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Diffstat (limited to 'libcr/coroutine.c')
| -rw-r--r-- | libcr/coroutine.c | 398 |
1 files changed, 398 insertions, 0 deletions
diff --git a/libcr/coroutine.c b/libcr/coroutine.c new file mode 100644 index 0000000..12d4a25 --- /dev/null +++ b/libcr/coroutine.c @@ -0,0 +1,398 @@ +/* coroutine.c - Simple embeddable coroutine implementation + * + * Copyright (C) 2024 Luke T. Shumaker <lukeshu@lukeshu.com> + * SPDX-Licence-Identifier: AGPL-3.0-or-later + */ + +#include <stdint.h> /* for uint8_t */ +#include <stdio.h> /* for printf(), fprintf(), stderr */ +#include <stdlib.h> /* for malloc(), free() */ +#include <assert.h> +#include <setjmp.h> + +#include "coroutine.h" + +/* Configuration **************************************************************/ + +#define USE_CONFIG_COROUTINE +#include "config.h" + +#ifndef CONFIG_COROUTINE_DEFAULT_STACK_SIZE +# error config.h must define CONFIG_COROUTINE_DEFAULT_STACK_SIZE +#endif +#ifndef CONFIG_COROUTINE_NUM +# error config.h must define CONFIG_COROUTINE_NUM +#endif +#ifndef CONFIG_COROUTINE_MEASURE_STACK +# error config.h must define CONFIG_COROUTINE_MEASURE_STACK +#endif +#ifndef CONFIG_COROUTINE_PROTECT_STACK +# error config.h must define CONFIG_COROUTINE_PROTECT_STACK +#endif +#ifndef CONFIG_COROUTINE_DEBUG +# error config.h must define CONFIG_COROUTINE_DEBUG +#endif + +/* Implementation *************************************************************/ + +/* + * Portability notes: + * + * - It uses GCC `__attribute__`s, though these can likely be easily + * swapped for other compilers. + * + * - It has a small bit of CPU-specific code (assembly, and definition + * of a STACK_GROWS_DOWNWARD={0,1} macro) in + * coroutine.c:call_with_stack(). Other than this, it should be + * portable to other CPUs. It currently contains implementations + * for __x86_64__ and __arm__, and should be fairly easy to add + * implementations for other CPUs. + * + * - It uses setjmp()/longjmp() in "unsafe" ways. POSIX-2017 + * longjmp(3) says + * + * > If the most recent invocation of setjmp() with the + * > corresponding jmp_buf ... or if the function containing the + * > invocation of setjmp() has terminated execution in the interim, + * > or if the invocation of setjmp() was within the scope of an + * > identifier with variably modified type and execution has left + * > that scope in the interim, the behavior is undefined. + * + * We use longjmp() both of these scenarios, but make it OK by using + * call_with_stack() to manage the stack ourselves, assuming the + * sole reason that longjmp() behavior is undefined in such cases is + * because the stack that its saved stack-pointer points to is no + * longer around. It seems absurd that an implementation would + * choose to do something else, but I'm calling it out here because + * you never know. + * + * Note that setjmp()/longjmp() are defined in 3 places: in the libc + * (glibc/newlib), as GCC intrinsics, and the lower-level GCC + * __builtin_{setjmp,newlib} which the libc and intrinsic versions + * likely use. Our assumptions seem to be valid for + * x86_64-pc-linux-gnu/gcc-14.2.1/glibc-2.40 and + * arm-none-eabi/gcc-14.1.0/newlib-4.4.0. + * + * Why not use <ucontext.h>, the now-deprecated (was in POSIX.1-2001, + * is gone in POSIX-2008) predecesor to <setjmp.h>? It would let us + * do this without any assembly or unsafe assumptions. Simply: + * because newlib does not provide it. + */ + +/* + * Design decisions and notes: + * + * - Coroutines are launched with a stack stack that is filled with + * known values (zero or a pre-determined pattern, depending on the + * config above). Because all stack variables should be initialized + * before they are read, this "shouldn't" make a difference, but: + * (1) Initializing it to a known value allows us to measure how + * much of the stack was written to, which is helpful to tune stack + * sizes. (2) Leaving it uninitialized just gives me the willies. + * + * - Because embedded programs should be adverse to using the heap, + * CONFIG_COROUTINE_NUM is fixed, instead of having coroutine_add() + * dynamically grow the coroutine_table as-needed. + * + * - On the flip-side, coroutine stacks are allocated on the heap + * instead of having them be statically-allocated along with + * coroutine_table. (1) This reduced the blast-area of damage for a + * stack-overflow; and indeed if the end of the stack alignes with a + * page-boundary then memory-protection can even detect the overflow + * for us. (2) Having different-looking addresses for stack-area vs + * static-area is handy for making things jump out at you when + * debugging. (3) This can likely also improve things with being + * page-aligned. + * + * - Coroutines must use cr_exit() instead of returning because if + * they return then they will return to call_with_stack() in + * coroutine_add() (not to after the longjmp() call in + * coroutine_main()), and besides being + * wrong-for-our-desired-flow-control, that's a stack location that + * no longer exists. + * + * Things to consider changing: + * + * - Consider having _cr_transition() go ahead and find the next + * coroutine to run and longjmp() direcly to it, instead of first + * jumping back to coroutine_main(). This could save a few cycles + * and a few bytes. + */ + +#if CONFIG_COROUTINE_DEBUG +# define debugf(...) printf("dbg: " __VA_ARGS__) +#else +# define debugf(...) +#endif + +static jmp_buf coroutine_add_env; +static jmp_buf coroutine_main_env; + +enum coroutine_state { + CR_NONE = 0, /* this slot in the table is empty */ + CR_INITIALIZING, /* running, before cr_begin() */ + CR_RUNNING, /* running, after cr_begin() */ + CR_RUNNABLE, /* not running, but runnable */ + CR_PAUSED, /* not running, and not runnable */ +}; + +/* + * Invariants (and non-invariants): + * + * - exactly 0 or 1 coroutines have state CR_INITIALIZING + * - exactly 0 or 1 coroutines have state CR_RUNNING + * - if coroutine_running is not zero, then + * coroutine_table[coroutine_running-1] is the currently-running + * coroutine + * - the coroutine_running coroutine either has state CR_RUNNING or + * CR_INITIALIZNG + * - a coroutine having state CR_RUNNING does *not* imply that + * coroutine_running points at that coroutine; if that coroutine is + * in the middle of coroutine_add(), it coroutine_running points at + * the CR_INITIALIZING child coroutine, while leaving the parent + * coroutine as CR_RUNNING. + */ + +struct coroutine { + volatile enum coroutine_state state; + volatile bool sig_unpause; + jmp_buf env; + size_t stack_size; + void *stack; +}; + +static struct coroutine coroutine_table[CONFIG_COROUTINE_NUM] = {0}; +static cid_t coroutine_running = 0; + +static void call_with_stack(void *stack, cr_fn_t fn, void *args) { + static void *saved_sp = NULL; + + /* As part of sbc-harness, this only really needs to support + * ARM-32, but being able to run it on x86-64 is useful for + * debugging. */ +#if __x86_64__ +#define STACK_GROWS_DOWNWARD 1 + asm