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));
[...]
As can be seen, read_seqretry() is never called if dentry != NULL;
consequently, current->kcsan_ctx.in_flat_atomic will never be reset to
false by read_seqretry().
Give up on the wrong assumption of "assume closing read_seqretry()", and
rely on the already-present annotations in read_seqcount_begin/retry().
Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* New patch.
---
include/linux/seqlock.h | 12 +-----------
1 file changed, 1 insertion(+), 11 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 45eee0e5dca0..5298765d6ca4 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -810,11 +810,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
-
- kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
- kcsan_flat_atomic_begin();
- return ret;
+ return read_seqcount_begin(&sl->seqcount);
}
/**
@@ -830,12 +826,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
*/
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
- /*
- * Assume not nested: read_seqretry() may be called multiple times when
- * completing read critical section.
- */
- kcsan_flat_atomic_end();
-
return read_seqcount_retry(&sl->seqcount, start);
}
--
2.47.0.163.g1226f6d8fa-googOn 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().
On Tue, 5 Nov 2024 at 10:34, Peter Zijlstra <peterz@infradead.org> 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/ ?
_loop seems straightforward.
> --- 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();
If we are guaranteed to always have one _begin paired by a matching
_end, we can s/kcsan_flat_atomic/kcsan_nestable_atomic/ for these.
> +}
> +
> +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))
> +
That's nice! I think before we fully moved over to C11, I recall Mark
and I discussed something like that (on IRC?) but gave up until C11
landed and then we forgot. ;-)
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.
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));
> [...]
>
> As can be seen, read_seqretry() is never called if dentry != NULL;
> consequently, current->kcsan_ctx.in_flat_atomic will never be reset to
> false by read_seqretry().
>
> Give up on the wrong assumption of "assume closing read_seqretry()", and
> rely on the already-present annotations in read_seqcount_begin/retry().
>
> Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v2:
> * New patch.
> ---
> include/linux/seqlock.h | 12 +-----------
> 1 file changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index 45eee0e5dca0..5298765d6ca4 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -810,11 +810,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
> */
> static inline unsigned read_seqbegin(const seqlock_t *sl)
> {
> - unsigned ret = read_seqcount_begin(&sl->seqcount);
> -
> - kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
> - kcsan_flat_atomic_begin();
> - return ret;
> + return read_seqcount_begin(&sl->seqcount);
> }
>
> /**
> @@ -830,12 +826,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
> */
> static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
> {
> - /*
> - * Assume not nested: read_seqretry() may be called multiple times when
> - * completing read critical section.
> - */
> - kcsan_flat_atomic_end();
> -
> return read_seqcount_retry(&sl->seqcount, start);
> }
OK, so this takes us back to kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)
and kcsan_atomic_next(0).
Which I suppose is safe, except it doesn't nest properly.
Anyway, these all look really nice, let me go queue them up.
Thanks!
On Tue, 5 Nov 2024 at 10:13, Peter Zijlstra <peterz@infradead.org> wrote:
> > static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
> > {
> > - /*
> > - * Assume not nested: read_seqretry() may be called multiple times when
> > - * completing read critical section.
> > - */
> > - kcsan_flat_atomic_end();
> > -
> > return read_seqcount_retry(&sl->seqcount, start);
> > }
>
> OK, so this takes us back to kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)
> and kcsan_atomic_next(0).
>
> Which I suppose is safe, except it doesn't nest properly.
Yes correct - we just give up on trying to be special here. It would
be nice to also have readers have a clear critical section, but that
seems a lot harder to enforce with a bunch of them having rather
convoluted control flow. :-/
> Anyway, these all look really nice, let me go queue them up.
Many thanks!
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 183ec5f26b2fc97a4a9871865bfe9b33c41fddb2
Gitweb: https://git.kernel.org/tip/183ec5f26b2fc97a4a9871865bfe9b33c41fddb2
Author: Marco Elver <elver@google.com>
AuthorDate: Mon, 04 Nov 2024 16:43:09 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 05 Nov 2024 12:55:35 +01:00
kcsan, seqlock: Fix incorrect assumption in read_seqbegin()
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));
[...]
As can be seen, read_seqretry() is never called if dentry != NULL;
consequently, current->kcsan_ctx.in_flat_atomic will never be reset to
false by read_seqretry().
Give up on the wrong assumption of "assume closing read_seqretry()", and
rely on the already-present annotations in read_seqcount_begin/retry().
Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241104161910.780003-6-elver@google.com
---
include/linux/seqlock.h | 12 +-----------
1 file changed, 1 insertion(+), 11 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 45eee0e..5298765 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -810,11 +810,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
-
- kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
- kcsan_flat_atomic_begin();
- return ret;
+ return read_seqcount_begin(&sl->seqcount);
}
/**
@@ -830,12 +826,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
*/
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
- /*
- * Assume not nested: read_seqretry() may be called multiple times when
- * completing read critical section.
- */
- kcsan_flat_atomic_end();
-
return read_seqcount_retry(&sl->seqcount, start);
}© 2016 - 2026 Red Hat, Inc.