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Received: from willy by casper.infradead.org with local (Exim 4.98 #2 (Red Hat Linux)) id 1tVXdR-00000001B8S-3BEj; Wed, 08 Jan 2025 15:06:17 +0000 Date: Wed, 8 Jan 2025 15:06:17 +0000 From: Matthew Wilcox To: Vlastimil Babka Cc: Suren Baghdasaryan , akpm@linux-foundation.org, peterz@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, mhocko@suse.com, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v7 11/17] refcount: introduce __refcount_{add|inc}_not_zero_limited Message-ID: References: <20241226170710.1159679-1-surenb@google.com> <20241226170710.1159679-12-surenb@google.com> <275bd492-7d7c-4b9a-9fce-fbe25639cbfb@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <275bd492-7d7c-4b9a-9fce-fbe25639cbfb@suse.cz> X-Stat-Signature: 38jgu4a8tcn5zfki36whagfef165kihw X-Rspam-User: X-Rspamd-Queue-Id: A988D1C0006 X-Rspamd-Server: rspam08 X-HE-Tag: 1736350303-278870 X-HE-Meta: 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 ZdV+0QJa ghPKtLpllQ+cv/N52VloAQGCUSZ3dDvj65vjHrHa4QMHeDvNNDFwo4CbmBUv6QAkTGQ1aYeM/miDwubMORG/tKGPDhoICjQWT/xTwCacNzeHjEW7de/BnpT2J72bHFw14oBj7CHePpFbz88QIwnXhhAr69q7skYj0UF8FfsZQeHCdVlwpyRQtLLyx6kPKEsoVOfVDCGZRdBdpT98Wa+hxXrQOeTV2ONp8jV1w8RrJFJrEyT+9P8yIYRRQYT03xC9NF7TAkkl0zn2g1+JbsSDxI9Eolw== 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: List-Subscribe: List-Unsubscribe: On Wed, Jan 08, 2025 at 10:16:04AM +0100, Vlastimil Babka wrote: > > static inline __must_check __signed_wrap > > -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp) > > +bool __refcount_add_not_zero_limited(int i, refcount_t *r, int *oldp, > > + int limit) > > { > > int old = refcount_read(r); > > > > do { > > if (!old) > > break; > > + if (limit && old + i > limit) { > > Should this be e.g. "old > limit - i" to avoid overflow and false negative > if someone sets limit close to INT_MAX? Although 'i' might also be INT_MAX, whereas we know that old < limit. So "i > limit - old" is the correct condition to check, IMO. I'd further suggest that using a limit of 0 to mean "unlimited" introduces an unnecessary arithmetic operation. Make 'limit' inclusive instead of exclusive, pass INT_MAX instead of 0, and Vlastimil's suggestion, and this becomes: if (i > limit - old) > > + if (oldp) > > + *oldp = old; > > + return false; > > + } > > } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i)); ... > > +static inline __must_check __signed_wrap > > +bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp) > > +{ > > + return __refcount_add_not_zero_limited(i, r, oldp, 0); Just to be clear, this becomes: return __refcount_add_not_zero_limited(i, r, oldp, INT_MAX);