From 563a8863b188e15be8027634bc3788c810e6ee74 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Fri, 17 Dec 2021 11:32:09 +0100 Subject: [PATCH 310/323] eventfd: Make signal recursion protection a task bit Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.215-rt107.tar.xz Upstream commit b542e383d8c005f06a131e2b40d5889b812f19c6 The recursion protection for eventfd_signal() is based on a per CPU variable and relies on the !RT semantics of spin_lock_irqsave() for protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither disables preemption nor interrupts which allows the spin lock held section to be preempted. If the preempting task invokes eventfd_signal() as well, then the recursion warning triggers. Paolo suggested to protect the per CPU variable with a local lock, but that's heavyweight and actually not necessary. The goal of this protection is to prevent the task stack from overflowing, which can be achieved with a per task recursion protection as well. Replace the per CPU variable with a per task bit similar to other recursion protection bits like task_struct::in_page_owner. This works on both !RT and RT kernels and removes as a side effect the extra per CPU storage. No functional change for !RT kernels. Reported-by: Daniel Bristot de Oliveira Signed-off-by: Thomas Gleixner Tested-by: Daniel Bristot de Oliveira Acked-by: Jason Wang Cc: Al Viro Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Luis Claudio R. Goncalves --- fs/aio.c | 2 +- fs/eventfd.c | 12 +++++------- include/linux/eventfd.h | 11 +++++------ include/linux/sched.h | 4 ++++ 4 files changed, 15 insertions(+), 14 deletions(-) diff --git a/fs/aio.c b/fs/aio.c index 43315af1c83ac..7030157e99338 100644 --- a/fs/aio.c +++ b/fs/aio.c @@ -1776,7 +1776,7 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync, list_del_init(&req->wait.entry); list_del(&iocb->ki_list); iocb->ki_res.res = mangle_poll(mask); - if (iocb->ki_eventfd && eventfd_signal_count()) { + if (iocb->ki_eventfd && eventfd_signal_allowed()) { iocb = NULL; INIT_WORK(&req->work, aio_poll_put_work); schedule_work(&req->work); diff --git a/fs/eventfd.c b/fs/eventfd.c index 3673eb8de0356..ce9ca07d6c004 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -25,8 +25,6 @@ #include #include -DEFINE_PER_CPU(int, eventfd_wake_count); - static DEFINE_IDA(eventfd_ida); struct eventfd_ctx { @@ -53,21 +51,21 @@ __u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, unsigned mask) * Deadlock or stack overflow issues can happen if we recurse here * through waitqueue wakeup handlers. If the caller users potentially * nested waitqueues with custom wakeup handlers, then it should - * check eventfd_signal_count() before calling this function. If - * it returns true, the eventfd_signal() call should be deferred to a + * check eventfd_signal_allowed() before calling this function. If + * it returns false, the eventfd_signal() call should be deferred to a * safe context. */ - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) + if (WARN_ON_ONCE(current->in_eventfd_signal)) return 0; spin_lock_irqsave(&ctx->wqh.lock, flags); - this_cpu_inc(eventfd_wake_count); + current->in_eventfd_signal = 1; if (ULLONG_MAX - ctx->count < n) n = ULLONG_MAX - ctx->count; ctx->count += n; if (waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask); - this_cpu_dec(eventfd_wake_count); + current->in_eventfd_signal = 0; spin_unlock_irqrestore(&ctx->wqh.lock, flags); return n; diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h index c1bd4883e2faf..842d223dfe175 100644 --- a/include/linux/eventfd.h +++ b/include/linux/eventfd.h @@ -14,6 +14,7 @@ #include #include #include +#include /* * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining @@ -44,11 +45,9 @@ int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *w __u64 *cnt); void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt); -DECLARE_PER_CPU(int, eventfd_wake_count); - -static inline bool eventfd_signal_count(void) +static inline bool eventfd_signal_allowed(void) { - return this_cpu_read(eventfd_wake_count); + return !current->in_eventfd_signal; } #else /* CONFIG_EVENTFD */ @@ -85,9 +84,9 @@ static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, return -ENOSYS; } -static inline bool eventfd_signal_count(void) +static inline bool eventfd_signal_allowed(void) { - return false; + return true; } static inline void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt) diff --git a/include/linux/sched.h b/include/linux/sched.h index a73528e8235d4..13d4957189bb9 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -848,6 +848,10 @@ struct task_struct { /* Stalled due to lack of memory */ unsigned in_memstall:1; #endif +#ifdef CONFIG_EVENTFD + /* Recursion prevention for eventfd_signal() */ + unsigned in_eventfd_signal:1; +#endif unsigned long atomic_flags; /* Flags requiring atomic access. */ -- 2.44.0