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Thu, 27 Nov 2025 03:35:49 -0800 (PST) X-Google-Smtp-Source: AGHT+IHw+losns/EbFf8Iuyd+Ic4CbDitf9tLRbOX3h2FYMV1qJKEuWEEDc8zOqFzENhs0ha+EaT5Q== X-Received: by 2002:a5d:5e01:0:b0:42b:3131:5435 with SMTP id ffacd0b85a97d-42cc1ac9de0mr24340515f8f.2.1764243349101; Thu, 27 Nov 2025 03:35:49 -0800 (PST) Received: from [192.168.88.32] ([212.105.155.212]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42e1ca1a2easm3317816f8f.23.2025.11.27.03.35.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Nov 2025 03:35:48 -0800 (PST) Message-ID: <0bcdd667-1811-4bde-8313-1a7e3abe55ad@redhat.com> Date: Thu, 27 Nov 2025 12:35:47 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v2 2/3] xsk: use atomic operations around cached_prod for copy mode To: Jason Xing , davem@davemloft.net, edumazet@google.com, kuba@kernel.org, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing References: <20251125085431.4039-1-kerneljasonxing@gmail.com> <20251125085431.4039-3-kerneljasonxing@gmail.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20251125085431.4039-3-kerneljasonxing@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/25/25 9:54 AM, Jason Xing wrote: > diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h > index 44cc01555c0b..3a023791b273 100644 > --- a/net/xdp/xsk_queue.h > +++ b/net/xdp/xsk_queue.h > @@ -402,13 +402,28 @@ static inline void xskq_prod_cancel_n(struct xsk_queue *q, u32 cnt) > q->cached_prod -= cnt; > } > > -static inline int xskq_prod_reserve(struct xsk_queue *q) > +static inline bool xsk_cq_cached_prod_nb_free(struct xsk_queue *q) > { > - if (xskq_prod_is_full(q)) > + u32 cached_prod = atomic_read(&q->cached_prod_atomic); > + u32 free_entries = q->nentries - (cached_prod - q->cached_cons); > + > + if (free_entries) > + return true; > + > + /* Refresh the local tail pointer */ > + q->cached_cons = READ_ONCE(q->ring->consumer); > + free_entries = q->nentries - (cached_prod - q->cached_cons); > + > + return free_entries ? true : false; > +} _If_ different CPUs can call xsk_cq_cached_prod_reserve() simultaneously (as the spinlock existence suggests) the above change introduce a race: xsk_cq_cached_prod_nb_free() can return true when num_free == 1 on CPU1, and xsk_cq_cached_prod_reserve increment cached_prod_atomic on CPU2 before CPU1 completed xsk_cq_cached_prod_reserve(). /P