eventfd_signal assumes that spin_lock_irqsave/spin_unlock_irqrestore is non-preemptable and therefore increments and decrements the percpu variable inside the critical section.

This obviously does not fly with PREEMPT_RT. If eventfd_signal is preempted and an unrelated thread calls eventfd_signal, the result is a spurious WARN. To avoid this, protect the percpu variable with a local_lock.

Reported-by: Daniel Bristot de Oliveira bristot@kernel.org Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth") Cc: He Zhe zhe.he@windriver.com Cc: Jens Axboe axboe@kernel.dk Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Thomas Gleixner tglx@linutronix.de Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: stable@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Co-developed-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Daniel Bristot de Oliveira bristot@kernel.org --- fs/eventfd.c | 27 ++++++++++++++++++++++----- include/linux/eventfd.h | 7 +------ 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c index e265b6dd4f34..9754fcd38690 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -12,6 +12,7 @@ #include <linux/fs.h> #include <linux/sched/signal.h> #include <linux/kernel.h> +#include <linux/local_lock.h> #include <linux/slab.h> #include <linux/list.h> #include <linux/spinlock.h> @@ -25,8 +26,6 @@ #include <linux/idr.h> #include <linux/uio.h>

-DEFINE_PER_CPU(int, eventfd_wake_count); - static DEFINE_IDA(eventfd_ida);

struct eventfd_ctx { @@ -45,6 +44,20 @@ struct eventfd_ctx { int id; };

+struct event_fd_recursion { + local_lock_t lock; + int count; +}; + +static DEFINE_PER_CPU(struct event_fd_recursion, event_fd_recursion) = { + .lock = INIT_LOCAL_LOCK(lock), +}; + +bool eventfd_signal_count(void) +{ + return this_cpu_read(event_fd_recursion.count); +} + /** * eventfd_signal - Adds @n to the eventfd counter. * @ctx: [in] Pointer to the eventfd context. @@ -71,18 +84,22 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) * it returns true, the eventfd_signal() call should be deferred to a * safe context. */ - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) + local_lock(&event_fd_recursion.lock); + if (WARN_ON_ONCE(this_cpu_read(event_fd_recursion.count))) { + local_unlock(&event_fd_recursion.lock); return 0; + }

spin_lock_irqsave(&ctx->wqh.lock, flags); - this_cpu_inc(eventfd_wake_count); + this_cpu_inc(event_fd_recursion.count); 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); - this_cpu_dec(eventfd_wake_count); + this_cpu_dec(event_fd_recursion.count); spin_unlock_irqrestore(&ctx->wqh.lock, flags); + local_unlock(&event_fd_recursion.lock);

return n; } diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h index fa0a524baed0..ca89d6c409c1 100644 --- a/include/linux/eventfd.h +++ b/include/linux/eventfd.h @@ -43,12 +43,7 @@ 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) -{ - return this_cpu_read(eventfd_wake_count); -} +bool eventfd_signal_count(void);

#else /* CONFIG_EVENTFD */