From: Davidlohr Bueso <dave@stgolabs.net>
To: Michal Hocko <mhocko@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>,
"David S. Miller" <davem@davemloft.net>,
Tony Luck <tony.luck@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Chris Zankel <chris@zankel.net>,
Max Filippov <jcmvbkbc@gmail.com>,
x86@kernel.org, linux-alpha@vger.kernel.org,
linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org,
linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org,
Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 02/11] locking, rwsem: drop explicit memory barriers
Date: Fri, 1 Apr 2016 18:17:53 -0700 [thread overview]
Message-ID: <20160402011753.GB5329@linux-uzut.site> (raw)
In-Reply-To: <1459508695-14915-3-git-send-email-mhocko@kernel.org>
On Fri, 01 Apr 2016, Michal Hocko wrote:
>From: Michal Hocko <mhocko@suse.com>
>
>sh and xtensa seem to be the only architectures which use explicit
>memory barriers for rw_semaphore operations even though they are not
>really needed because there is the full memory barrier is always implied
>by atomic_{inc,dec,add,sub}_return resp. cmpxchg. Remove them.
Heh, and sh even defines smp_store_mb() with xchg(), which makes the above
even more so.
>
>Signed-off-by: Michal Hocko <mhocko@suse.com>
>---
> arch/sh/include/asm/rwsem.h | 14 ++------------
> arch/xtensa/include/asm/rwsem.h | 14 ++------------
> 2 files changed, 4 insertions(+), 24 deletions(-)
I think we can actually get rid of these files altogether. While I don't know the archs,
it does not appear to be implementing any kind of workaround/optimization, which is obviously
the whole purpose of defining per-arch rwsem internals, unless the redundant barriers are
actually the purpose. So we could use the generic rwsem.h (which has optimizations and ACQUIRE/
RELEASE semantics, although nops for either sh or xtensa specifically). A first inspection shows
that the code is in fact the same and continue to use 32bit rwsems.
Thanks,
Davidlohr
>diff --git a/arch/sh/include/asm/rwsem.h b/arch/sh/include/asm/rwsem.h
>index a5104bebd1eb..f6c951c7a875 100644
>--- a/arch/sh/include/asm/rwsem.h
>+++ b/arch/sh/include/asm/rwsem.h
>@@ -24,9 +24,7 @@
> */
> static inline void __down_read(struct rw_semaphore *sem)
> {
>- if (atomic_inc_return((atomic_t *)(&sem->count)) > 0)
>- smp_wmb();
>- else
>+ if (atomic_inc_return((atomic_t *)(&sem->count)) <= 0)
> rwsem_down_read_failed(sem);
> }
>
>@@ -37,7 +35,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> while ((tmp = sem->count) >= 0) {
> if (tmp == cmpxchg(&sem->count, tmp,
> tmp + RWSEM_ACTIVE_READ_BIAS)) {
>- smp_wmb();
> return 1;
> }
> }
>@@ -53,9 +50,7 @@ static inline void __down_write(struct rw_semaphore *sem)
>
> tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count));
>- if (tmp == RWSEM_ACTIVE_WRITE_BIAS)
>- smp_wmb();
>- else
>+ if (tmp != RWSEM_ACTIVE_WRITE_BIAS)
> rwsem_down_write_failed(sem);
> }
>
>@@ -65,7 +60,6 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
>
> tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> RWSEM_ACTIVE_WRITE_BIAS);
>- smp_wmb();
> return tmp == RWSEM_UNLOCKED_VALUE;
> }
>
>@@ -76,7 +70,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_dec_return((atomic_t *)(&sem->count));
> if (tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0)
> rwsem_wake(sem);
>@@ -87,7 +80,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> */
> static inline void __up_write(struct rw_semaphore *sem)
> {
>- smp_wmb();
> if (atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count)) < 0)
> rwsem_wake(sem);
>@@ -108,7 +100,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
> if (tmp < 0)
> rwsem_downgrade_wake(sem);
>@@ -119,7 +110,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> */
> static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
> {
>- smp_mb();
> return atomic_add_return(delta, (atomic_t *)(&sem->count));
> }
>
>diff --git a/arch/xtensa/include/asm/rwsem.h b/arch/xtensa/include/asm/rwsem.h
>index 249619e7e7f2..593483f6e1ff 100644
>--- a/arch/xtensa/include/asm/rwsem.h
>+++ b/arch/xtensa/include/asm/rwsem.h
>@@ -29,9 +29,7 @@
> */
> static inline void __down_read(struct rw_semaphore *sem)
> {
>- if (atomic_add_return(1,(atomic_t *)(&sem->count)) > 0)
>- smp_wmb();
>- else
>+ if (atomic_add_return(1,(atomic_t *)(&sem->count)) <= 0)
> rwsem_down_read_failed(sem);
> }
>
>@@ -42,7 +40,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> while ((tmp = sem->count) >= 0) {
