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Date: Fri, 1 Apr 2016 18:17:53 -0700 From: Davidlohr Bueso <dave@...olabs.net> To: Michal Hocko <mhocko@...nel.org> Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, "H. Peter Anvin" <hpa@...or.com>, "David S. Miller" <davem@...emloft.net>, Tony Luck <tony.luck@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>, x86@...nel.org, linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org, linux-sh@...r.kernel.org, sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org, linux-arch@...r.kernel.org, Michal Hocko <mhocko@...e.com> Subject: Re: [PATCH 02/11] locking, rwsem: drop explicit memory barriers On Fri, 01 Apr 2016, Michal Hocko wrote: >From: Michal Hocko <mhocko@...e.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@...e.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 >
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