Re: [PATCH v6 07/12] mmap locking API: add mmap_read_trylock_non_owner()

[Davidlohr Bueso]

On Tue, 19 May 2020, Michel Lespinasse wrote:

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.

Signed-off-by: Michel Lespinasse <wal...@google.com>
Reviewed-by: Daniel Jordan <daniel.m.jor...@oracle.com>
Reviewed-by: Vlastimil Babka <vba...@suse.cz>

Sigh, bpf, but ok.

Reviewed-by: Davidlohr Bueso <dbu...@suse.de>

---
include/linux/mmap_lock.h | 14 ++++++++++++++
kernel/bpf/stackmap.c     | 17 +++++------------
2 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index a757cb30ae77..d1826ce42f00 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -56,4 +56,18 @@ static inline void mmap_read_unlock(struct mm_struct *mm)
        up_read(&mm->mmap_sem);
}

+static inline bool mmap_read_trylock_non_owner(struct mm_struct *mm)
+{
+       if (down_read_trylock(&mm->mmap_sem)) {
+               rwsem_release(&mm->mmap_sem.dep_map, _RET_IP_);
+               return true;
+       }
+       return false;
+}
+
+static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
+{
+       up_read_non_owner(&mm->mmap_sem);
+}
+
#endif /* _LINUX_MMAP_LOCK_H */
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 11d41f0c7005..998968659892 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -33,7 +33,7 @@ struct bpf_stack_map {
/* 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)
@@ -44,8 +44,7 @@ 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);
-       work->sem = NULL;
+       mmap_read_unlock_non_owner(work->mm);
}

static DEFINE_PER_CPU(struct stack_map_irq_work, up_read_work);
@@ -317,7 +316,7 @@ static void stack_map_get_build_id_offset(struct 
bpf_stack_build_id *id_offs,
         * 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;
@@ -342,16 +341,10 @@ static void stack_map_get_build_id_offset(struct 
bpf_stack_build_id *id_offs,
        }

        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_);
        }
}

--
2.26.2.761.g0e0b3e54be-goog