Making Sure All Locks Are Released

assert(NO_LOCKS_HELD);

LockLint recognizes this assertion to mean that, when this point in the code is reached, no locks should be held by the thread executing this test. Violations are reported during analysis. A routine that blocks might want to use such an assertion to ensure that no locks are held when a thread blocks or exits.

The assertion also clearly serves as a reminder to someone modifying the code that any locks acquired must be released at that point.

It is really only necessary to use this assertion in leaf-level functions that block. If a function blocks only inasmuch as it calls another function that blocks, the caller need not contain this assertion as long as the callee does. Therefore this assertion probably sees its heaviest use in versions of libraries (for example, libc) written specifically for LockLint (like lint libraries).

The file synch.h defines NO_LOCKS_HELD as 1 if it has not already been otherwise defined, causing the assertion to succeed; that is, the assertion is effectively ignored at runtime. You can override this default runtime meaning by defining NO_LOCKS_HELD before you include either note.h or synch.h (which may be included in either order). For example, if a body of code uses only two locks called a and b, the following definition would probably suffice:

#define NO_LOCKS_HELD (!MUTEX_HELD(&a) && !MUTEX_HELD(&b))
#include <note.h>
#include <synch.h>

Doing so does not affect LockLint's testing of the assertion; that is, LockLint still complains if any locks are held (not just a or b).