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Message-ID: <20241105093400.GA10375@noisy.programming.kicks-ass.net>
Date: Tue, 5 Nov 2024 10:34:00 +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 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/ ?
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -829,6 +829,33 @@ static inline unsigned read_seqretry(con
return read_seqcount_retry(&sl->seqcount, start);
}
+
+static inline unsigned read_seq_scope_begin(const struct seqlock_t *sl)
+{
+ unsigned ret = read_seqcount_begin(&sl->seqcount);
+ kcsan_atomic_next(0);
+ kcsan_flat_atomic_begin();
+ return ret;
+}
+
+static inline void read_seq_scope_end(unsigned *seq)
+{
+ kcsan_flat_atomic_end();
+}
+
+static inline bool read_seq_scope_retry(const struct seqlock_t *sl, unsigned *seq)
+{
+ bool done = !read_seqcount_retry(&sl->seqcount, *seq);
+ if (!done)
+ *seq = read_seqcount_begin(&sl->seqcount);
+ return done;
+}
+
+#define read_seqcount_scope(sl) \
+ for (unsigned seq __cleanup(read_seq_scope_end) = \
+ read_seq_scope_begin(sl), done = 0; \
+ !done; done = read_seq_scope_retry(sl, &seq))
+
/*
* For all seqlock_t write side functions, use the internal
* do_write_seqcount_begin() instead of generic write_seqcount_begin().
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