uprobes: Document uprobe_register() vs uprobe_mmap() race

Because the mind is treacherous and makes us forget we need to
write stuff down.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anton Arapov <anton@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20120615154339.GA9591@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 28 insertions, 3 deletions

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 897417dbca8e..2671d9ad49be 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -44,6 +44,23 @@ static DEFINE_SPINLOCK(uprobes_treelock);	/* serialize rbtree access */
 
 #define UPROBES_HASH_SZ 13
 
+/*
+ * We need separate register/unregister and mmap/munmap lock hashes because
+ * of mmap_sem nesting.
+ *
+ * uprobe_register() needs to install probes on (potentially) all processes
+ * and thus needs to acquire multiple mmap_sems (consequtively, not
+ * concurrently), whereas uprobe_mmap() is called while holding mmap_sem
+ * for the particular process doing the mmap.
+ *
+ * uprobe_register()->register_for_each_vma() needs to drop/acquire mmap_sem
+ * because of lock order against i_mmap_mutex. This means there's a hole in
+ * the register vma iteration where a mmap() can happen.
+ *
+ * Thus uprobe_register() can race with uprobe_mmap() and we can try and
+ * install a probe where one is already installed.
+ */
+
 /* serialize (un)register */
 static struct mutex uprobes_mutex[UPROBES_HASH_SZ];
 
@@ -339,7 +356,9 @@ out:
 int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr)
 {
         int result;
-
+        /*
+         * See the comment near uprobes_hash().
+         */
         result = is_swbp_at_addr(mm, vaddr);
         if (result == 1)
                 return -EEXIST;
@@ -845,6 +864,10 @@ static int register_for_each_vma(struct uprobe *uprobe, bool is_register)
 
                 if (is_register) {
                         err = install_breakpoint(uprobe, mm, vma, info->vaddr);
+                        /*
+                         * We can race against uprobe_mmap(), see the
+                         * comment near uprobe_hash().
+                         */
                         if (err == -EEXIST)
                                 err = 0;
                 } else {
@@ -1054,8 +1077,10 @@ int uprobe_mmap(struct vm_area_struct *vma)
                         }
 
                         ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);
-
-                        /* Ignore double add: */
+                        /*
+                         * We can race against uprobe_register(), see the
+                         * comment near uprobe_hash().
+                         */
                         if (ret == -EEXIST) {
                                 ret = 0;