diff options
Diffstat (limited to '')
| -rw-r--r-- | debian/patches-rt/0076-printk-nbcon-Add-acquire-release-logic.patch | 705 |
1 files changed, 705 insertions, 0 deletions
diff --git a/debian/patches-rt/0076-printk-nbcon-Add-acquire-release-logic.patch b/debian/patches-rt/0076-printk-nbcon-Add-acquire-release-logic.patch new file mode 100644 index 000000000..271837903 --- /dev/null +++ b/debian/patches-rt/0076-printk-nbcon-Add-acquire-release-logic.patch @@ -0,0 +1,705 @@ +From: Thomas Gleixner <tglx@linutronix.de> +Date: Sat, 16 Sep 2023 21:26:01 +0206 +Subject: [PATCH 076/134] printk: nbcon: Add acquire/release logic +Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz + +Add per console acquire/release functionality. + +The state of the console is maintained in the "nbcon_state" atomic +variable. + +The console is locked when: + + - The 'prio' field contains the priority of the context that owns the + console. Only higher priority contexts are allowed to take over the + lock. A value of 0 (NBCON_PRIO_NONE) means the console is not locked. + + - The 'cpu' field denotes on which CPU the console is locked. It is used + to prevent busy waiting on the same CPU. Also it informs the lock owner + that it has lost the lock in a more complex scenario when the lock was + taken over by a higher priority context, released, and taken on another + CPU with the same priority as the interrupted owner. + +The acquire mechanism uses a few more fields: + + - The 'req_prio' field is used by the handover approach to make the + current owner aware that there is a context with a higher priority + waiting for the friendly handover. + + - The 'unsafe' field allows to take over the console in a safe way in the + middle of emitting a message. The field is set only when accessing some + shared resources or when the console device is manipulated. It can be + cleared, for example, after emitting one character when the console + device is in a consistent state. + + - The 'unsafe_takeover' field is set when a hostile takeover took the + console in an unsafe state. The console will stay in the unsafe state + until re-initialized. + +The acquire mechanism uses three approaches: + + 1) Direct acquire when the console is not owned or is owned by a lower + priority context and is in a safe state. + + 2) Friendly handover mechanism uses a request/grant handshake. It is used + when the current owner has lower priority and the console is in an + unsafe state. + + The requesting context: + + a) Sets its priority into the 'req_prio' field. + + b) Waits (with a timeout) for the owning context to unlock the + console. + + c) Takes the lock and clears the 'req_prio' field. + + The owning context: + + a) Observes the 'req_prio' field set on exit from the unsafe + console state. + + b) Gives up console ownership by clearing the 'prio' field. + + 3) Unsafe hostile takeover allows to take over the lock even when the + console is an unsafe state. It is used only in panic() by the final + attempt to flush consoles in a try and hope mode. + + Note that separate record buffers are used in panic(). As a result, + the messages can be read and formatted without any risk even after + using the hostile takeover in unsafe state. + +The release function simply clears the 'prio' field. + +All operations on @console::nbcon_state are atomic cmpxchg based to +handle concurrency. + +The acquire/release functions implement only minimal policies: + + - Preference for higher priority contexts. + - Protection of the panic CPU. + +All other policy decisions must be made at the call sites: + + - What is marked as an unsafe section. + - Whether to spin-wait if there is already an owner and the console is + in an unsafe state. + - Whether to attempt an unsafe hostile takeover. + +The design allows to implement the well known: + + acquire() + output_one_printk_record() + release() + +The output of one printk record might be interrupted with a higher priority +context. The new owner is supposed to reprint the entire interrupted record +from scratch. + +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> +Signed-off-by: Petr Mladek <pmladek@suse.com> +Link: https://lore.kernel.org/r/20230916192007.608398-3-john.ogness@linutronix.de +Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> +--- + include/linux/console.h | 56 +++++ + kernel/printk/nbcon.c | 497 ++++++++++++++++++++++++++++++++++++++++++++++++ + 2 files changed, 553 insertions(+) + +--- a/include/linux/console.h ++++ b/include/linux/console.h +@@ -175,13 +175,29 @@ enum cons_flags { + * struct nbcon_state - console state for nbcon consoles + * @atom: Compound of the state fields for atomic operations + * ++ * @req_prio: The priority of a handover request ++ * @prio: The priority of the current owner ++ * @unsafe: Console is busy in a non takeover region ++ * @unsafe_takeover: A hostile takeover in an unsafe state happened in the ++ * past. The console cannot be safe until re-initialized. ++ * @cpu: The CPU on which the owner runs ++ * + * To be used for reading and preparing of the value stored in the nbcon + * state variable @console::nbcon_state. ++ * ++ * The @prio and @req_prio fields are particularly important to allow ++ * spin-waiting to timeout and give up without the risk of a waiter being ++ * assigned the lock after giving up. + */ + struct nbcon_state { + union { + unsigned int atom; + struct { ++ unsigned int prio : 2; ++ unsigned int req_prio : 2; ++ unsigned int unsafe : 1; ++ unsigned int unsafe_takeover : 1; ++ unsigned int cpu : 24; + }; + }; + }; +@@ -195,6 +211,46 @@ struct nbcon_state { + static_assert(sizeof(struct nbcon_state) <= sizeof(int)); + + /** ++ * nbcon_prio - console owner priority for nbcon consoles ++ * @NBCON_PRIO_NONE: Unused ++ * @NBCON_PRIO_NORMAL: Normal (non-emergency) usage ++ * @NBCON_PRIO_EMERGENCY: Emergency output (WARN/OOPS...) ++ * @NBCON_PRIO_PANIC: Panic output ++ * @NBCON_PRIO_MAX: The number of priority levels ++ * ++ * A higher priority context can takeover the console when it is ++ * in the safe state. The final attempt to flush consoles in panic() ++ * can be allowed to do so even in an unsafe state (Hope and pray). ++ */ ++enum nbcon_prio { ++ NBCON_PRIO_NONE = 0, ++ NBCON_PRIO_NORMAL, ++ NBCON_PRIO_EMERGENCY, ++ NBCON_PRIO_PANIC, ++ NBCON_PRIO_MAX, ++}; ++ ++struct console; ++ ++/** ++ * struct nbcon_context - Context for console acquire/release ++ * @console: The associated console ++ * @spinwait_max_us: Limit for spin-wait acquire ++ * @prio: Priority of the context ++ * @allow_unsafe_takeover: Allow performing takeover even if unsafe. Can ++ * be used only with NBCON_PRIO_PANIC @prio. It ++ * might cause a system freeze when the console ++ * is used later. ++ */ ++struct nbcon_context { ++ /* members set by caller */ ++ struct console *console; ++ unsigned int spinwait_max_us; ++ enum nbcon_prio prio; ++ unsigned int allow_unsafe_takeover : 1; ++}; ++ ++/** + * struct console - The console descriptor structure + * @name: The name of the console driver + * @write: Write callback to output messages (Optional) +--- a/kernel/printk/nbcon.c ++++ b/kernel/printk/nbcon.c +@@ -4,10 +4,98 @@ + + #include <linux/kernel.h> + #include <linux/console.h> ++#include <linux/delay.h> + #include "internal.h" + /* + * Printk console printing implementation for consoles which does not depend + * on the legacy style console_lock mechanism. ++ * ++ * The state of the console is maintained in the "nbcon_state" atomic ++ * variable. ++ * ++ * The console is locked when: ++ * ++ * - The 'prio' field contains the priority of the context that owns the ++ * console. Only higher priority contexts are allowed to take over the ++ * lock. A value of 0 (NBCON_PRIO_NONE) means the console is not locked. ++ * ++ * - The 'cpu' field denotes on which CPU the console is locked. It is used ++ * to prevent busy waiting on the same CPU. Also it informs the lock owner ++ * that it has lost the lock in a more complex scenario when the lock was ++ * taken over by a higher priority context, released, and taken on another ++ * CPU with the same priority as the interrupted owner. ++ * ++ * The acquire mechanism uses a few more fields: ++ * ++ * - The 'req_prio' field is used by the handover approach to make the ++ * current owner aware that there is a context with a higher priority ++ * waiting for the friendly handover. ++ * ++ * - The 'unsafe' field allows to take over the console in a safe way in the ++ * middle of emitting a message. The field is set only when accessing some ++ * shared resources or when the console device is manipulated. It can be ++ * cleared, for example, after emitting one character when the console ++ * device is in a consistent state. ++ * ++ * - The 'unsafe_takeover' field is set when a hostile takeover took the ++ * console in an unsafe state. The console will stay in the unsafe state ++ * until re-initialized. ++ * ++ * The acquire mechanism uses three approaches: ++ * ++ * 1) Direct acquire when the console is not owned or is owned by a lower ++ * priority context and is in a safe state. ++ * ++ * 2) Friendly handover mechanism uses a request/grant handshake. It is used ++ * when the current owner has lower priority and the console is in an ++ * unsafe state. ++ * ++ * The requesting context: ++ * ++ * a) Sets its priority into the 'req_prio' field. ++ * ++ * b) Waits (with a timeout) for the owning context to unlock the ++ * console. ++ * ++ * c) Takes the lock and clears the 'req_prio' field. ++ * ++ * The owning context: ++ * ++ * a) Observes the 'req_prio' field set on exit from the unsafe ++ * console state. ++ * ++ * b) Gives up console ownership by clearing the 'prio' field. ++ * ++ * 3) Unsafe hostile takeover allows to take over the lock even when the ++ * console is an unsafe state. It is used only in panic() by the final ++ * attempt to flush consoles in a try and hope mode. ++ * ++ * The release function simply clears the 'prio' field. ++ * ++ * All operations on @console::nbcon_state are atomic cmpxchg based to ++ * handle concurrency. ++ * ++ * The acquire/release functions implement only minimal policies: ++ * ++ * - Preference for higher priority contexts. ++ * - Protection of the panic CPU. ++ * ++ * All other policy decisions must be made at the call sites: ++ * ++ * - What is marked as an unsafe section. ++ * - Whether to spin-wait if there is already an owner and the console is ++ * in an unsafe state. ++ * - Whether to attempt an unsafe hostile takeover. ++ * ++ * The design allows to implement the well known: ++ * ++ * acquire() ++ * output_one_printk_record() ++ * release() ++ * ++ * The output of one printk record might be interrupted with a higher priority ++ * context. The new owner is supposed to reprint the entire interrupted record ++ * from scratch. + */ + + /** +@@ -48,6 +136,415 @@ static inline bool nbcon_state_try_cmpxc + } + + /** ++ * nbcon_context_try_acquire_direct - Try to acquire directly ++ * @ctxt: The context of the caller ++ * @cur: The current console state ++ * ++ * Acquire the console when it is released. Also acquire the console when ++ * the current owner has a lower priority and the console is in a safe state. ++ * ++ * Return: 0 on success. Otherwise, an error code on failure. Also @cur ++ * is updated to the latest state when failed to modify it. ++ * ++ * Errors: ++ * ++ * -EPERM: A panic is in progress and this is not the panic CPU. ++ * Or the current owner or waiter has the same or higher ++ * priority. No acquire method can be successful