linux-kernel - Re: [PATCH v2 5/5] kcsan, seqlock: Fix incorrect assumption in read

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Message-ID: <20241105094115.GX33184@noisy.programming.kicks-ass.net>
Date: Tue, 5 Nov 2024 10:41:15 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Marco Elver <elver@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mark Rutland <mark.rutland@....com>,
	Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 5/5] kcsan, seqlock: Fix incorrect assumption in
 read_seqbegin()

On Tue, Nov 05, 2024 at 10:34:00AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 04, 2024 at 04:43:09PM +0100, Marco Elver wrote:
> > During testing of the preceding changes, I noticed that in some cases,
> > current->kcsan_ctx.in_flat_atomic remained true until task exit. This is
> > obviously wrong, because _all_ accesses for the given task will be
> > treated as atomic, resulting in false negatives i.e. missed data races.
> > 
> > Debugging led to fs/dcache.c, where we can see this usage of seqlock:
> > 
> > 	struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
> > 	{
> > 		struct dentry *dentry;
> > 		unsigned seq;
> > 
> > 		do {
> > 			seq = read_seqbegin(&rename_lock);
> > 			dentry = __d_lookup(parent, name);
> > 			if (dentry)
> > 				break;
> > 		} while (read_seqretry(&rename_lock, seq));
> > 	[...]
> 
> 
> How's something like this completely untested hack?
> 
> 
> 	struct dentry *dentry;
> 
> 	read_seqcount_scope (&rename_lock) {
> 		dentry = __d_lookup(parent, name);
> 		if (dentry)
> 			break;
> 	}
> 
> 
> But perhaps naming isn't right, s/_scope/_loop/ ?

It is also confused between seqcount and seqlock. So perhaps it should
read:

	read_seqcount_loop (&rename_lock.seqcount) {
	   ...
	}

instead.