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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1rDRzx-002B4P-Kt; Wed, 13 Dec 2023 16:22:13 +0000 Date: Wed, 13 Dec 2023 16:22:13 +0000 From: Matthew Wilcox To: Maria Yu Cc: ebiederm@xmission.com, kernel@quicinc.com, quic_pkondeti@quicinc.com, keescook@chromium.or, viro@zeniv.linux.org.uk, brauner@kernel.org, oleg@redhat.com, dhowells@redhat.com, jarkko@kernel.org, paul@paul-moore.com, jmorris@namei.org, serge@hallyn.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module@vger.kernel.org, linux-arm-msm@vger.kernel.org Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist_lock Message-ID: References: <20231213101745.4526-1-quic_aiquny@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231213101745.4526-1-quic_aiquny@quicinc.com> X-Rspamd-Queue-Id: 1689F40014 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: ojzhp6capyrjosmodgyqgjce1u14asew X-HE-Tag: 1702484557-274379 X-HE-Meta: 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 UbSqae00 xWLUxBPFGy67vJZxaOeuR9Fb9b+jJXhyasfWVmfSySduMdRjOmquFz5nhOXYnvLzTqBL/UEIsIpZSfHbXX67CEKtRWYBR2bFSSC9ERskQKVJot8TjZxJwApeCV63Zz+TlIvHMXbG+N3NCr/sSoOVal2M4Hzm76+cgIBaMm2nXgBWzal/maD3W4ol+4qEDdxgZxEnm9KfsxOY2Uq0rf3zMjSHKtQ== 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, Dec 13, 2023 at 06:17:45PM +0800, Maria Yu wrote: > +static inline void write_lock_tasklist_lock(void) > +{ > + while (1) { > + local_irq_disable(); > + if (write_trylock(&tasklist_lock)) > + break; > + local_irq_enable(); > + cpu_relax(); This is a bad implementation though. You don't set the _QW_WAITING flag so readers don't know that there's a pending writer. Also, I've seen cpu_relax() pessimise CPU behaviour; putting it into a low-power mode that takes a while to wake up from. I think the right way to fix this is to pass a boolean flag to queued_write_lock_slowpath() to let it know whether it can re-enable interrupts while checking whether _QW_WAITING is set.