mmap locking API: add mmap_read_trylock_non_owner()

Add a couple APIs used by kernel/bpf/stackmap.c only: - mmap_read_trylock_non_owner() - mmap_read_unlock_non_owner() (may be called from a work queue). It's still not ideal that bpf/stackmap subverts the lock ownership in this way. Thanks to Peter Zijlstra for suggesting this API as the least-ugly way of addressing this in the short term.

Michel Lespinasse committed 6 years ago

Commit 0cc55a01 parent aaa2cc52 files · +19 −12Browse files

Unified · Split

2 files

include/linux/mmap_lock.h +14 −0
kernel/bpf/stackmap.c +5 −12

include/linux/mmap_lock.h+14−0

@@ -56,4 +56,18 @@ static inline void mmap_read_unlock(struct mm_struct *mm) · 55 unmodified lines
56	up_read(&mm->mmap_sem);	56	up_read(&mm->mmap_sem);
57	}	57	}
58		58
		59	static inline bool mmap_read_trylock_non_owner(struct mm_struct *mm)
		60	{
		61	if (down_read_trylock(&mm->mmap_sem)) {
		62	rwsem_release(&mm->mmap_sem.dep_map, _RET_IP_);
		63	return true;
		64	}
		65	return false;
		66	}
		67
		68	static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
		69	{
		70	up_read_non_owner(&mm->mmap_sem);
		71	}
		72
59	#endif /* _LINUX_MMAP_LOCK_H */	73	#endif /* _LINUX_MMAP_LOCK_H */

kernel/bpf/stackmap.c+5−12

 /* irq_work to run up_read() for build_id lookup in nmi context */
 struct stack_map_irq_work {
 	struct irq_work irq_work;
-	struct rw_semaphore *sem;
+	struct mm_struct *mm;
 };
 static void do_up_read(struct irq_work *entry)
 		return;
 	work = container_of(entry, struct stack_map_irq_work, irq_work);
-	up_read_non_owner(work->sem);
+	mmap_read_unlock_non_owner(work->mm);
-	work->sem = NULL;
 }
 static DEFINE_PER_CPU(struct stack_map_irq_work, up_read_work);
 	 * with build_id.
 	 */
 	if (!user || !current || !current->mm || irq_work_busy ||
-	    mmap_read_trylock(current->mm) == 0) {
+	    !mmap_read_trylock_non_owner(current->mm)) {
 		/* cannot access current->mm, fall back to ips */
 		for (i = 0; i < trace_nr; i++) {
 			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
 	}
 	if (!work) {
-		mmap_read_unlock(current->mm);
+		mmap_read_unlock_non_owner(current->mm);
 	} else {
-		work->sem = &current->mm->mmap_sem;
+		work->mm = current->mm;
 		irq_work_queue(&work->irq_work);
-		/*
-		 * The irq_work will release the mmap_sem with
-		 * up_read_non_owner(). The rwsem_release() is called
-		 * here to release the lock from lockdep's perspective.
-		 */
-		rwsem_release(&current->mm->mmap_sem.dep_map, _RET_IP_);
 	}
 }