uprobes: allow put_uprobe() from non-sleepable softirq context

Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
makes it unsuitable to be called from more restricted context like softirq.

Let's make put_uprobe() agnostic to the context in which it is called,
and use work queue to defer the mutex-protected clean up steps.

RB tree removal step is also moved into work-deferred callback to avoid
potential deadlock between softirq-based timer callback, added in the
next patch, and the rest of uprobe code.

We can rework locking altogher as a follow up, but that's significantly
more tricky, so warrants its own patch set. For now, we need to make
sure that changes in the next patch that add timer thread work correctly
with existing approach, while concentrating on SRCU + timeout logic.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241024044159.3156646-2-andrii@kernel.org

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 4ef4b51776ebe0d90b997281fcb32bd549c2d34e..d7e4892466083820fbeb47f102cf2674a60a2e7e 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -27,6 +27,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/khugepaged.h>
 #include <linux/rcupdate_trace.h>
+#include <linux/workqueue.h>
 
 #include <linux/uprobes.h>
 
@@ -61,7 +62,10 @@ struct uprobe {
        struct list_head        pending_list;
        struct list_head        consumers;
        struct inode            *inode;         /* Also hold a ref to inode */
-       struct rcu_head         rcu;
+       union {
+               struct rcu_head         rcu;
+               struct work_struct      work;
+       };
        loff_t                  offset;
        loff_t                  ref_ctr_offset;
        unsigned long           flags;          /* "unsigned long" so bitops work */
@@ -625,10 +629,9 @@ static void uprobe_free_rcu(struct rcu_head *rcu)
        kfree(uprobe);
 }
 
-static void put_uprobe(struct uprobe *uprobe)
+static void uprobe_free_deferred(struct work_struct *work)
 {
-       if (!refcount_dec_and_test(&uprobe->ref))
-               return;
+       struct uprobe *uprobe = container_of(work, struct uprobe, work);
 
        write_lock(&uprobes_treelock);
 
@@ -652,6 +655,15 @@ static void put_uprobe(struct uprobe *uprobe)
        call_rcu_tasks_trace(&uprobe->rcu, uprobe_free_rcu);
 }
 
+static void put_uprobe(struct uprobe *uprobe)
+{
+       if (!refcount_dec_and_test(&uprobe->ref))
+               return;
+
+       INIT_WORK(&uprobe->work, uprobe_free_deferred);
+       schedule_work(&uprobe->work);
+}
+
 static __always_inline
 int uprobe_cmp(const struct inode *l_inode, const loff_t l_offset,
               const struct uprobe *r)