Re: =?gb18030?q?=BB=D8=B8=B4=A3=BA=5BPATCH_1/3_v3=5D_dcach?==?gb18030?q?e=3A_Don=27t_take_unnecessary_lock_in_d=5Fcount_?==?gb18030?q?update?=

From: Waiman Long
Date: Thu May 23 2013 - 16:52:04 EST

On 05/23/2013 05:09 AM, remaper wrote:
maybe you can use the atomic_dec_and_lock(&dentry->d_count,&dentry->d_lock) here, right ?

------------------ Origin ------------------
The current code takes the dentry's d_lock lock whenever the d_count
reference count is being updated. In reality, nothing big really
happens until d_count goes to 0 in dput(). So it is not necessary to
take the lock if the reference count won't go to 0.
Thank for the suggestion.

First of all, the d_count field is not an atomic type. Changing it to atomic will require touching quite a large number of source files even though that change won't change the size of the dentry. Secondly, the function internally uses cmpxchg to do its work. There isn't any performance advantage of using it. I also need the count to go in both direction, not just going down.

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