From: Andrey Konovalov <[email protected]>
Accesses to slab_index are protected by slab_lock everywhere except
in a sanity check in stack_depot_fetch. The read access there can race
with the write access in depot_alloc_stack.
Use WRITE/READ_ONCE() to annotate the racy accesses.
As the sanity check is only used to print a warning in case of a
violation of the stack depot interface usage, it does not make a lot
of sense to use proper synchronization.
Signed-off-by: Andrey Konovalov <[email protected]>
---
lib/stackdepot.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index f291ad6a4e72..cc2fe8563af4 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
return NULL;
}
- /* Move on to the next slab. */
- slab_index++;
+ /*
+ * Move on to the next slab.
+ * WRITE_ONCE annotates a race with stack_depot_fetch.
+ */
+ WRITE_ONCE(slab_index, slab_index + 1);
slab_offset = 0;
/*
* smp_store_release() here pairs with smp_load_acquire() in
@@ -492,6 +495,8 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
unsigned long **entries)
{
union handle_parts parts = { .handle = handle };
+ /* READ_ONCE annotates a race with depot_alloc_stack. */
+ int slab_index_cached = READ_ONCE(slab_index);
void *slab;
size_t offset = parts.offset << DEPOT_STACK_ALIGN;
struct stack_record *stack;
@@ -500,9 +505,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
if (!handle)
return 0;
- if (parts.slab_index > slab_index) {
+ if (parts.slab_index > slab_index_cached) {
WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
- parts.slab_index, slab_index, handle);
+ parts.slab_index, slab_index_cached, handle);
return 0;
}
slab = stack_slabs[parts.slab_index];
--
2.25.1
On Mon, 30 Jan 2023 at 21:51, <[email protected]> wrote:
>
> From: Andrey Konovalov <[email protected]>
>
> Accesses to slab_index are protected by slab_lock everywhere except
> in a sanity check in stack_depot_fetch. The read access there can race
> with the write access in depot_alloc_stack.
>
> Use WRITE/READ_ONCE() to annotate the racy accesses.
>
> As the sanity check is only used to print a warning in case of a
> violation of the stack depot interface usage, it does not make a lot
> of sense to use proper synchronization.
>
> Signed-off-by: Andrey Konovalov <[email protected]>
> ---
> lib/stackdepot.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index f291ad6a4e72..cc2fe8563af4 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> return NULL;
> }
>
> - /* Move on to the next slab. */
> - slab_index++;
> + /*
> + * Move on to the next slab.
> + * WRITE_ONCE annotates a race with stack_depot_fetch.
"Pairs with potential concurrent read in stack_depot_fetch()." would be clearer.
I wouldn't say WRITE_ONCE annotates a race (race = involves 2+
accesses, but here's just 1), it just marks this access here which
itself is paired with the potential racing read in the other function.
> + */
> + WRITE_ONCE(slab_index, slab_index + 1);
> slab_offset = 0;
> /*
> * smp_store_release() here pairs with smp_load_acquire() in
> @@ -492,6 +495,8 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
> unsigned long **entries)
> {
> union handle_parts parts = { .handle = handle };
> + /* READ_ONCE annotates a race with depot_alloc_stack. */
> + int slab_index_cached = READ_ONCE(slab_index);
> void *slab;
> size_t offset = parts.offset << DEPOT_STACK_ALIGN;
> struct stack_record *stack;
> @@ -500,9 +505,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
> if (!handle)
> return 0;
>
> - if (parts.slab_index > slab_index) {
> + if (parts.slab_index > slab_index_cached) {
> WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
> - parts.slab_index, slab_index, handle);
> + parts.slab_index, slab_index_cached, handle);
> return 0;
> }
> slab = stack_slabs[parts.slab_index];
> --
> 2.25.1
>
On Tue, Jan 31, 2023 at 9:41 AM Marco Elver <[email protected]> wrote:
>
> > diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> > index f291ad6a4e72..cc2fe8563af4 100644
> > --- a/lib/stackdepot.c
> > +++ b/lib/stackdepot.c
> > @@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> > return NULL;
> > }
> >
> > - /* Move on to the next slab. */
> > - slab_index++;
> > + /*
> > + * Move on to the next slab.
> > + * WRITE_ONCE annotates a race with stack_depot_fetch.
>
> "Pairs with potential concurrent read in stack_depot_fetch()." would be clearer.
>
> I wouldn't say WRITE_ONCE annotates a race (race = involves 2+
> accesses, but here's just 1), it just marks this access here which
> itself is paired with the potential racing read in the other function.
Will do in v2. Thanks!
On Tue, 31 Jan 2023 19:57:58 +0100 Andrey Konovalov <[email protected]> wrote:
> On Tue, Jan 31, 2023 at 9:41 AM Marco Elver <[email protected]> wrote:
> >
> > > diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> > > index f291ad6a4e72..cc2fe8563af4 100644
> > > --- a/lib/stackdepot.c
> > > +++ b/lib/stackdepot.c
> > > @@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> > > return NULL;
> > > }
> > >
> > > - /* Move on to the next slab. */
> > > - slab_index++;
> > > + /*
> > > + * Move on to the next slab.
> > > + * WRITE_ONCE annotates a race with stack_depot_fetch.
> >
> > "Pairs with potential concurrent read in stack_depot_fetch()." would be clearer.
> >
> > I wouldn't say WRITE_ONCE annotates a race (race = involves 2+
> > accesses, but here's just 1), it just marks this access here which
> > itself is paired with the potential racing read in the other function.
>
> Will do in v2. Thanks!
Please let's not redo an 18-patch series for a single line comment
change. If there are more substantial changes then OK.
I queued this as a to-be-squashed fixup against "/stackdepot: annotate
racy slab_index accesses":
From: Andrew Morton <[email protected]>
Subject: lib-stackdepot-annotate-racy-slab_index-accesses-fix
Date: Tue Jan 31 01:10:50 PM PST 2023
enhance comment, per Marco
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Evgenii Stepanov <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---
lib/stackdepot.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/lib/stackdepot.c~lib-stackdepot-annotate-racy-slab_index-accesses-fix
+++ a/lib/stackdepot.c
@@ -271,7 +271,8 @@ depot_alloc_stack(unsigned long *entries
/*
* Move on to the next slab.
- * WRITE_ONCE annotates a race with stack_depot_fetch.
+ * WRITE_ONCE pairs with potential concurrent read in
+ * stack_depot_fetch().
*/
WRITE_ONCE(slab_index, slab_index + 1);
slab_offset = 0;
_