volatile ("movq %%rsp , %0\n\t" /* saved_sp = sp */ + "movq %1 , %%rsp\n\t" /* sp = stack */ + "movq %3 , %%rdi\n\t" /* arg0 = args */ + "call *%2\n\t" /* fn() */ + "movq %0 , %%rsp" /* sp = saved_sp */ + : + : /* %0 */"m"(saved_sp), + /* %1 */"r"(stack), + /* %2 */"r"(fn), + /* %3 */"r"(args) + : "rdi" + ); +#elif __arm__ +#define STACK_GROWS_DOWNWARD 1 + /* str/ldr can only work with a "lo" register, which sp is + * not, so we use r0 as an intermediate because we're going to + * clobber it with args anyway. */ + asm volatile ("mov r0, sp\n\t" /* [saved_sp = sp */ + "str r0, %0\n\t" /* ] */ + "mov sp, %1\n\t" /* [sp = stack] */ + "mov r0, %3\n\t" /* [arg0 = args] */ + "blx %2\n\t" /* [fn()] */ + "ldr r0, %0\n\t" /* [sp = staved_sp */ + "mov sp, r0" /* ] */ + : + : /* %0 */"m"(saved_sp), + /* %1 */"r"(stack), + /* %2 */"r"(fn), + /* %3 */"r"(args) + : "r0" + ); +#else +# error unsupported architecture +#endif +} + +#if CONFIG_COROUTINE_MEASURE_STACK || CONFIG_COROUTINE_PROTECT_STACK +/* We just need a pattern that is unlikely to occur naturaly; this is + * just a few bytes that I read from /dev/random. */ +static const uint8_t stack_pattern[] = {0x1e, 0x15, 0x16, 0x0a, 0xcc, 0x52, 0x7e, 0xb7}; +#endif + +static void inline assert_cid(cid_t cid) { + assert(cid > 0); + assert(cid <= CONFIG_COROUTINE_NUM); +#if CONFIG_COROUTINE_PROTECT_STACK + assert(coroutine_table[cid-1].stack_size); + assert(coroutine_table[cid-1].stack); + for (size_t i = 0; i < sizeof(stack_pattern); i++) { + size_t j = coroutine_table[cid-1].stack_size - (i+1); + assert(((uint8_t*)coroutine_table[cid-1].stack)[i] == stack_pattern[i]); + assert(((uint8_t*)coroutine_table[cid-1].stack)[j] == stack_pattern[j%sizeof(stack_pattern)]); + } +#endif +} + +#define assert_cid_state(cid, opstate) do { \ + assert_cid(cid); \ + assert(coroutine_table[(cid)-1].state opstate); \ + } while (0) + +cid_t coroutine_add_with_stack_size(size_t stack_size, cr_fn_t fn, void *args) { + static cid_t last_created = 0; + cid_t parent = coroutine_running; + + if (!stack_size) + stack_size = CONFIG_COROUTINE_DEFAULT_STACK_SIZE; + + if (parent) + assert_cid_state(parent, == CR_RUNNING); + assert(stack_size); + assert(fn); + debugf("coroutine_add_with_stack_size(%zu, %#p, %#p)...\n", stack_size, fn, args); + + cid_t child; + { + size_t idx_base = last_created; + for (size_t idx_shift = 0; idx_shift < CONFIG_COROUTINE_NUM; idx_shift++) { + child = ((idx_base + idx_shift) % CONFIG_COROUTINE_NUM) + 1; + if (coroutine_table[child-1].state == CR_NONE) + goto found; + } + return 0; + found: + } + debugf("...child=%zu\n", child); + + last_created = child; + + coroutine_table[child-1].stack_size = stack_size; + coroutine_table[child-1].stack = malloc(stack_size); +#if CONFIG_COROUTINE_MEASURE_STACK || CONFIG_COROUTINE_PROTECT_STACK + for (size_t i = 0; i < stack_size; i++) + ((uint8_t*)coroutine_table[child-1].stack)[i] = stack_pattern[i%sizeof(stack_pattern)]; +#endif + + coroutine_running = child; + coroutine_table[child-1].state = CR_INITIALIZING; + if (!setjmp(coroutine_add_env)) { /* point=a */ + void *stack_base = coroutine_table[child-1].stack + (STACK_GROWS_DOWNWARD ? stack_size : 0); +#if CONFIG_COROUTINE_PROTECT_STACK +# if STACK_GROWS_DOWNWARD + stack_base -= sizeof(stack_pattern); +# else + stack_base += sizeof(stack_pattern); +# endif +#endif + debugf("...stack =%#p\n", coroutine_table[child-1].stack); + debugf("...stack_base=%#p\n", stack_base); + /* run until cr_begin() */ + call_with_stack(stack_base, fn, args); + assert(false); /* should cr_begin() instead of returning */ + } + assert_cid_state(child, == CR_RUNNABLE); + if (parent) + assert_cid_state(parent, == CR_RUNNING); + coroutine_running = parent; + + return child; +} + +void coroutine_main(void) { + debugf("coroutine_main()\n"); + bool ran; + for (coroutine_running = 1;; coroutine_running = (coroutine_running%CONFIG_COROUTINE_NUM)+1) { + if (coroutine_running == 1) + ran = false; + struct coroutine *cr = &coroutine_table[coroutine_running-1]; + if (cr->state == CR_RUNNABLE) { + debugf("running cid=%zu...\n", coroutine_running); + ran = true; + cr->state = CR_RUNNING; + if (!setjmp(coroutine_main_env)) { /* point=b */ + longjmp(cr->env, 1); /* jump to point=c */ + assert(false); /* should cr_exit() instead of returning */ + } + assert_cid_state(coroutine_running, != CR_RUNNING); + if (cr->state == CR_NONE) { +#if CONFIG_COROUTINE_MEASURE_STACK + size_t stack_size = cr->stack_size - (CONFIG_COROUTINE_PROTECT_STACK ? 2*sizeof(stack_pattern) : 0); + size_t stack_used = stack_size; + for (;;) { + size_t i = STACK_GROWS_DOWNWARD + ? (CONFIG_COROUTINE_PROTECT_STACK ? sizeof(stack_pattern) : 0) + stack_size - stack_used + : stack_used - 1 - (CONFIG_COROUTINE_PROTECT_STACK ? sizeof(stack_pattern) : 0); + if (stack_used == 0 || ((uint8_t*)cr->stack)[i] != stack_pattern[i%sizeof(stack_pattern)]) + break; + stack_used--; + } + printf("info: cid=%zu: exited having used %zu B stack space\n", coroutine_running, stack_used); +#endif + free(cr->stack); + coroutine_table[coroutine_running-1] = (struct coroutine){0}; + } + } + if (coroutine_running == CONFIG_COROUTINE_NUM && !ran) { + fprintf(stderr, "error: no runnable coroutines\n"); + return; + } + } +} + +void cr_begin(void) { + assert_cid_state(coroutine_running, == CR_INITIALIZING); + + coroutine_table[coroutine_running-1].state = CR_RUNNABLE; + if (!setjmp(coroutine_table[coroutine_running-1].env)) /* point=c1 */ + longjmp(coroutine_add_env, 1); /* jump to point=a */ +} + +static inline void _cr_transition(enum coroutine_state state) { + debugf("cid=%zu: transition %i->%i\n", coroutine_running, coroutine_table[coroutine_running-1].state, state); + + coroutine_table[coroutine_running-1].state = state; + if (!setjmp(coroutine_table[coroutine_running-1].env)) /* point=c2 */ + longjmp(coroutine_main_env, 1); /* jump to point=b */ +} + +void cr_yield(void) { + assert_cid_state(coroutine_running, == CR_RUNNING); + _cr_transition(CR_RUNNABLE); +} + +void cr_pause_and_yield(void) { + assert_cid_state(coroutine_running, == CR_RUNNING); + if (coroutine_table[cid-1].sig_unpause) + _cr_transition(CR_RUNNABLE); + else + _cr_transition(CR_PAUSED); + coroutine_table[cid-1].sig_unpause = false; +} + +void cr_exit(void) { + assert_cid_state(coroutine_running, == CR_RUNNING); + debugf("cid=%zu: exit\n", coroutine_running); + + coroutine_table[coroutine_running-1].state = CR_NONE; + longjmp(coroutine_main_env, 1); /* jump to point=b */ +} + +void cr_unpause(cid_t cid) { + assert_cid_state(cid, == CR_PAUSED); + debugf("cr_unpause(%zu)\n", cid); + + coroutine_table[cid-1].sig_unpause = false; + coroutine_table[cid-1].state = CR_RUNNABLE; +} + +void cr_unpause_from_sighandler(cid_t cid) { + assert_cid(cid); + + switch (coroutine_table[cid-1].state) { + case CR_RUNNING: + coroutine_table[cid-1].sig_unpause = true; + case CR_PAUSED: + coroutine_table[cid-1].state = CR_RUNNABLE; + default: + assert(false); + } +} + +cid_t cr_getcid(void) { + return coroutine_running; +} |