From: Peter Zijlstra Subject: Re: [PATCH, RFC] percpu_counters: make fbc->count read atomic on 32 bit architecture Date: Tue, 07 Oct 2008 11:55:47 +0200 Message-ID: <1223373347.26330.19.camel@lappy.programming.kicks-ass.net> References: <20081006232322.c383fac5.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Theodore Ts'o , "Aneesh Kumar K.V" , linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org To: Andrew Morton Return-path: In-Reply-To: <20081006232322.c383fac5.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, 2008-10-06 at 23:23 -0700, Andrew Morton wrote: > On Sun, 05 Oct 2008 21:28:10 -0400 "Theodore Ts'o" wrote: > > > The following patch has been sitting in the ext4 patch queue for about > > six weeks. It was there it was a suspected cause for block allocation > > bug. As I recall, it we found the true root cause since then, but this > > has stuck around since it's a potential problem. Andrew has expressed > > concerns that this patch might have performance impacts. > > Performace impacts I guess we'll just have to put up with. iirc I was > thinking that this implementation should be pushed down to a kernel-wide > atomic64_t and then the percpu_counters would just use that type. something like so? or should I use a global seqlock hash table? i386 could then do a version using cmpxchg8, although I'm not sure that's a win, as I've heard thats an awefuly expensive op to use. --- Subject: From: Peter Zijlstra Date: Tue Oct 07 11:52:37 CEST 2008 Signed-off-by: Peter Zijlstra --- include/asm-generic/atomic_64.h | 132 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 132 insertions(+) Index: linux-2.6/include/asm-generic/atomic_64.h =================================================================== --- /dev/null +++ linux-2.6/include/asm-generic/atomic_64.h @@ -0,0 +1,132 @@ +#ifndef _ASM_GENERIC_ATOMIC64_H +#define _ASM_GENERIC_ATOMIC64_H + +#include + +#if BITS_PER_LONG == 32 + +#include + +typedef struct { + s64 counter; + seqlock_t slock; +} atomic64_t; + +ATOMIC64_INIT(i) { .counter = (i), .slock = __SEQLOCK_UNLOCKED(i.slock) } + +static inline s64 atomic64_read(atomic64_t *v) +{ + unsigned seq = read_seqbegin(&v->slock); + s64 val; + + do { + val = v->counter; + } while (read_seqretry(&v->slock, seq)); + + return val; +} + +static inline void atomic64_set(s64 i, atomic64_t *v) +{ + write_seqlock(&v->slock); + v->counter = i; + write_sequnlock(&v->slock); +} + +static inline void atomic64_add(s64 i, atomic64_t *v) +{ + write_seqlock(&v->slock); + v->counter += i; + write_sequnlock(&v->slock); +} + +static inline void atomic64_sub(s64 i, atomic64_t *v) +{ + write_seqlock(&v->slock); + v->counter -= i; + write_sequnlock(&v->slock); +} + +static inline int atomic64_sub_and_test(s64 i, atomic64_t *v) +{ + int ret; + + write_seqlock(&v->slock); + v->counter -= i; + ret = !v->counter; + write_sequnlock(&v->slock); + + return ret; +} + +static inline void atomic64_inc(atomic64_t *v) +{ + write_seqlock(&v->slock); + v->counter++; + write_sequnlock(&v->slock); +} + +static inline void atomic64_dec(atomic64_t *v) +{ + write_seqlock(&v->slock); + v->counter--; + write_sequnlock(&v->slock); +} + +static inline int atomic64_dec_and_test(atomic64_t *v) +{ + int ret; + + write_seqlock(&v->slock); + v->counter--; + ret = !v->counter; + write_sequnlock(&v->slock); + + return ret; +} + +static inline int atomic64_add_and_test(atomic64_t *v) +{ + int ret; + + write_seqlock(&v->slock); + v->counter++; + ret = !v->counter; + write_sequnlock(&v->slock); + + return ret; +} + +static inline int atomic64_add_negative(s64 i, atomic64_t *v) +{ + int ret; + + write_seqlock(&v->slock); + v->counter += i; + ret = v->counter < 0; + write_sequnlock(&v->slock); + + return ret; +} + +static inline s64 atomic64_add_return(s64 i, atomic64_t *v) +{ + s64 val; + + write_seqlock(&v->slock); + v->counter += i; + val = v->counter; + write_sequnlock(&v->slock); + + return val; +} + +static inline s64 atomic64_sub_return(s64 i, atomic64_t *v) +{ + return atomic64_add_return(-i, v); +} + +#define atomic64_inc_return(v) (atomic64_add_return(1, (v))) +#define atomic64_dec_return(v) (atomic64_sub_return(1, (v))) + +#endif