lib/stackdepot: annotate racy pool_index accesses

Accesses to pool_index are protected by pool_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.

[andreyknvl@google.com: s/pool_index/pool_index_cached/ in stack_depot_fetch()]
Link: https://lkml.kernel.org/r/95cf53f0da2c112aa2cc54456cbcd6975c3ff343.1676129911.git.andreyknvl@google.com
Link: https://lkml.kernel.org/r/359ac9c13cd0869c56740fb2029f505e41593830.1676063693.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 8c6e4e9..b625090 100644 (file)
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -278,8 +278,12 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
                        return NULL;
                }
 
-               /* Move on to the next pool. */
-               pool_index++;
+               /*
+                * Move on to the next pool.
+                * WRITE_ONCE pairs with potential concurrent read in
+                * stack_depot_fetch().
+                */
+               WRITE_ONCE(pool_index, pool_index + 1);
                pool_offset = 0;
                /*
                 * If the maximum number of pools is not reached, take note
@@ -502,6 +506,11 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
                               unsigned long **entries)
 {
        union handle_parts parts = { .handle = handle };
+       /*
+        * READ_ONCE pairs with potential concurrent write in
+        * depot_alloc_stack.
+        */
+       int pool_index_cached = READ_ONCE(pool_index);
        void *pool;
        size_t offset = parts.offset << DEPOT_STACK_ALIGN;
        struct stack_record *stack;
@@ -510,9 +519,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
        if (!handle)
                return 0;
 
-       if (parts.pool_index > pool_index) {
+       if (parts.pool_index > pool_index_cached) {
                WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
-                       parts.pool_index, pool_index, handle);
+                       parts.pool_index, pool_index_cached, handle);
                return 0;
        }
        pool = stack_pools[parts.pool_index];