From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793874335; b=INE4QXp14So0JutxnEVxr5zApi4mv0jS9H0sBjPbrRrjfvkB1WMoZiMDZ2fwmmkQX8cS v+fvslum6jhOmG6STuGrNggZ/c1z7S67cH4JAbOfiu68dl/MrRBRDFRFlckh9ya3iAVFi T+dxL5+eEr7XmMb+ylhK0NBvWGagxZhdIJ3fuStIyMb6ZNl7GHw0v9BrJDW/Mzfhuq4WY v42v8l6iQd9paQYWsZqNXwX/4SeFm0deg7oRecTEopQvdX5GRNcb57lEz41aDPgPJ73k9 woaVGcF7KRDF6udBtK7tBKaVPbu4U6MMwD55LBZMHE4ddN53VhMQotnlcC9Mfvx5gtw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793874335; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=CTZjVvkIUqA1hYxbAMBN5D0S+bQTUaargjdOqGhgIgw=; b=IZxeKe72Wyyov0LYs3JTcuSApXa4F6Jy03TO2TSlRCoEYmdu4R5J0Y6ZF9tEq1Th19Gb wycRTltCqw4jZFyt0xN9yIczmDi2fXz54Mdk27PfOeBcDNHd7vHWi/Mdf8Jm7uJgqt50O TFus/NkA98TOJyYJ95N6rKkrzo1cFmC1kBhtNjwmzcpQzFH4vNsylbQ95pZWbxIFgAnHl uJ09jw5yuATFlAZQyuytZJyg4cY0y6ajt3VuaaJOS54qdQh7akJdL191kgrZtnBV6A68H 5LNEPta8t+LdeRQ6wiXmtAwmUQaU1CHwbf1P33D2xvf0hkBgpHfEEukL4YrsCK010Aw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=AWA5EUum; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 87F1F606A31B; Thu, 5 Nov 2026 10:25:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=AWA5EUum Received: by smtp.kernel.org (Postfix) with ESMTPSA id BC3DA1F000C1; Thu, 5 Nov 2026 10:25:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793874335; bh=CTZjVvkIUqA1hYxbAMBN5D0S+bQTUaargjdOqGhgIgw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=AWA5EUumNdwSBWn4g6tCydb8yT3P9NIpU1AhnI/l68ypPPQQjsmlpmZcmjqP0LN7w N27be4rfuMZiWwBCq7her7lB+sc11HSH4D/It973/gw2n+J+yJdXPUKAwXRjqOXG6y +3pvrnmk/IkTbMiVT+f7swrdRCPqMW2Hvq2OBNpXeaZjFaLDXFyS2ZLfADRs+mUY4L zAGr34i+l3CSk3NcxiMWRysp3rMsXyT3OAyIDd0GQElcCJpX8c7qsNkVzOhFqRdS82 7Ro2+vUiGHSoWSeJ0BfSs4RidofOfhFtdjMtqubRz5QJeJjYyTeuR/NxOeDYB3wbBP DHBSE9YQBSBJQ== Message-ID: <9c367177-1822-4b6d-8bee-d283eef7b895@kernel.org> Date: Thu, 5 Nov 2026 11:25:25 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/5] locking/rwsem: add down_read[_killable]_nosteal() To: "Lorenzo Stoakes (ARM)" , Peter Zijlstra Cc: Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Andrew Morton , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Suren Baghdasaryan , Vlastimil Babka , Shakeel Butt , Yonghong Song , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai , linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org References: <20261103-khugepaged-latency-fix-v1-0-a07ab16ac860@kernel.org> <20261103-khugepaged-latency-fix-v1-3-a07ab16ac860@kernel.org> <20261103115159.GK1813469@noisy.programming.kicks-ass.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/3/26 13:24, Lorenzo Stoakes (ARM) wrote: > On Tue, Nov 03, 2026 at 12:51:59PM +0100, Peter Zijlstra wrote: >> On Tue, Nov 03, 2026 at 08:27:44AM +0000, Lorenzo Stoakes (ARM) wrote: >>> Since commit 1a728dff855a ("locking/rwsem: Enable reader optimistic lock >>> stealing") a sleeping, queued writer whose handoff timeout hasn't expired >>> can have its lock stolen by a read lock issued before a woken queued writer >>> was able to acquire a lock. >>> >>> To avoid write lock starvation, this is only permitted if no other read >>> locks are held. >>> >>> However, this behaviour defeats the purpose of a reader periodically >>> dropping the lock to allow writers in, as doing so only works if the write >>> lock is acquired first, another read lock is held, or a queued writer's >>> handoff timeout has expired. >>> >>> Provide a means of indicating this intent via down_read_nosteal() and >>> down_read_killable_nosteal(). >>> >>> Implement it by exposing a steal parameter to rwsem_down_read_slowpath(), >>> update the starvation logic to modify this parameter and modify the steal >>> branch to check it first. >>> >>> Also drop the duplicated RWSEM_WRITER_LOCKED check and remove the >>> goto as the steal condition makes this redundant. >>> >>> Exposing this function is safe as it only makes the lock more pessimistic, >>> so does not impact lock contention. >> >> Not generally a fan of this -- can't we tweak the steal thing to never >> steal if there are pending writers? > > Oh, well, I had assumed I couldn't fiddle with that behaviour across > rwsem's in general, so if I can yeah that's preferable! > > I could send that change separately then and drop it out of this series. Just so I understand correctly, this patch here would be gone entirely? But #4 depends on it, so now I'm confused :) -- Cheers, David