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[65.112.8.51]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-899c716e09csm22299906d6.19.2026.02.26.08.42.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Feb 2026 08:42:04 -0800 (PST) Date: Thu, 26 Feb 2026 11:42:02 -0500 From: Alan Stern To: Harry Yoo Cc: linux-mm@kvack.org, Dmitry Vyukov , lkmm@lists.linux.dev, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Joel Fernandes , Daniel Lustig , Akira Yokosawa , "Paul E. McKenney" , Luc Maranget , Jade Alglave , David Howells , Nicholas Piggin , Boqun Feng , Peter Zijlstra , Will Deacon , Andrea Parri , Pedro Falcato , Vlastimil Babka , Christoph Lameter , David Rientjes , Roman Gushchin , Hao Li , Shakeel Butt , Venkat Rao Bagalkote , Mateusz Guzik , Suren Baghdasaryan , Marco Elver Subject: Re: [BUG] Memory ordering between kmalloc() and kfree()? it's confusing! Message-ID: References: <9dcd02b9-42da-455d-aa08-165e6ff0b921@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: pa9odztqje4zto8bzm8uafzqkxfonw93 X-Rspam-User: X-Rspamd-Queue-Id: 3E5C8A0011 X-Rspamd-Server: rspam02 X-HE-Tag: 1772124126-103247 X-HE-Meta: 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 Xtzgp31q 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Feb 27, 2026 at 01:17:52AM +0900, Harry Yoo wrote: > On Thu, Feb 26, 2026 at 10:45:55AM -0500, Alan Stern wrote: > > On Thu, Feb 26, 2026 at 03:35:08PM +0900, Harry Yoo wrote: > > > Because the slab allocator itself doesn't guarantee that such > > > barriers are invoked within the allocator, it relies on users to > > > do this when needed. > > > > It doesn't? Then how does the slab allocator guarantee that two > > different CPUs won't try to perform allocations or deallocations from > > the same slab at the same time, messing everything up? > > Ah, alloc/free slowpaths do use cmpxchg128 or spinlock and > don't mess things up. > > But fastpath allocs/frees are served from percpu array that is protected > by a local_lock. local_lock has a compiler barrier in it, but that's > not enough. If those things rely on a percpu array, how can one CPU possibly manipulate a resource (slab or something else) that was changed by a different CPU? The whole point of percpu data structures is that each CPU gets its own copy. Alan Stern