in ++ * this case. ++ * ++ * -EBUSY: The current owner has a lower priority but the console ++ * in an unsafe state. The caller should try using ++ * the handover acquire method. ++ */ ++static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt, ++ struct nbcon_state *cur) ++{ ++ unsigned int cpu = smp_processor_id(); ++ struct console *con = ctxt->console; ++ struct nbcon_state new; ++ ++ do { ++ if (other_cpu_in_panic()) ++ return -EPERM; ++ ++ if (ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio) ++ return -EPERM; ++ ++ if (cur->unsafe) ++ return -EBUSY; ++ ++ /* ++ * The console should never be safe for a direct acquire ++ * if an unsafe hostile takeover has ever happened. ++ */ ++ WARN_ON_ONCE(cur->unsafe_takeover); ++ ++ new.atom = cur->atom; ++ new.prio = ctxt->prio; ++ new.req_prio = NBCON_PRIO_NONE; ++ new.unsafe = cur->unsafe_takeover; ++ new.cpu = cpu; ++ ++ } while (!nbcon_state_try_cmpxchg(con, cur, &new)); ++ ++ return 0; ++} ++ ++static bool nbcon_waiter_matches(struct nbcon_state *cur, int expected_prio) ++{ ++ /* ++ * The request context is well defined by the @req_prio because: ++ * ++ * - Only a context with a higher priority can take over the request. ++ * - There are only three priorities. ++ * - Only one CPU is allowed to request PANIC priority. ++ * - Lower priorities are ignored during panic() until reboot. ++ * ++ * As a result, the following scenario is *not* possible: ++ * ++ * 1. Another context with a higher priority directly takes ownership. ++ * 2. The higher priority context releases the ownership. ++ * 3. A lower priority context takes the ownership. ++ * 4. Another context with the same priority as this context ++ * creates a request and starts waiting. ++ */ ++ ++ return (cur->req_prio == expected_prio); ++} ++ ++/** ++ * nbcon_context_try_acquire_requested - Try to acquire after having ++ * requested a handover ++ * @ctxt: The context of the caller ++ * @cur: The current console state ++ * ++ * This is a helper function for nbcon_context_try_acquire_handover(). ++ * It is called when the console is in an unsafe state. The current ++ * owner will release the console on exit from the unsafe region. ++ * ++ * Return: 0 on success and @cur is updated to the new console state. ++ * Otherwise an error code on failure. ++ * ++ * Errors: ++ * ++ * -EPERM: A panic is in progress and this is not the panic CPU ++ * or this context is no longer the waiter. ++ * ++ * -EBUSY: The console is still locked. The caller should ++ * continue waiting. ++ * ++ * Note: The caller must still remove the request when an error has occurred ++ * except when this context is no longer the waiter. ++ */ ++static int nbcon_context_try_acquire_requested(struct nbcon_context *ctxt, ++ struct nbcon_state *cur) ++{ ++ unsigned int cpu = smp_processor_id(); ++ struct console *con = ctxt->console; ++ struct nbcon_state new; ++ ++ /* Note that the caller must still remove the request! */ ++ if (other_cpu_in_panic()) ++ return -EPERM; ++ ++ /* ++ * Note that the waiter will also change if there was an unsafe ++ * hostile takeover. ++ */ ++ if (!nbcon_waiter_matches(cur, ctxt->prio)) ++ return -EPERM; ++ ++ /* If still locked, caller should continue waiting. */ ++ if (cur->prio != NBCON_PRIO_NONE) ++ return -EBUSY; ++ ++ /* ++ * The previous owner should have never released ownership ++ * in an unsafe region. ++ */ ++ WARN_ON_ONCE(cur->unsafe); ++ ++ new.atom = cur->atom; ++ new.prio = ctxt->prio; ++ new.req_prio = NBCON_PRIO_NONE; ++ new.unsafe = cur->unsafe_takeover; ++ new.cpu = cpu; ++ ++ if (!nbcon_state_try_cmpxchg(con, cur, &new)) { ++ /* ++ * The acquire could fail only when it has been taken ++ * over by a higher priority context. ++ */ ++ WARN_ON_ONCE(nbcon_waiter_matches(cur, ctxt->prio)); ++ return -EPERM; ++ } ++ ++ /* Handover success. This context now owns