[PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[Will Deacon]

The asm-generic rwsem implementation directly acceses sem->cnt when
performing a __down_read_trylock operation. Whilst this is probably safe
on all architectures, we should stick to the atomic_long_* API and use
atomic_long_read instead.

Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 include/asm-generic/rwsem.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index bb1e2cdeb9bf..75af612f54f8 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 {
 	long tmp;
 
-	while ((tmp = sem->count) >= 0) {
+	while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {
 		if (tmp == cmpxchg(&sem->count, tmp,
 				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
 			return 1;
-- 
1.8.2.2

[PATCH 2/2] asm-generic: rwsem: de-PPCify rwsem.h

[Will Deacon]

asm-generic/rwsem.h used to live under arch/powerpc. During its
liberation to common code, a few references to its former home where
preserved, in particular the definition of RWSEM_ACTIVE_MASK is
predicated on CONFIG_PPC64.

This patch updates the ifdefs and comments to architecturally neutral
versions.

Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 include/asm-generic/rwsem.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index 75af612f54f8..603a0a11e592 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -1,5 +1,5 @@
-#ifndef _ASM_POWERPC_RWSEM_H
-#define _ASM_POWERPC_RWSEM_H
+#ifndef _ASM_GENERIC_RWSEM_H
+#define _ASM_GENERIC_RWSEM_H
 
 #ifndef _LINUX_RWSEM_H
 #error "Please don't include <asm/rwsem.h> directly, use <linux/rwsem.h> instead."
@@ -8,7 +8,7 @@
 #ifdef __KERNEL__
 
 /*
- * R/W semaphores for PPC using the stuff in lib/rwsem.c.
+ * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
  * Adapted largely from include/asm-i386/rwsem.h
  * by Paul Mackerras <paulus@samba.org>.
  */
@@ -16,7 +16,7 @@
 /*
  * the semaphore definition
  */
-#ifdef CONFIG_PPC64
+#ifdef CONFIG_64BIT
 # define RWSEM_ACTIVE_MASK		0xffffffffL
 #else
 # define RWSEM_ACTIVE_MASK		0x0000ffffL
@@ -129,4 +129,4 @@ static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
 }
 
 #endif	/* __KERNEL__ */
-#endif	/* _ASM_POWERPC_RWSEM_H */
+#endif	/* _ASM_GENERIC_RWSEM_H */
-- 
1.8.2.2

Re: [PATCH 2/2] asm-generic: rwsem: de-PPCify rwsem.h

[rkuo]

On Fri, Feb 21, 2014 at 05:22:27PM +0000, Will Deacon wrote:
> asm-generic/rwsem.h used to live under arch/powerpc. During its
> liberation to common code, a few references to its former home where
> preserved, in particular the definition of RWSEM_ACTIVE_MASK is
> predicated on CONFIG_PPC64.
> 
> This patch updates the ifdefs and comments to architecturally neutral
> versions.
> 
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
>  include/asm-generic/rwsem.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 

Thanks; I probably should have noticed these when I copied the file over.

Acked-by: Richard Kuo <rkuo@codeaurora.org>

-- 

Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
hosted by The Linux Foundation

Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[Davidlohr Bueso]

On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote:
> The asm-generic rwsem implementation directly acceses sem->cnt when
> performing a __down_read_trylock operation. Whilst this is probably safe
> on all architectures, we should stick to the atomic_long_* API and use
> atomic_long_read instead.
> 
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
>  include/asm-generic/rwsem.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index bb1e2cdeb9bf..75af612f54f8 100644
> --- a/include/asm-generic/rwsem.h
> +++ b/include/asm-generic/rwsem.h
> @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
>  {
>  	long tmp;
>  
> -	while ((tmp = sem->count) >= 0) {
> +	while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {

That's pretty ugly, how about having infinite look and just do the tmp
assign separately from the conditional?

It also looks like a cpu_relax() could help here between iterations.
Other than that:

Reviewed-by: Davidlohr Bueso <davidlohr@hp.com>

Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[Will Deacon]

On Thu, Feb 27, 2014 at 05:28:24AM +0000, Davidlohr Bueso wrote:
> On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote:
> > The asm-generic rwsem implementation directly acceses sem->cnt when
> > performing a __down_read_trylock operation. Whilst this is probably safe
> > on all architectures, we should stick to the atomic_long_* API and use
> > atomic_long_read instead.
> > 
> > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > ---
> >  include/asm-generic/rwsem.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> > index bb1e2cdeb9bf..75af612f54f8 100644
> > --- a/include/asm-generic/rwsem.h
> > +++ b/include/asm-generic/rwsem.h
> > @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> >  {
> >  	long tmp;
> >  
> > -	while ((tmp = sem->count) >= 0) {
> > +	while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {
> 
> That's pretty ugly, how about having infinite look and just do the tmp
> assign separately from the conditional?
> 
> It also looks like a cpu_relax() could help here between iterations.

Sure, I can do these in a separate patch (this patch is just a simple fix).

