um: Fix incorrect __acquires/__releases annotations

With Clang's context analysis, the compiler is a bit more strict about
what goes into the __acquires/__releases annotations and can't refer to
non-existent variables.

On an UM build, mm_id.h is transitively included into mm_types.h, and we
can observe the following error (if context analysis is enabled in e.g.
stackdepot.c):

   In file included from lib/stackdepot.c:17:
   In file included from include/linux/debugfs.h:15:
   In file included from include/linux/fs.h:5:
   In file included from include/linux/fs/super.h:5:
   In file included from include/linux/fs/super_types.h:7:
   In file included from include/linux/list_lru.h:14:
   In file included from include/linux/xarray.h:16:
   In file included from include/linux/gfp.h:7:
   In file included from include/linux/mmzone.h:22:
   In file included from include/linux/mm_types.h:26:
   In file included from arch/um/include/asm/mmu.h:12:
>> arch/um/include/shared/skas/mm_id.h:24:54: error: use of undeclared identifier 'turnstile'
      24 | void enter_turnstile(struct mm_id *mm_id) __acquires(turnstile);
         |                                                      ^~~~~~~~~
   arch/um/include/shared/skas/mm_id.h:25:53: error: use of undeclared identifier 'turnstile'
      25 | void exit_turnstile(struct mm_id *mm_id) __releases(turnstile);
         |                                                     ^~~~~~~~~

One (discarded) option was to use token_context_lock(turnstile) to just
define a token with the already used name, but that would not allow the
compiler to distinguish between different mm_id-dependent instances.

Another constraint is that struct mm_id is only declared and incomplete
in the header, so even if we tried to construct an expression to get to
the mutex instance, this would fail (including more headers transitively
everywhere should also be avoided).

Instead, just declare an mm_id-dependent helper to return the mutex, and
use the mm_id-dependent call expression in the __acquires/__releases
attributes; the compiler will consider the identity of the mutex to be
the call expression. Then using __get_turnstile() in the lock/unlock
wrappers (with context analysis enabled for mmu.c) the compiler will be
able to verify the implementation of the wrappers as-is.

We leave context analysis disabled in arch/um/kernel/skas/ for now. This
change is a preparatory change to allow enabling context analysis in
subsystems that include any of the above headers.

No functional change intended.

Closes: https://lore.kernel.org/oe-kbuild-all/202512171220.vHlvhpCr-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-23-elver@google.com

diff --git a/arch/um/include/shared/skas/mm_id.h b/arch/um/include/shared/skas/mm_id.h
index fb96c0bd82228ed7098effabe9b2c7d7d6c154d7..18c0621430d2caaf28aa21f907a191dc5d97c1da 100644 (file)
--- a/arch/um/include/shared/skas/mm_id.h
+++ b/arch/um/include/shared/skas/mm_id.h
@@ -21,8 +21,9 @@ struct mm_id {
        int syscall_fd_map[STUB_MAX_FDS];
 };
 
-void enter_turnstile(struct mm_id *mm_id) __acquires(turnstile);
-void exit_turnstile(struct mm_id *mm_id) __releases(turnstile);
+struct mutex *__get_turnstile(struct mm_id *mm_id);
+void enter_turnstile(struct mm_id *mm_id) __acquires(__get_turnstile(mm_id));
+void exit_turnstile(struct mm_id *mm_id) __releases(__get_turnstile(mm_id));
 
 void notify_mm_kill(int pid);
 

diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index 00957788591b3dd91a99e5d6fd6df306ee0ecc38..b5017096028ba166d7c87bef345952517757ee13 100644 (file)
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -23,18 +23,21 @@ static_assert(sizeof(struct stub_data) == STUB_DATA_PAGES * UM_KERN_PAGE_SIZE);
 static spinlock_t mm_list_lock;
 static struct list_head mm_list;
 
-void enter_turnstile(struct mm_id *mm_id) __acquires(turnstile)
+struct mutex *__get_turnstile(struct mm_id *mm_id)
 {
        struct mm_context *ctx = container_of(mm_id, struct mm_context, id);
 
-       mutex_lock(&ctx->turnstile);
+       return &ctx->turnstile;
 }
 
-void exit_turnstile(struct mm_id *mm_id) __releases(turnstile)
+void enter_turnstile(struct mm_id *mm_id)
 {
-       struct mm_context *ctx = container_of(mm_id, struct mm_context, id);
+       mutex_lock(__get_turnstile(mm_id));
+}
 
-       mutex_unlock(&ctx->turnstile);
+void exit_turnstile(struct mm_id *mm_id)
+{
+       mutex_unlock(__get_turnstile(mm_id));
 }
 
 int init_new_context(struct task_struct *task, struct mm_struct *mm)