On Tue, 09 Sep 2014 16:21:14 -0700, Andrew Morton said: > On Tue, 9 Sep 2014 23:25:28 +0200 (CEST) Jiri Kosina <jkosina@xxxxxxx> wrote: > kfree() is quite a hot path to which this will add overhead. And we > have (as far as we know) no code which will actually use this at > present. We already do a check for ZERO_SIZE_PTR, and given that dereferencing *that* is instant death for the kernel, and we see it very rarely, I'm going to guess that IS_ERR(ptr) *has* to be true more often than ZERO_SIZE_PTR, and thus even more advantageous to short-circuit. I guess it depends on a few things: 1) How many instances of 'if (!IS_ERR(foo)) kfree(foo);' are in the tree, and what percent of kfree() calls executed have the guard on them 2) How many of the hot calls can/will get the guard removed. 3) How many cycles, if any, this adds to the path (a non-trivial question on superscalar architectures), compared with doing a test before calling kfree() I unfortunately have no earthly clue what the values of any of those three quantities are....
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