From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B000621257E for ; Sun, 12 Apr 2026 13:16:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775999760; cv=none; b=M685qbZySf46L8SsWJy+cz5Cx0SgakKUnMsAU5XvpTAlSjj0rnr6JvdZD9sE+hkn+xr8yD1zs7I9JM/YdFf3b0Y6ZzaxjbCJlEmTfPAEpygtOSLGJ4C7TAch3QCmK9IPn4EdpYpEk5ckyeAhhwTQ529smHKSM7Be6XuEnTlyeXo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775999760; c=relaxed/simple; bh=tmZGN3DkNkJrxDZkEqenppuLHjwCsg2Wr81mtFU95sY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=A85qMZSMa+ZXWScpfmgeBt+h66qirzumI9kRc6dDLziL0WBkIAo25S5FaRJeq6Ah0/v9xo1TMRLB42xlE04BnZMl/6RjTJXp/CXwoq6An/ia5C4YS4L2RWUitBiTAlEDioKY5kFHi52WcmrbO6xT/ZjyM9RRu8+IhMlEmFgKQ1E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oZcSN8bX; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="oZcSN8bX" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DD512C19424; Sun, 12 Apr 2026 13:15:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775999760; bh=tmZGN3DkNkJrxDZkEqenppuLHjwCsg2Wr81mtFU95sY=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=oZcSN8bXbzXnR3+3tW1ugrgKhe4rnccebqJjfEKeSdEMrUVU9ddDqTEXcgzbxsv4C rEZrzEMMp/x8Ag6ENCqcpH52qyGFeJ4FWjStj9W2Yt2CL4UTDmWUUKzLhBoseKUEiW XnWNtIJ64smT4otOivAS+zI4HwGMyfXDVrIkvNcVbRjNQ6I/aLpBPUOEfCwJK+u1Sf oyd4pMejZVSxgo0OZmVOK+HSb+ZdN0qh+yHrZLeB0Vbvtf9xoAl+SWJrNY6ttW0ZnJ lBKiOSK2hZG9vOJldSjzGbf22eoceEJ/NmPMfvDQnz28uu4oP/WjELbKRh6dIRz+yT iBXwMCfyG3NDA== Message-ID: <72698ebb266c70c36910d401d03f45ac0fc33fc0.camel@kernel.org> Subject: Re: [RFC][PATCH] make sure that lock_for_kill() callers drop the locks in safe order From: Jeff Layton To: Al Viro , Linus Torvalds Cc: "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , linux-fsdevel@vger.kernel.org, Christian Brauner , Jan Kara , Nikolay Borisov , Max Kellermann , Eric Sandeen , Paulo Alcantara Date: Sun, 12 Apr 2026 09:15:56 -0400 In-Reply-To: <20260411213348.GB3836593@ZenIV> References: <20260404080751.2526990-1-viro@zeniv.linux.org.uk> <53b4795292d1df2fe1569fc724325ab52fcab322.camel@kernel.org> <20260409190213.GQ3836593@ZenIV> <41cfd0f95b7fde411c0d59463dce979be89cb8ef.camel@kernel.org> <20260409215733.GS3836593@ZenIV> <20260410084839.GA1310153@ZenIV> <20260410185243.GU3836593@ZenIV> <20260410202404.GW3836593@ZenIV> <20260411213348.GB3836593@ZenIV> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sat, 2026-04-11 at 22:33 +0100, Al Viro wrote: > On Fri, Apr 10, 2026 at 09:24:04PM +0100, Al Viro wrote: > > On Fri, Apr 10, 2026 at 12:30:13PM -0700, Linus Torvalds wrote: > >=20 > > > The reason it exists is because lock_for_kill() can drop d_lock(), bu= t > > > that's in the unlikely case that we cn't just immediately get the > > > inode lock. > > >=20 > > > So honestly, I think that rcu_read_lock() should be inside > > > lock_for_kill(), rather than in the caller as a "just in case things > > > go down". > >=20 > > Yup, in the cascade of followups I've mentioned... >=20 > FWIW, see #work.dcache-cleanups (on top of #work.dcache-busy-wait). That= 's > obviously next cycle fodder, and it needs review and testing (at the mome= nt > I've only build-tested that). >=20 > Branch is in git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #= work.dcache-cleanups > individual patches in followups. >=20 > I think the eviction machinery got easier to read, if