Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance

[Shaokun Zhang]

Hi Al,

在 2020/8/27 22:28, Al Viro 写道:
> On Thu, Aug 27, 2020 at 06:19:44PM +0800, Shaokun Zhang wrote:
>> From: Yuqi Jin <jinyuqi@xxxxxxxxxx>
>>
>> It is well known that the performance of atomic_add is better than that of
>> atomic_cmpxchg.
>> The initial value of @f_count is 1. While @f_count is increased by 1 in
>> __fget_files, it will go through three phases: > 0, < 0, and = 0. When the
>> fixed value 0 is used as the condition for terminating the increase of 1,
>> only atomic_cmpxchg can be used. When we use < 0 as the condition for
>> stopping plus 1, we can use atomic_add to obtain better performance.
> 
> Suppose another thread has just removed it from the descriptor table.
> 
>> +static inline bool get_file_unless_negative(atomic_long_t *v, long a)
>> +{
>> +	long c = atomic_long_read(v);
>> +
>> +	if (c <= 0)
>> +		return 0;
> 
> Still 1.  Now the other thread has gotten to dropping the last reference,
> decremented counter to zero and committed to freeing the struct file.
> 

Apologies that I missed it.

>> +
>> +	return atomic_long_add_return(a, v) - 1;
> 
> ... and you increment that sucker back to 1.  Sure, you return 0, so the
> caller does nothing to that struct file.  Which includes undoing the
> changes to its refecount.
> 
> In the meanwhile, the third thread does fget on the same descriptor,
> and there we end up bumping the refcount to 2 and succeeding.  Which
> leaves the caller with reference to already doomed struct file...
> 
> 	IOW, NAK - this is completely broken.  The whole point of
> atomic_long_add_unless() is that the check and conditional increment
> are atomic.  Together.  That's what your optimization takes out.
> 

How about this? We try to replace atomic_cmpxchg with atomic_add to improve
performance. The atomic_add does not check the current f_count value.
Therefore, the number of online CPUs is reserved to prevent multi-core
competition.

+
+static inline bool get_file_unless(atomic_long_t *v, long a)
+{
+       long cpus = num_online_cpus();
+       long c = atomic_long_read(v);
+       long ret;
+
+       if (c > cpus || c < -cpus)
+               ret = atomic_long_add_return(a, v) - a;
+       else
+               ret = atomic_long_add_unless(v, a, 0);
+
+       return ret;
+}
+
 #define get_file_rcu_many(x, cnt)      \
-       atomic_long_add_unless(&(x)->f_count, (cnt), 0)
+       get_file_unless(&(x)->f_count, (cnt))

Thanks,
Shaokun

> .
>