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Subject[PATCH v6 07/12] mmap locking API: add mmap_read_trylock_non_owner()
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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 <walken@google.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
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
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