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However sometimes different eventfds may be used in parallel. Specifically, when heavy network load goes through kvm and vhost, working as below, it would trigger the following call trace. - 100.00% - 66.51% ret_from_fork kthread - vhost_worker - 33.47% handle_tx_kick handle_tx handle_tx_copy vhost_tx_batch.isra.0 vhost_add_used_and_signal_n eventfd_signal - 33.05% handle_rx_net handle_rx vhost_add_used_and_signal_n eventfd_signal - 33.49% ioctl entry_SYSCALL_64_after_hwframe do_syscall_64 __x64_sys_ioctl ksys_ioctl do_vfs_ioctl kvm_vcpu_ioctl kvm_arch_vcpu_ioctl_run vmx_handle_exit handle_ept_misconfig kvm_io_bus_write __kvm_io_bus_write eventfd_signal 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0 ---- snip ---- 001: Call Trace: 001: vhost_signal+0x15e/0x1b0 [vhost] 001: vhost_add_used_and_signal_n+0x2b/0x40 [vhost] 001: handle_rx+0xb9/0x900 [vhost_net] 001: handle_rx_net+0x15/0x20 [vhost_net] 001: vhost_worker+0xbe/0x120 [vhost] 001: kthread+0x106/0x140 001: ? log_used.part.0+0x20/0x20 [vhost] 001: ? kthread_park+0x90/0x90 001: ret_from_fork+0x35/0x40 001: ---[ end trace 0000000000000003 ]--- This patch enlarges the limit to 1 which is the maximum recursion depth we have found so far. The credit of modification for eventfd_signal_count goes to Xie Yongji Signed-off-by: He Zhe --- fs/eventfd.c | 3 ++- include/linux/eventfd.h | 5 ++++- 2 files changed, 6 insertions(+), 2 deletions(-) diff --git a/fs/eventfd.c b/fs/eventfd.c index e265b6dd4f34..add6af91cacf 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -71,7 +71,8 @@ __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))) + if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) > + EFD_WAKE_COUNT_MAX)) return 0; spin_lock_irqsave(&ctx->wqh.lock, flags); diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h index fa0a524baed0..74be152ebe87 100644 --- a/include/linux/eventfd.h +++ b/include/linux/eventfd.h @@ -29,6 +29,9 @@ #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK) #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE) +/* This is the maximum recursion depth we find so far */ +#define EFD_WAKE_COUNT_MAX 1 + struct eventfd_ctx; struct file; @@ -47,7 +50,7 @@ DECLARE_PER_CPU(int, eventfd_wake_count); static inline bool eventfd_signal_count(void) { - return this_cpu_read(eventfd_wake_count); + return this_cpu_read(eventfd_wake_count) > EFD_WAKE_COUNT_MAX; } #else /* CONFIG_EVENTFD */ -- 2.17.1