* Peter Zijlstra (pet...@infradead.org) wrote: > On Wed, Sep 11, 2013 at 08:48:11PM -0400, Mathieu Desnoyers wrote: > > Thoughts ? > > > struct foo { > ... > }; > > spinlock_t foo_lock; > unsigned int foo_head = 0; > unsigned int foo_tail = 0; > struct foo foo_array[2]; > > void foo_assign(struct foo f) > { > spin_lock(&foo_lock); > foo_head++; > smp_wmb(); > foo_array[foo_head & 1] = f; > smp_wmb(); > foo_tail++; > spin_unlock(&foo_lock); > } > > struct foo foo_get(void) > { > unsigned int tail, head; > struct foo ret; > > again: > tail = ACCESS_ONCE(foo_tail); > smp_rmb(); > ret = foo_array[tail & 1]; > smp_rmb(); > head = ACCESS_ONCE(foo_head); > if (head - tail >= 2) > goto again; > > return ret; > } > > Should work and get you the most recent 'complete' foo even when > foo_get() is called nested inside foo_assign().
Cool! Your design looks good to me. It reminds me of a latch. My only fear is that struct timekeeper is probably too large to be copied every time on the read path. Here is a slightly reworked version that would allow in-place read of "foo" without copy. struct foo { ... }; struct latchfoo { unsigned int head, tail; spinlock_t write_lock; struct foo data[2]; }; static void foo_update(struct latchfoo *lf, void cb(struct foo *foo), void *ctx) { spin_lock(&lf->write_lock); lf->head++; smp_wmb(); lf->data[lf->head & 1] = lf->data[lf->tail & 1]; cb(&lf->data[lf->head & 1], ctx); smp_wmb(); lf->tail++; spin_unlock(&lock->write_lock); } static unsigned int foo_read_begin(struct latchfoo *lf) { unsigned int ret; ret = ACCESS_ONCE(lf->tail); smp_rmb(); return ret; } static struct foo *foo_read_get(struct latchfoo *lf, unsigned int tail) { return &lf->data[tail & 1]; } static int foo_read_retry(struct latchfoo *lf, unsigned int tail) { smp_rmb(); return (ACCESS_ONCE(lf->head) - tail >= 2); } Comments are welcome, Thanks! Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/