Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

From: Paolo Bonzini
Date: Thu Jul 15 2021 - 07:06:06 EST


On 15/07/21 12:10, He Zhe wrote:
The following was provided in this thread. The commit log contains the call traces that I met and fixed back to Apr. 2020.

https://lore.kernel.org/lkml/20210618084412.18257-1-zhe.he@xxxxxxxxxxxxx/

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

This call trace is not of a reentrant call; there is only one call to eventfd_signal. It does fit the symptoms that Daniel reported for PREEMPT_RT though.

https://lore.kernel.org/lkml/beac2025-2e11-8ed0-61e2-9f6e633482e8@xxxxxxxxxx/

This one is about PREEMPT_RT, so it would be fixed by local_lock.

There _may_ be two bugs, so let's start by fixing this one. Once this one is fixed, we will examine the call stacks of any further reports, and diagnose whether the second bug (if it exists) is related to vDUSE, PREEMPT_RT or neeither.

Paolo