Messages in this thread | | | Date | Tue, 21 Jan 2014 10:45:42 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v9 4/5] qrwlock: Use smp_store_release() in write_unlock() |
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On 01/20/2014 10:18 AM, Peter Zijlstra wrote: > On Tue, Jan 14, 2014 at 11:44:06PM -0500, Waiman Long wrote: >> This patch modifies the queue_write_unlock() function to use the new >> smp_store_release() function (currently in tip). It also removes the >> temporary implementation of smp_load_acquire() and smp_store_release() >> function in qrwlock.c. >> >> This patch will use atomic subtraction instead if the writer field is >> not atomic. >> >> Signed-off-by: Waiman Long<Waiman.Long@hp.com> >> --- >> include/asm-generic/qrwlock.h | 10 ++++++---- >> kernel/locking/qrwlock.c | 34 ---------------------------------- >> 2 files changed, 6 insertions(+), 38 deletions(-) >> >> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h >> index 5abb6ca..68f488b 100644 >> --- a/include/asm-generic/qrwlock.h >> +++ b/include/asm-generic/qrwlock.h >> @@ -181,11 +181,13 @@ static inline void queue_read_unlock(struct qrwlock *lock) >> static inline void queue_write_unlock(struct qrwlock *lock) >> { >> /* >> - * Make sure that none of the critical section will be leaked out. >> + * If the writer field is atomic, it can be cleared directly. >> + * Otherwise, an atomic subtraction will be used to clear it. >> */ >> - smp_mb__before_clear_bit(); >> - ACCESS_ONCE(lock->cnts.writer) = 0; >> - smp_mb__after_clear_bit(); >> + if (__native_word(lock->cnts.writer)) >> + smp_store_release(&lock->cnts.writer, 0); >> + else >> + atomic_sub(_QW_LOCKED,&lock->cnts.rwa); >> } > If we're a writer, read-count must be zero. The only way for that not to > be zero is a concurrent read-(try)lock. If you move all the > read-(try)locks over to cmpxchg() you can avoid this afaict:
That is not true. A reader may transiently set the reader count to a non-zero value in the fast path. Also, a reader in interrupt context will force a non-zero reader count to take the read lock as soon as the writer is done.
> > static inline void queue_read_trylock(struct qrwlock *lock) > { > union qrwcnts cnts > > cnts = ACCESS_ONCE(lock->cnts); > if (!cnts.writer) { > if (cmpxchg(&lock->cnts.rwc, cnts.rwc, cnts.rwc + _QR_BIAS) == cnts.rwc) > return 1; > } > > return 0; > } > > static inline void queue_read_lock(struct qrwlock *lock) > { > if (!queue_read_trylock(lock)) > queue_read_lock_slowpath(); // XXX do not assume extra _QR_BIAS > } > > At which point you have the guarantee that read-count == 0, and you can > write: > > static inline void queue_write_unlock(struct qrwlock *lock) > { > smp_store_release(&lock->cnts.rwc, 0); > } > > No? >
The current code is optimized for the reader-heavy case. So I used xadd for incrementing reader count to reduce the chance of retry due to concurrent reader count updates. The downside is the need to back out if a writer is here.
I can change the logic to use only cmpxchg for readers, but I don't see a compelling reason to do so.
-Longman
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