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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20200721195132.GJ10769@hirez.programming.kicks-ass.net> References: <20200721195132.GJ10769@hirez.programming.kicks-ass.net> <202006142054.C00B3E9C9@keescook> <20200612183450.4189588-1-keescook@chromium.org> <7be4d56b-0406-099b-e505-02e074c5173e@huawei.com> <544539.1595328664@warthog.procyon.org.uk> <202007211144.A68C31D@keescook> To: Peter Zijlstra Cc: dhowells@redhat.com, Kees Cook , Xiaoming Ni , David Windsor , Hans Liljestrand , Elena Reshetova , Paul Moore , edumazet@google.com, paulmck@kernel.org, shakeelb@google.com, James Morris , alex.huangjianhui@huawei.com, dylix.dailei@huawei.com, chenzefeng2@huawei.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/3] Convert nsproxy, groups, and creds to refcount_t MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <3211865.1595933798.1@warthog.procyon.org.uk> Date: Tue, 28 Jul 2020 11:56:38 +0100 Message-ID: <3211866.1595933798@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra wrote: > > Please do not _undo_ the changes; just add the API you need. > > add_return and sub_return are horrible interface for refcount, which is > the problem. > > If you meant: refcount_dec(), but want the old value for tracing, you > want a different ordering than if you wanted to do > refcount_dec_and_test(); dec_return can't know this. > > David, would something like a __refcount_*() API work where there is a > 3rd argument (int *), which, if !NULL, will be assigned the old value? That would be fine, though the number needs to be something I can interpret easily when looking through the traces. It would also be okay if there was an interpretation function that I could use in the trace point when setting the variable. Say: void rxrpc_get_call(struct rxrpc_call *call, enum rxrpc_call_trace op) { const void *here = __builtin_return_address(0); unsigned int n; refcount_inc_return(&call->usage, &n); trace_rxrpc_call(call->debug_id, op, n, here, NULL); } then: TRACE_EVENT(rxrpc_call, TP_PROTO(..., int usage, ...), TP_ARGS(...), TP_STRUCT__entry( ... __field(int, usage) ... ), TP_fast_assign( ... __entry->usage = refcount_interpret(usage); ... ), TP_printk("... u=%d ...", ... __entry->usage, ...) ); so that it looks like the refcount is 'complete' at 0. David