[PATCH] Documentation/livepatch: remove the limitation for schedule() patching

[Miroslav Benes <mbenes@suse.cz>]

The Limitations section of the documentation describes the impossibility
to livepatch anything that is inlined to __schedule() function. This had
been true till 4.9 kernel came. Thanks to commit 0100301bfdf5
("sched/x86: Rewrite the switch_to() code") from Brian Gerst there is
__switch_to_asm function now (implemented in assembly) called properly
from context_switch(). RIP is thus saved on the stack and a task would
return to proper version of __schedule() et al. functions.

Of course __switch_to_asm() is not patchable for the reason described in
the section. But there is no __fentry__ call and I cannot imagine a
reason to do it anyway.

Therefore, remove the paragraphs from the section.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
---
FWIW, I also tested this to be sure on top of the consistency model
patch set. I patched schedule() function which calls __schedule() (it is
impossible to patch it directly due to notrace attribute). It works well
except...

1. the patching process does not finish, because many tasks sleep in
schedule. STOP/CONT signal does not help. I'll investigate.

2. reversion of the process does not work as expected. The kernel
crashes after the removal of the module. A task very likely slept in
schedule and was not migrated properly. It might be because of the races
in klp_reverse_transition() described by Petr, or might be somewhere
else. I'll look into it.

 Documentation/livepatch/livepatch.txt | 19 -------------------
 1 file changed, 19 deletions(-)

diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt
index f5967316deb9..7f04e13ec53d 100644
--- a/Documentation/livepatch/livepatch.txt
+++ b/Documentation/livepatch/livepatch.txt
@@ -329,25 +329,6 @@ See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
     by "notrace".
 
 
-  + Anything inlined into __schedule() can not be patched.
-
-    The switch_to macro is inlined into __schedule(). It switches the
-    context between two processes in the middle of the macro. It does
-    not save RIP in x86_64 version (contrary to 32-bit version). Instead,
-    the currently used __schedule()/switch_to() handles both processes.
-
-    Now, let's have two different tasks. One calls the original
-    __schedule(), its registers are stored in a defined order and it
-    goes to sleep in the switch_to macro and some other task is restored
-    using the original __schedule(). Then there is the second task which
-    calls patched__schedule(), it goes to sleep there and the first task
-    is picked by the patched__schedule(). Its RSP is restored and now
-    the registers should be restored as well. But the order is different
-    in the new patched__schedule(), so...
-
-    There is work in progress to remove this limitation.
-
-
   + Livepatch modules can not be removed.
 
     The current implementation just redirects the functions at the very
-- 
2.11.0