the console. */ ++ return 0; ++} ++ ++/** ++ * nbcon_context_try_acquire_handover - Try to acquire via handover ++ * @ctxt: The context of the caller ++ * @cur: The current console state ++ * ++ * The function must be called only when the context has higher priority ++ * than the current owner and the console is in an unsafe state. ++ * It is the case when nbcon_context_try_acquire_direct() returns -EBUSY. ++ * ++ * The function sets "req_prio" field to make the current owner aware of ++ * the request. Then it waits until the current owner releases the console, ++ * or an even higher context takes over the request, or timeout expires. ++ * ++ * The current owner checks the "req_prio" field on exit from the unsafe ++ * region and releases the console. It does not touch the "req_prio" field ++ * so that the console stays reserved for the waiter. ++ * ++ * Return: 0 on success. Otherwise, an error code on failure. Also @cur ++ * is updated to the latest state when failed to modify it. ++ * ++ * Errors: ++ * ++ * -EPERM: A panic is in progress and this is not the panic CPU. ++ * Or a higher priority context has taken over the ++ * console or the handover request. ++ * ++ * -EBUSY: The current owner is on the same CPU so that the hand ++ * shake could not work. Or the current owner is not ++ * willing to wait (zero timeout). Or the console does ++ * not enter the safe state before timeout passed. The ++ * caller might still use the unsafe hostile takeover ++ * when allowed. ++ * ++ * -EAGAIN: @cur has changed when creating the handover request. ++ * The caller should retry with direct acquire. ++ */ ++static int nbcon_context_try_acquire_handover(struct nbcon_context *ctxt, ++ struct nbcon_state *cur) ++{ ++ unsigned int cpu = smp_processor_id(); ++ struct console *con = ctxt->console; ++ struct nbcon_state new; ++ int timeout; ++ int request_err = -EBUSY; ++ ++ /* ++ * Check that the handover is called when the direct acquire failed ++ * with -EBUSY. ++ */ ++ WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio); ++ WARN_ON_ONCE(!cur->unsafe); ++ ++ /* Handover is not possible on the same CPU. */ ++ if (cur->cpu == cpu) ++ return -EBUSY; ++ ++ /* ++ * Console stays unsafe after an unsafe takeover until re-initialized. ++ * Waiting is not going to help in this case. ++ */ ++ if (cur->unsafe_takeover) ++ return -EBUSY; ++ ++ /* Is the caller willing to wait? */ ++ if (ctxt->spinwait_max_us == 0) ++ return -EBUSY; ++ ++ /* ++ * Setup a request for the handover. The caller should try to acquire ++ * the console directly when the current state has been modified. ++ */ ++ new.atom = cur->atom; ++ new.req_prio = ctxt->prio; ++ if (!nbcon_state_try_cmpxchg(con, cur, &new)) ++ return -EAGAIN; ++ ++ cur->atom = new.atom; ++ ++ /* Wait until there is no owner and then acquire the console. */ ++ for (timeout = ctxt->spinwait_max_us; timeout >= 0; timeout--) { ++ /* On successful acquire, this request is cleared. */ ++ request_err = nbcon_context_try_acquire_requested(ctxt, cur); ++ if (!request_err) ++ return 0; ++ ++ /* ++ * If the acquire should be aborted, it must be ensured ++ * that the request is removed before returning to caller. ++ */ ++ if (request_err == -EPERM) ++ break; ++ ++ udelay(1); ++ ++ /* Re-read the state because some time has passed. */ ++ nbcon_state_read(con, cur); ++ } ++ ++ /* Timed out or aborted. Carefully remove handover request. */ ++ do { ++ /* ++ * No need to remove request if there is a new waiter. This ++ * can only happen if a higher priority context has taken over ++ * the console or the handover request. ++ */ ++ if (!nbcon_waiter_matches(cur, ctxt->prio)) ++ return -EPERM; ++ ++ /* Unset request for handover. */ ++ new.atom = cur->atom; ++ new.req_prio = NBCON_PRIO_NONE; ++ if (nbcon_state_try_cmpxchg(con, cur, &new)) { ++ /* ++ * Request successfully