Messages in this thread Patch in this message | | | From | John Ogness <> | Subject | [PATCH printk v3 4/7] printk: nbcon: Add ownership state functions | Date | Sun, 3 Sep 2023 17:11:36 +0206 |
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From: Thomas Gleixner <tglx@linutronix.de>
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_update_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. This function is also a cancellation point where loss of ownership can occur.
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 <john.ogness@linutronix.de> Signed-off-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de> --- kernel/printk/nbcon.c | 107 +++++++++++++++++++++++++++++++++++++++++- 1 file changed, 106 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c index c148467b39b8..76e39538bdec 100644 --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -490,7 +490,6 @@ 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(); @@ -518,6 +517,112 @@ 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 after the record was dumped into the assigned buffer + * and at appropriate safe places in the driver. + * + * 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 known whether the handover succeeded. The outermost + * callsite has to make the final decision whether printing + * should proceed or not (via reacquire, possibly hostile). The + * console is now unlocked so go back all the way instead of + * trying to implement heuristics in tons of places. + */ + return 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 initialize -- 2.39.2
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