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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHqfl-009uMY-Sl; Tue, 17 Jan 2023 18:27:02 +0000 Date: Tue, 17 Jan 2023 18:27:01 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: Hillf Danton , vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, peterz@infradead.org, hughd@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock Message-ID: References: <20230109205336.3665937-42-surenb@google.com> <20230116140649.2012-1-hdanton@sina.com> <20230117031632.2321-1-hdanton@sina.com> <20230117083355.2374-1-hdanton@sina.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D1093180003 X-Stat-Signature: dty3e6n7h7s6u6iikumh1n9p8fj38azd X-HE-Tag: 1673980026-770071 X-HE-Meta: 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 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, Jan 17, 2023 at 10:21:28AM -0800, Suren Baghdasaryan wrote: > static inline bool vma_read_trylock(struct vm_area_struct *vma) > { > int count, new; > > /* Check before locking. A race might cause false locked result. */ > if (READ_ONCE(vma->vm_lock->lock_seq) == > READ_ONCE(vma->vm_mm->mm_lock_seq)) > return false; > > count = atomic_read(&vma->vm_lock->count); > for (;;) { > /* > * Is VMA is write-locked? Overflow might produce false > locked result. > * False unlocked result is impossible because we modify and check > * vma->vm_lock_seq under vma->vm_lock protection and > mm->mm_lock_seq > * modification invalidates all existing locks. > */ > if (count < 0) > return false; > > new = count + 1; > /* If atomic_t overflows, fail to lock. */ > if (new < 0) > return false; > > /* > * Atomic RMW will provide implicit mb on success to pair > with smp_wmb in > * vma_write_lock, on failure we retry. > */ > new = atomic_cmpxchg(&vma->vm_lock->count, count, new); > if (new == count) > break; > count = new; > cpu_relax(); The cpu_relax() is exactly the wrong thing to do here. See this thread: https://lore.kernel.org/linux-fsdevel/20230113184447.1707316-1-mjguzik@gmail.com/