unset. Report failure of ++ * acquiring via handover. ++ */ ++ cur->atom = new.atom; ++ return request_err; ++ } ++ ++ /* ++ * Unable to remove request. Try to acquire in case ++ * the owner has released the lock. ++ */ ++ } while (nbcon_context_try_acquire_requested(ctxt, cur)); ++ ++ /* Lucky timing. The acquire succeeded while removing the request. */ ++ return 0; ++} ++ ++/** ++ * nbcon_context_try_acquire_hostile - Acquire via unsafe hostile takeover ++ * @ctxt: The context of the caller ++ * @cur: The current console state ++ * ++ * Acquire the console even in the unsafe state. ++ * ++ * It can be permitted by setting the 'allow_unsafe_takeover' field only ++ * by the final attempt to flush messages in panic(). ++ * ++ * Return: 0 on success. -EPERM when not allowed by the context. ++ */ ++static int nbcon_context_try_acquire_hostile(struct nbcon_context *ctxt, ++ struct nbcon_state *cur) ++{ ++ unsigned int cpu = smp_processor_id(); ++ struct console *con = ctxt->console; ++ struct nbcon_state new; ++ ++ if (!ctxt->allow_unsafe_takeover) ++ return -EPERM; ++ ++ /* Ensure caller is allowed to perform unsafe hostile takeovers. */ ++ if (WARN_ON_ONCE(ctxt->prio != NBCON_PRIO_PANIC)) ++ return -EPERM; ++ ++ /* ++ * Check that try_acquire_direct() and try_acquire_handover() returned ++ * -EBUSY in the right situation. ++ */ ++ WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio); ++ WARN_ON_ONCE(cur->unsafe != true); ++ ++ do { ++ new.atom = cur->atom; ++ new.cpu = cpu; ++ new.prio = ctxt->prio; ++ new.unsafe |= cur->unsafe_takeover; ++ new.unsafe_takeover |= cur->unsafe; ++ ++ } while (!nbcon_state_try_cmpxchg(con, cur, &new)); ++ ++ return 0; ++} ++ ++/** ++ * nbcon_context_try_acquire - Try to acquire nbcon console ++ * @ctxt: The context of the caller ++ * ++ * Return: True if the console was acquired. False otherwise. ++ * ++ * If the caller allowed an unsafe hostile takeover, on success the ++ * caller should check the current console state to see if it is ++ * in an unsafe state. Otherwise, on success the caller may assume ++ * the console is not in an unsafe state. ++ */ ++__maybe_unused ++static bool nbcon_context_try_acquire(struct nbcon_context *ctxt) ++{ ++ struct console *con = ctxt->console; ++ struct nbcon_state cur; ++ int err; ++ ++ nbcon_state_read(con, &cur); ++try_again: ++ err = nbcon_context_try_acquire_direct(ctxt, &cur); ++ if (err != -EBUSY) ++ goto out; ++ ++ err = nbcon_context_try_acquire_handover(ctxt, &cur); ++ if (err == -EAGAIN) ++ goto try_again; ++ if (err != -EBUSY) ++ goto out; ++ ++ err = nbcon_context_try_acquire_hostile(ctxt, &cur); ++out: ++ return !err; ++} ++ ++static bool nbcon_owner_matches(struct nbcon_state *cur, int expected_cpu, ++ int expected_prio) ++{ ++ /* ++ * Since consoles can only be acquired by higher priorities, ++ * owning contexts are uniquely identified by @prio. However, ++ * since contexts can unexpectedly lose ownership, it is ++ * possible that later another owner appears with the same ++ * priority. For this reason @cpu is also needed. ++ */ ++ ++ if (cur->prio != expected_prio) ++ return false; ++ ++ if (cur->cpu != expected_cpu) ++ return false; ++ ++ return true; ++} ++ ++/** ++ * 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(); ++ struct console *con = ctxt->console; ++ struct nbcon_state cur; ++ struct nbcon_state new; ++ ++ nbcon_state_read(con, &cur); ++ ++ do { ++ if (!nbcon_owner_matches(&cur, cpu, ctxt->prio)) ++ return; ++ ++ new.atom = cur.atom; ++ new.prio = NBCON_PRIO_NONE; ++ ++ /* ++ * If @unsafe_takeover is set, it is kept set so that ++ * the state remains permanently unsafe. ++ */ ++ new.unsafe |= cur.unsafe_takeover; ++ ++ } while (!nbcon_state_try_cmpxchg(con, &cur, &new)); ++} ++ ++/** + * nbcon_init - Initialize the nbcon console specific data + * @con: Console to initialize + */ |