Messages in this thread | | | Date | Fri, 19 Oct 2012 19:49:47 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] brw_mutex: big read-write mutex |
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On 10/19, Mikulas Patocka wrote: > > synchronize_rcu() is way slower than msleep(1) -
This depends, I guess. but this doesn't mmatter,
> so I don't see a reason > why should it be complicated to avoid msleep(1).
I don't think this really needs complications. Please look at this patch for example. Or initial (single writer) version below. It is not finished and lacks the barriers too, but I do not think it is more complex.
Oleg.
struct brw_sem { long __percpu *read_ctr; wait_queue_head_t read_waitq; struct mutex writer_mutex; struct task_struct *writer; };
int brw_init(struct brw_sem *brw) { brw->writer = NULL; mutex_init(&brw->writer_mutex); init_waitqueue_head(&brw->read_waitq); brw->read_ctr = alloc_percpu(long); return brw->read_ctr ? 0 : -ENOMEM; }
void brw_down_read(struct brw_sem *brw) { for (;;) { bool done = false;
preempt_disable(); if (likely(!brw->writer)) { __this_cpu_inc(*brw->read_ctr); done = true; } preempt_enable();
if (likely(done)) break;
__wait_event(brw->read_waitq, !brw->writer); } }
void brw_up_read(struct brw_sem *brw) { struct task_struct *writer;
preempt_disable(); __this_cpu_dec(*brw->read_ctr); writer = ACCESS_ONCE(brw->writer); if (unlikely(writer)) wake_up_process(writer); preempt_enable(); }
static inline long brw_read_ctr(struct brw_sem *brw) { long sum = 0; int cpu;
for_each_possible_cpu(cpu) sum += per_cpu(*brw->read_ctr, cpu);
return sum; }
void brw_down_write(struct brw_sem *brw) { mutex_lock(&brw->writer_mutex); brw->writer = current; synchronize_sched(); /* * Thereafter brw_*_read() must see ->writer != NULL, * and we should see the result of __this_cpu_inc(). */ for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); if (brw_read_ctr(brw) == 0) break; schedule(); } __set_current_state(TASK_RUNNING); /* * We can add another synchronize_sched() to avoid the * spurious wakeups from brw_up_read() after return. */ }
void brw_up_write(struct brw_sem *brw) { brw->writer = NULL; synchronize_sched(); wake_up_all(&brw->read_waitq); mutex_unlock(&brw->writer_mutex); }
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