nothing else, and > rcu_read_lock() scopes are saner now. OTOH, if we *do* have RCU bugs wrt > overlapping scopes, we'll probably see a lot of UAF on that - rcu_read_lo= ck() > scopes got aggressively minimized... >=20 > I've split the rcu_read_lock() massage into a series of small steps after > getting confused while trying to do them at once; not sure if it needs > to be carved up so much, but... >=20 > Shortlog: > Al Viro (11): > shrink_dentry_list(): start with removing from shrink list > fold lock_for_kill() into shrink_kill() > fold lock_for_kill() and __dentry_kill() into common helper > reducing rcu_read_lock() scopes in dput and friends, step 1 > reducing rcu_read_lock() scopes in dput and friends, step 2 > reducing rcu_read_lock() scopes in dput and friends, step 3 > reducing rcu_read_lock() scopes in dput and friends, step 4 > reducing rcu_read_lock() scopes in dput and friends, step 5 > reducing rcu_read_lock() scopes in dput and friends, step 6 > adjust calling conventions of lock_for_kill(), fold __dentry_kill()= into dentry_kill() > document dentry_kill() >=20 > Diffstat: > fs/dcache.c | 214 ++++++++++++++++++++++++++++++++++++------------------= ------ > 1 file changed, 129 insertions(+), 85 deletions(-) > 63 lines of comments added - outside of comments it's -20LoC... FWIW, I ran fstests + the earlier reproducer on a v6.19-ish kernel without any of your fixes, and with "slub_debug=3DP,dentry" on the kernel command line. It also had the patch below. I got no warnings back: --------------------------8<-------------------------- diff --git a/fs/Kconfig b/fs/Kconfig index 0bfdaecaa877..f3a228cd14e8 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -9,6 +9,15 @@ menu "File systems" config DCACHE_WORD_ACCESS bool =20 +config DCACHE_SHRINK_RACE_DEBUG + bool "Debug: inject delay in __dentry_kill to widen race window" + depends on DEBUG_KERNEL + default n + help + Inject a probabilistic delay in __dentry_kill() between releasing + d_lock and re-acquiring it, to make dcache shrink races reproducible + in test environments. Only enable for testing. + config VALIDATE_FS_PARSER bool "Validate filesystem parameter description" help diff --git a/fs/dcache.c b/fs/dcache.c index 66dd1bb830d1..b1625f7e30e1 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -32,6 +32,8 @@ #include #include #include +#include +#include #include "internal.h" #include "mount.h" =20 @@ -675,6 +677,16 @@ static struct dentry *__dentry_kill(struct dentry *den= try) dentry->d_op->d_release(dentry); =20 cond_resched(); +#ifdef CONFIG_DCACHE_SHRINK_RACE_DEBUG + /* + * Probabilistic delay to widen the race window in __dentry_kill() + * between dropping d_lock and re-acquiring it. 30% chance of 5ms + * delay for proc dentries. + */ + if (dentry->d_sb->s_magic =3D=3D 0x9fa0 /* PROC_SUPER_MAGIC */ && + get_random_u32_below(100) < 30) + mdelay(5); +#endif /* now that it's negative, ->d_parent is stable */ if (!IS_ROOT(dentry)) { parent =3D dentry->d_parent; @@ -2722,6 +2734,9 @@ static wait_queue_head_t *__d_lookup_unhash(struct de= ntry *dentry) =20 lockdep_assert_held(&dentry->d_lock); =20 + if (WARN_ON(!(dentry->d_flags & DCACHE_PAR_LOOKUP))) + return NULL; + b =3D in_lookup_hash(dentry->d_parent, dentry->d_name.hash); hlist_bl_lock(b); dentry->d_flags &=3D ~DCACHE_PAR_LOOKUP;