From 7dbefb9f38e74b14037b33fd82bdd405fc7aa501 Mon Sep 17 00:00:00 2001 From: Taylor R Campbell Date: Fri, 11 Feb 2022 15:52:09 +0000 Subject: [PATCH] ucas(9): Membar audit. - Omit needless membar_enter before ipi_trigger_broadcast. This was presumably intended to imply a happens-before relation for the following two CPUs: /* CPU doing ucas */ ucas_critical_enter() ucas_critical_pausing_cpus = ncpu - 1 (A) ipi_trigger_broadcast() /* other CPU walking by whistling innocently */ IPI handler ucas_critical_cpu_gate() load ucas_critical_pausing_cpus (B) That is, this was presumably meant to ensure (A) happens-before (B). This relation is already guaranteed by ipi(9), so there is no need for any explicit memory barrier. - Issue a store-release in ucas_critical_cpu_gate so we have the following happens-before relation which was otherwise not guaranteed except if __HAVE_ATOMIC_AS_MEMBAR: /* other CPU walking by whistling innocently */ ...other logic touching the target ucas word... (A) IPI handler ucas_critical_cpu_gate() ... atomic_dec_uint(&ucas_critical_pausing_cpus) happens-before /* CPU doing ucas */ ucas_critical_enter() -> ucas_critical_wait(); ...touching the word with ufetch/ustore... (B) We need to ensure the logic (A) on another CPU touching the target ucas word happens-before we actually do the ucas at (B). (a) This requires the other CPU to do a store-release on ucas_critical_pausing_cpus in ucas_critical_cpu_gate, and (b) this requires the ucas CPU to do a load-acquire on ucas_critical_pausing_cpus in ucas_critical_wait. Without _both_ sides -- store-release and then load-acquire -- there is no such happens-before guarantee; another CPU may have a buffered store, for instance, that clobbers the ucas. For now, do the store-release with membar_exit conditional on __HAVE_ATOMIC_AS_MEMBAR and then atomic_dec_uint -- later with the C11 API we can dispense with the #ifdef and just use atomic_fetch_add_explicit(..., memory_order_release). The load-acquire we can do with atomic_load_acquire. - Issue a load-acquire in ucas_critical_cpu_gate so we have the following happens-before relation which was otherwise not guaranteed: /* CPU doing ucas */ ...ufetch/ustore... (A) ucas_critical_exit() ucas_critical_pausing_cpus = -1; /* other CPU walking by whistling innocently */ IPI handler ucas_critical_cpu_gate() ... while (ucas_critical_pausing_cpus != -1) spin; ...other logic touching the target ucas word... (B) We need to ensure the logic (A) to do the ucas happens-before logic that might use it on another CPU at (B). (a) This requires that the ucas CPU do a store-release on ucas_critical_pausing_cpus in ucas_critical_exit, and (b) this requires that the other CPU do a load-acquire on ucas_critical_pausing_cpus in ucas_critical_cpu_gate. Without _both_ sides -- store-release and then load-acquire -- there is no such happens-before guarantee; the other CPU might witness a cached stale value of the target location but a new value of some other location in the wrong order. - Use atomic_load/store_* to avoid the appearance of races, e.g. for sanitizers. - Document which barriers pair up with which barriers and what they're doing. --- sys/kern/subr_copy.c | 54 ++++++++++++++++++++++++++++++++++++++------ 1 file changed, 47 insertions(+), 7 deletions(-) diff --git a/sys/kern/subr_copy.c b/sys/kern/subr_copy.c index d1ca943de0e7..09e37150c201 100644 --- a/sys/kern/subr_copy.c +++ b/sys/kern/subr_copy.c @@ -400,9 +400,32 @@ ucas_critical_cpu_gate(void *arg __unused) { int count = SPINLOCK_BACKOFF_MIN; - KASSERT(ucas_critical_pausing_cpus > 0); + KASSERT(atomic_load_relaxed(&ucas_critical_pausing_cpus) > 0); + + /* + * Notify ucas_critical_wait that we have stopped. Using + * store-release ensures all our memory operations up to the + * IPI happen before the ucas -- no buffered stores on our end + * can clobber it later on, for instance. + * + * Matches atomic_load_acquire in ucas_critical_wait -- turns + * the following atomic_dec_uint into a store-release. + */ +#ifndef __HAVE_ATOMIC_AS_MEMBAR + membar_exit(); +#endif atomic_dec_uint(&ucas_critical_pausing_cpus); - while (ucas_critical_pausing_cpus != (u_int)-1) { + + /* + * Wait for ucas_critical_exit to reopen the gate and let us + * proceed. Using a load-acquire ensures the ucas happens + * before any of our memory operations when we return from the + * IPI and proceed -- we won't observe any stale cached value + * that the ucas overwrote, for instance. + * + * Matches atomic_store_release in ucas_critical_exit. + */ + while (atomic_load_acquire(&ucas_critical_pausing_cpus) != (u_int)-1) { SPINLOCK_BACKOFF(count); } } @@ -410,6 +433,7 @@ ucas_critical_cpu_gate(void *arg __unused) static int ucas_critical_init(void) { + ucas_critical_ipi = ipi_register(ucas_critical_cpu_gate, NULL); return 0; } @@ -419,7 +443,16 @@ ucas_critical_wait(void) { int count = SPINLOCK_BACKOFF_MIN; - while (ucas_critical_pausing_cpus > 0) { + /* + * Wait for all CPUs to stop at the gate. Using a load-acquire + * ensures all memory operations before they stop at the gate + * happen before the ucas -- no buffered stores in other CPUs + * can clobber it later on, for instance. + * + * Matches membar_exit/atomic_dec_uint (store-release) in + * ucas_critical_cpu_gate. + */ + while (atomic_load_acquire(&ucas_critical_pausing_cpus) > 0) { SPINLOCK_BACKOFF(count); } } @@ -444,8 +477,6 @@ ucas_critical_enter(lwp_t * const l) mutex_enter(&cpu_lock); ucas_critical_splcookie = splhigh(); ucas_critical_pausing_cpus = ncpu - 1; - membar_enter(); - ipi_trigger_broadcast(ucas_critical_ipi, true); ucas_critical_wait(); return; @@ -461,8 +492,17 @@ ucas_critical_exit(lwp_t * const l) #if !defined(__HAVE_UCAS_MP) && defined(MULTIPROCESSOR) if (ncpu > 1) { - membar_exit(); - ucas_critical_pausing_cpus = (u_int)-1; + /* + * Open the gate and notify all CPUs in + * ucas_critical_cpu_gate that they can now proceed. + * Using a store-release ensures the ucas happens + * before any memory operations they issue after the + * IPI -- they won't observe any stale cache of the + * target word, for instance. + * + * Matches atomic_load_acquire in ucas_critical_cpu_gate. + */ + atomic_store_release(&ucas_critical_pausing_cpus, (u_int)-1); splx(ucas_critical_splcookie); mutex_exit(&cpu_lock); return;