulmck@linux.ibm.com>
Date:   Wed Jun 26 10:16:19 2019 -0700

    doc: RCU callback locks need only _bh, not necessarily _irq
    
    The UP.txt file calls for locks acquired within RCU callback functions
    to use _irq variants (spin_lock_irqsave() or similar), which does work,
    but can be overkill.  This commit therefore instead calls for _bh variants
    (spin_lock_bh() or similar), while noting that _irq does work.
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt
index 53bde717017b..0edd8c5af0b5 100644
--- a/Documentation/RCU/UP.txt
+++ b/Documentation/RCU/UP.txt
@@ -104,12 +104,13 @@ Answer to Quick Quiz #1:
 Answer to Quick Quiz #2:
 	What locking restriction must RCU callbacks respect?
 
-	Any lock that is acquired within an RCU callback must be
-	acquired elsewhere using an _irq variant of the spinlock
-	primitive.  For example, if "mylock" is acquired by an
-	RCU callback, then a process-context acquisition of this
-	lock must use something like spin_lock_irqsave() to
-	acquire the lock.
+	Any lock that is acquired within an RCU callback must be acquired
+	elsewhere using an _bh variant of the spinlock primitive.
+	For example, if "mylock" is acquired by an RCU callback, then
+	a process-context acquisition of this lock must use something
+	like spin_lock_bh() to acquire the lock.  Please note that
+	it is also OK to use _irq variants of spinlocks, for example,
+	spin_lock_irqsave().
 
 	If the process-context code were to simply use spin_lock(),
 	then, since RCU callbacks can be invoked from softirq context,

Re: [Linux-kernel-mentees][PATCH] doc: RCU callback locks need only _bh, not necessarily _irq

[Jonathan Corbet]

On Thu, 27 Jun 2019 16:01:47 -0500
Jiunn Chang <c0d1n61at3@gmail.com> wrote:

> The UP.rst file calls for locks acquired within RCU callback functions
> to use _irq variants (spin_lock_irqsave() or similar), which does work,
> but can be overkill.  This commit therefore instead calls for _bh variants
> (spin_lock_bh() or similar), while noting that _irq does work.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
> ---
>  Documentation/RCU/UP.rst | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)

Applied, thanks.

jon