> if (tmp == cmpxchg(&sem->count, tmp,
> tmp + RWSEM_ACTIVE_READ_BIAS)) {
>- smp_wmb();
> return 1;
> }
> }
>@@ -58,9 +55,7 @@ static inline void __down_write(struct rw_semaphore *sem)
>
> tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count));
>- if (tmp == RWSEM_ACTIVE_WRITE_BIAS)
>- smp_wmb();
>- else
>+ if (tmp != RWSEM_ACTIVE_WRITE_BIAS)
> rwsem_down_write_failed(sem);
> }
>
>@@ -70,7 +65,6 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
>
> tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> RWSEM_ACTIVE_WRITE_BIAS);
>- smp_wmb();
> return tmp == RWSEM_UNLOCKED_VALUE;
> }
>
>@@ -81,7 +75,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_sub_return(1,(atomic_t *)(&sem->count));
> if (tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0)
> rwsem_wake(sem);
>@@ -92,7 +85,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> */
> static inline void __up_write(struct rw_semaphore *sem)
> {
>- smp_wmb();
> if (atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count)) < 0)
> rwsem_wake(sem);
>@@ -113,7 +105,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
> if (tmp < 0)
> rwsem_downgrade_wake(sem);
>@@ -124,7 +115,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> */
> static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
> {
>- smp_mb();
> return atomic_add_return(delta, (atomic_t *)(&sem->count));
> }
>
>--
>2.8.0.rc3
>
next prev parent reply other threads:[~2016-04-02 1:18 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-01 11:04 [PATCH 0/11] introduce down_write_killable for rw_semaphore v2 Michal Hocko
2016-04-01 11:04 ` [PATCH 01/11] locking, rwsem: get rid of __down_write_nested Michal Hocko
2016-04-02 0:28 ` Davidlohr Bueso
2016-04-01 11:04 ` [PATCH 02/11] locking, rwsem: drop explicit memory barriers Michal Hocko
2016-04-02 1:17 ` Davidlohr Bueso [this message]
2016-04-04 9:03 ` Michal Hocko
2016-04-04 9:06 ` [PATCH 1/2] xtensa, rwsem: drop superfluous arch specific implementation Michal Hocko
2016-04-04 9:06 ` [PATCH 2/2] sh, " Michal Hocko
2016-04-06 9:26 ` Peter Zijlstra
2016-04-06 9:50 ` Geert Uytterhoeven
2016-04-06 10:27 ` Peter Zijlstra
2016-04-04 10:23 ` [PATCH 1/2] xtensa, " Max Filippov
2016-04-06 9:06 ` [PATCH] sparc, " Michal Hocko
2016-04-01 11:04 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Michal Hocko
2016-04-02 4:41 ` Davidlohr Bueso
2016-04-04 9:17 ` Michal Hocko
2016-04-04 9:21 ` Peter Zijlstra
2016-04-07 6:58 ` Davidlohr Bueso
2016-04-07 7:38 ` Michal Hocko
2016-05-10 10:43 ` Tetsuo Handa
2016-05-10 11:53 ` Michal Hocko
2016-05-10 12:38 ` Peter Zijlstra
2016-05-10 13:57 ` Tetsuo Handa
2016-05-11 7:23 ` Michal Hocko
2016-05-11 8:28 ` Michal Hocko
2016-05-11 8:44 ` Peter Zijlstra
2016-05-11 9:04 ` Michal Hocko
2016-05-11 9:17 ` Peter Zijlstra
2016-05-11 9:31 ` Michal Hocko
2016-05-11 9:41 ` Peter Zijlstra
2016-05-11 13:59 ` Michal Hocko
2016-05-11 18:03 ` Michal Hocko
2016-05-12 11:57 ` [PATCH] locking, rwsem: Fix down_write_killable() Peter Zijlstra
2016-05-12 12:15 ` [tip:locking/rwsem] locking/rwsem: " tip-bot for Peter Zijlstra
2016-05-12 16:59 ` [PATCH] locking, rwsem: " Michal Hocko
2016-05-15 20:57 ` [tip:locking/rwsem] locking/rwsem: " tip-bot for Peter Zijlstra
2016-05-12 12:12 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Peter Zijlstra
2016-05-12 12:19 ` Michal Hocko
2016-05-12 13:58 ` Peter Zijlstra
2016-05-12 19:42 ` Waiman Long
2016-05-11 8:35 ` Peter Zijlstra
2016-05-11 9:02 ` Michal Hocko
2016-04-01 11:04 ` [PATCH 04/11] alpha, rwsem: provide __down_write_killable Michal Hocko
2016-04-01 11:04 ` [PATCH 05/11] ia64, " Michal Hocko
2016-04-01 11:04 ` [PATCH 06/11] s390, " Michal Hocko
2016-04-01 11:04 ` [PATCH 07/11] sh, " Michal Hocko
2016-04-01 11:04 ` [PATCH 08/11] sparc, " Michal Hocko
2016-04-01 11:04 ` [PATCH 09/11] xtensa, " Michal Hocko
2016-04-01 11:04 ` [PATCH 10/11] x86, " Michal Hocko
2016-04-06 18:31 ` Peter Zijlstra
2016-04-01 11:04 ` [PATCH 11/11] locking, rwsem: provide down_write_killable Michal Hocko
-- strict thread matches above, loose matches on Subject: below --
2016-04-07 15:12 [PATCH 0/11] introduce down_write_killable for rw_semaphore v3 Michal Hocko
2016-04-07 15:12 ` [PATCH 02/11] locking, rwsem: drop explicit memory barriers Michal Hocko
2016-02-29 12:58 [PATCH 0/11] introduce down_write_killable for rw_semaphore Michal Hocko
2016-02-29 12:58 ` [PATCH 02/11] locking, rwsem: drop explicit memory barriers Michal Hocko
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