> Reviewed-by: Davidlohr Bueso <davidlohr@hp.com>

Thanks,

Will

Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[Will Deacon]

On Thu, Feb 27, 2014 at 05:28:24AM +0000, Davidlohr Bueso wrote:
> On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote:
> > The asm-generic rwsem implementation directly acceses sem->cnt when
> > performing a __down_read_trylock operation. Whilst this is probably safe
> > on all architectures, we should stick to the atomic_long_* API and use
> > atomic_long_read instead.
> > 
> > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > ---
> >  include/asm-generic/rwsem.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> > index bb1e2cdeb9bf..75af612f54f8 100644
> > --- a/include/asm-generic/rwsem.h
> > +++ b/include/asm-generic/rwsem.h
> > @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> >  {
> >  	long tmp;
> >  
> > -	while ((tmp = sem->count) >= 0) {
> > +	while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {
> 
> That's pretty ugly, how about having infinite look and just do the tmp
> assign separately from the conditional?
> 
> It also looks like a cpu_relax() could help here between iterations.
> Other than that:
> 
> Reviewed-by: Davidlohr Bueso <davidlohr@hp.com>

Actually, we should make that cmpxchg an atomic_long_cmpxchg, so the extra
diff ends up looking like below. It's ugly adding a cpu_relax(), since you
only want it when the cmpxchg fails (and we don't have such logic in the
asm-generic __atomic_add_unless, for example).

Will

--->8

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index 603a0a11e592..2b6401f9e428 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -40,14 +40,16 @@ static inline void __down_read(struct rw_semaphore *sem)
 static inline int __down_read_trylock(struct rw_semaphore *sem)
 {
        long tmp;
+       atomic_long_t *cnt = (atomic_long_t *)&sem->count;
 
-       while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {
-               if (tmp == cmpxchg(&sem->count, tmp,
-                                  tmp + RWSEM_ACTIVE_READ_BIAS)) {
-                       return 1;
-               }
-       }
-       return 0;
+       do {
+               tmp = atomic_long_read(cnt);
+               if (tmp < 0)
+                       return 0;
+       } while (tmp != atomic_long_cmpxchg(cnt, tmp,
+                                           tmp + RWSEM_ACTIVE_READ_BIAS));
+
+       return 1;
 }
 
 /*

Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[Peter Hurley]

On 02/28/2014 07:13 AM, Will Deacon wrote:
> On Thu, Feb 27, 2014 at 05:28:24AM +0000, Davidlohr Bueso wrote:
>> On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote:
>>> The asm-generic rwsem implementation directly acceses sem->cnt when
>>> performing a __down_read_trylock operation. Whilst this is probably safe
>>> on all architectures, we should stick to the atomic_long_* API and use
>>> atomic_long_read instead.
>>>
>>> Signed-off-by: Will Deacon <will.deacon@arm.com>
>>> ---
>>>   include/asm-generic/rwsem.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
>>> index bb1e2cdeb9bf..75af612f54f8 100644
>>> --- a/include/asm-generic/rwsem.h
>>> +++ b/include/asm-generic/rwsem.h
>>> @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
>>>   {
>>>   	long tmp;
>>>
>>> -	while ((tmp = sem->count) >= 0) {
>>> +	while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {
>>
>> That's pretty ugly, how about having infinite look and just do the tmp
>> assign separately from the conditional?
>>
>> It also looks like a cpu_relax() could help here between iterations.

This is the read trylock so no cpu_relax().

>> Other than that:
>>
>> Reviewed-by: Davidlohr Bueso <davidlohr@hp.com>
>
> Actually, we should make that cmpxchg an atomic_long_cmpxchg, so the extra
> diff ends up looking like below. It's ugly adding a cpu_relax(), since you
> only want it when the cmpxchg fails (and we don't have such logic in the
> asm-generic __atomic_add_unless, for example).
>
> Will
>
> --->8
>
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index 603a0a11e592..2b6401f9e428 100644
> --- a/include/asm-generic/rwsem.h
> +++ b/include/asm-generic/rwsem.h
> @@ -40,14 +40,16 @@ static inline void __down_read(struct rw_semaphore *sem)
>   static inline int __down_read_trylock(struct rw_semaphore *sem)
>   {
>          long tmp;
> +       atomic_long_t *cnt = (atomic_long_t *)&sem->count;

The shared rwsem failure paths (kernel/locking/rwsem_xadd.c) peek at
sem->count as long type, so this isn't really necessary.

>
> -       while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) {
> -               if (tmp == cmpxchg(&sem->count, tmp,
> -                                  tmp + RWSEM_ACTIVE_READ_BIAS)) {
> -                       return 1;
> -               }
> -       }
> -       return 0;
> +       do {
> +               tmp = atomic_long_read(cnt);
> +               if (tmp < 0)
> +                       return 0;
> +       } while (tmp != atomic_long_cmpxchg(cnt, tmp,
> +                                           tmp + RWSEM_ACTIVE_READ_BIAS));
> +
> +       return 1;
>   }

Regards,
Peter Hurley

Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

[Will Deacon]

On Fri, Feb 28, 2014 at 12:50:06PM +0000, Peter Hurley wrote:
> On 02/28/2014 07:13 AM, Will Deacon wrote:
> > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> > index 603a0a11e592..2b6401f9e428 100644
> > --- a/include/asm-generic/rwsem.h
> > +++ b/include/asm-generic/rwsem.h
> > @@ -40,14 +40,16 @@ static inline void __down_read(struct rw_semaphore *sem)
> >   static inline int __down_read_trylock(struct rw_semaphore *sem)
> >   {
> >          long tmp;
> > +       atomic_long_t *cnt = (atomic_long_t *)&sem->count;
> 
> The shared rwsem failure paths (kernel/locking/rwsem_xadd.c) peek at
> sem->count as long type, so this isn't really necessary.

Yeah, none of this patch is necessary from a technical perspective. I was
just in the area and tried to make the file at least self-consistent wrt the
atomic API. It's a harmless cleanup.

Will