From: Thomas Gleixner <[email protected]>
Provide functions that are related to the safe handover mechanism
and allow console drivers to dynamically specify unsafe regions:
- nbcon_context_can_proceed()
Invoked by a console owner to check whether a handover request
is pending or whether the console has been taken over by another
context. If a handover request is pending, this function will
also perform the handover, thus cancelling its own ownership.
- nbcon_context_enter_unsafe()/nbcon_context_exit_unsafe()
Invoked by a console owner to denote that the driver is about
to enter or leave a critical region where a take over is unsafe.
The exit variant is the point where the current owner releases
the lock for a higher priority context which asked for the
friendly handover.
The unsafe state is stored in the console state and allows a
new context to make informed decisions whether to attempt a
takeover of such a console. The unsafe state is also available
to the driver so that it can make informed decisions about the
required actions and possibly take a special emergency path.
Co-developed-by: John Ogness <[email protected]>
Signed-off-by: John Ogness <[email protected]>
Signed-off-by: Thomas Gleixner (Intel) <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
---
kernel/printk/nbcon.c | 123 +++++++++++++++++++++++++++++++++++++++++-
1 file changed, 122 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index ba1febf15db6..98e4be5429f0 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -537,7 +537,6 @@ static bool nbcon_owner_matches(struct nbcon_state *cur, int expected_cpu,
* nbcon_context_release - Release the console
* @ctxt: The nbcon context from nbcon_context_try_acquire()
*/
-__maybe_unused
static void nbcon_context_release(struct nbcon_context *ctxt)
{
unsigned int cpu = smp_processor_id();
@@ -565,6 +564,128 @@ static void nbcon_context_release(struct nbcon_context *ctxt)
ctxt->pbufs = NULL;
}
+/**
+ * nbcon_context_can_proceed - Check whether ownership can proceed
+ * @ctxt: The nbcon context from nbcon_context_try_acquire()
+ * @cur: The current console state
+ *
+ * Return: True if this context still owns the console. False if
+ * ownership was handed over or taken.
+ *
+ * Must be invoked when entering the unsafe state to make sure that it still
+ * owns the lock. Also must be invoked when exiting the unsafe context
+ * to eventually free the lock for a higher priority context which asked
+ * for the friendly handover.
+ *
+ * It can be called inside an unsafe section when the console is just
+ * temporary in safe state instead of exiting and entering the unsafe
+ * state.
+ *
+ * Also it can be called in the safe context before doing an expensive
+ * safe operation. It does not make sense to do the operation when
+ * a higher priority context took the lock.
+ *
+ * When this function returns false then the calling context no longer owns
+ * the console and is no longer allowed to go forward. In this case it must
+ * back out immediately and carefully. The buffer content is also no longer
+ * trusted since it no longer belongs to the calling context.
+ */
+static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur)
+{
+ unsigned int cpu = smp_processor_id();
+
+ /* Make sure this context still owns the console. */
+ if (!nbcon_owner_matches(cur, cpu, ctxt->prio))
+ return false;
+
+ /* The console owner can proceed if there is no waiter. */
+ if (cur->req_prio == NBCON_PRIO_NONE)
+ return true;
+
+ /*
+ * A console owner within an unsafe region is always allowed to
+ * proceed, even if there are waiters. It can perform a handover
+ * when exiting the unsafe region. Otherwise the waiter will
+ * need to perform an unsafe hostile takeover.
+ */
+ if (cur->unsafe)
+ return true;
+
+ /* Waiters always have higher priorities than owners. */
+ WARN_ON_ONCE(cur->req_prio <= cur->prio);
+
+ /*
+ * Having a safe point for take over and eventually a few
+ * duplicated characters or a full line is way better than a
+ * hostile takeover. Post processing can take care of the garbage.
+ * Release and hand over.
+ */
+ nbcon_context_release(ctxt);
+
+ /*
+ * It is not clear whether the waiter really took over ownership. The
+ * outermost callsite must make the final decision whether console
+ * ownership is needed for it to proceed. If yes, it must reacquire
+ * ownership (possibly hostile) before carefully proceeding.
+ *
+ * The calling context no longer owns the console so go back all the
+ * way instead of trying to implement reacquire heuristics in tons of
+ * places.
+ */
+ return false;
+}
+
+#define nbcon_context_enter_unsafe(c) __nbcon_context_update_unsafe(c, true)
+#define nbcon_context_exit_unsafe(c) __nbcon_context_update_unsafe(c, false)
+
+/**
+ * __nbcon_context_update_unsafe - Update the unsafe bit in @con->nbcon_state
+ * @ctxt: The nbcon context from nbcon_context_try_acquire()
+ * @unsafe: The new value for the unsafe bit
+ *
+ * Return: True if the unsafe state was updated and this context still
+ * owns the console. Otherwise false if ownership was handed
+ * over or taken.
+ *
+ * This function allows console owners to modify the unsafe status of the
+ * console.
+ *
+ * When this function returns false then the calling context no longer owns
+ * the console and is no longer allowed to go forward. In this case it must
+ * back out immediately and carefully. The buffer content is also no longer
+ * trusted since it no longer belongs to the calling context.
+ *
+ * Internal helper to avoid duplicated code.
+ */
+__maybe_unused
+static bool __nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
+{
+ struct console *con = ctxt->console;
+ struct nbcon_state cur;
+ struct nbcon_state new;
+
+ nbcon_state_read(con, &cur);
+
+ do {
+ /*
+ * The unsafe bit must not be cleared if an
+ * unsafe hostile takeover has occurred.
+ */
+ if (!unsafe && cur.unsafe_takeover)
+ goto out;
+
+ if (!nbcon_context_can_proceed(ctxt, &cur))
+ return false;
+
+ new.atom = cur.atom;
+ new.unsafe = unsafe;
+ } while (!nbcon_state_try_cmpxchg(con, &cur, &new));
+
+ cur.atom = new.atom;
+out:
+ return nbcon_context_can_proceed(ctxt, &cur);
+}
+
/**
* nbcon_alloc - Allocate buffers needed by the nbcon console
* @con: Console to allocate buffers for
--
2.39.2