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Wong" , linux-xfs@vger.kernel.org Subject: Re: [PATCH 1/5] locking: Add rwsem_is_write_locked() Message-ID: <20230912142300.GC22127@noisy.programming.kicks-ass.net> References: <20230908104434.GB24372@noisy.programming.kicks-ass.net> <70d89bf4-708b-f131-f90e-5250b6804d48@redhat.com> <20230912090342.GC35261@noisy.programming.kicks-ass.net> <20230912135213.GA22127@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 4055840010 X-Stat-Signature: ids4h7sd97q4jw5omqor4nuegpcjrcmo X-Rspam-User: X-HE-Tag: 1694528585-548734 X-HE-Meta: U2FsdGVkX18YdPm86MKD6W6q315S2+Q0VkxTmdI0fe5s5jWYH2kL3St0UOTIrj05wJUcB9d5f/MLdNUaVGyAqyd6rvWyXUe/++L4wY13776AanbX7zYjJ6pMCM7FaiENcDfVdRcsLmMYsYPQy1lFpWUvAazrh5YWoqSWB/SChiUBVnKzWs4/wQn3fzZtSEdk2QrIh8WDm89X5HMdLdRtc0Q4UUNs861qlggtwfGQl3YFZycGHJbO7KJeVLHYTaKfLC0sVHonhYUIx9aWtJEK0RVV7gQXfzx/3UE8WiBWetnmu98q/m22C0zqrBTVxoFOamE9BgUcO6cuwfuM1MvoKiCWRG5B8p08DgrR4Yq5QAOwqBFb+8EqzDQ6245DjeIUBZR1jiKKjExTOn3XLBldJ/P7bpSzUN52gvkeQV/mKP5w/ga5uIC3kN8MQp12b8jghRXd8GsPwH9ZUVjJ85RtJcS1nzWrPrJ3VZMvrfprpHmrrJzNFlG8zRtzE4hW1dud0CBp7AyFfbS0C9UllpoQNksOqfVslAdUE+emQqUnc82yAsTYCf8J7SaHRDgB3LAhpy5nEhSZ0aFIcQG67hQgD3VpLhzIWjaXA7szXVgt0BjpqH1hDZQH1mN3G8oiZ6uFx4qOlnF+zxpp53y8tz9gys6nYLUcYP7CPlF62zaaEpGsOIqTfv8MUyfjm43bPm7AaeqMr4bSgnkukiD7mgBGiE2QUzFgdUnqmJqf+zkEuffdyY9pmikc6C6lrj4tJ3JaVsYx876OlDHlcErWFLSydRPzea7AsMiT5J64uL7pkbm8z7pR68ObSvRrCkRLWNhGZUITN6ku0wFMLcCLTO1NvtB4f+NsqSP7PWwwj6aYbV3xQaK5lnKUSt05rauQVBC2DTBHqFM88bL1nAWWruvXDB7hUWZc/nSkdnsERDwD8vHNZsGInnXWtUrnfF2JrQGySeXknYOzhhhR4uUbJoZ Ejt6g9xf kirU0vJ6iqC3IAA+Fbh2CJ95V2C4ABGzuWKnNL4LoSsYAAic/WcksKXier4MoEFxOqRRwULigw06qj3bur1pOplKEBogHMNwBqW06vV9HRYz1UaI4Sei1OCcko/Gey0jSSc0vQBoZ2H2zI1zejtKvkDfD/Gr5RuyXXd1IZXxEhIdSsrx4yQXkUn8lig== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Sep 12, 2023 at 02:58:32PM +0100, Matthew Wilcox wrote: > On Tue, Sep 12, 2023 at 03:52:13PM +0200, Peter Zijlstra wrote: > > On Tue, Sep 12, 2023 at 01:28:13PM +0100, Matthew Wilcox wrote: > > > On Tue, Sep 12, 2023 at 11:03:42AM +0200, Peter Zijlstra wrote: > > > > If not, then sure we can do this; it's not like I managed to get rid of > > > > muteX_is_locked() -- and I actually tried at some point :/ > > > > > > > > And just now I grepped for it, and look what I find: > > > > > > > > drivers/hid/hid-nintendo.c: if (unlikely(mutex_is_locked(&ctlr->output_mutex))) { > > > > drivers/nvdimm/btt.c: if (mutex_is_locked(&arena->err_lock) > > > > > > > > And there's more :-( > > > > > > Are these actually abuse? I looked at these two, and they both seem to > > > be asking "Does somebody else currently have this mutex?" rather than > > > "Do I have this mutex?". > > > > It's effectively a random number generator in that capacity. Someone > > might have it or might have had it when you looked and no longer have > > it, or might have it now but not when you asked. > > Well, no. > > if (mutex_is_locked(&arena->err_lock) > || arena->freelist[lane].has_err) { > nd_region_release_lane(btt->nd_region, lane); > > ret = arena_clear_freelist_error(arena, lane); > > So that's "Is somebody currently processing an error, or have they > already finished setting an error". Sure, it's somewhat racy, but > it looks like a performance optimisation, not something that needs > 100% accuracy. We're arguing past one another I think. Yes mutex_is_locked() is a random number generator when asked for something you don't own. But it might not be a bug because the code is ok with races. It is still fully dodgy IMO, such usage is pretty close to UB.