Re: [PATCH v2] net: make unregister netdev warning timeout configurable

On Tue, Mar 23, 2021 at 7:49 AM Dmitry Vyukov [off-list ref] wrote:

netdev_wait_allrefs() issues a warning if refcount does not drop to 0
after 10 seconds. While 10 second wait generally should not happen
under normal workload in normal environment, it seems to fire falsely
very often during fuzzing and/or in qemu emulation (~10x slower).
At least it's not possible to understand if it's really a false
positive or not. Automated testing generally bumps all timeouts
to very high values to avoid flake failures.
Add net.core.netdev_unregister_timeout_secs sysctl to make
the timeout configurable for automated testing systems.
Lowering the timeout may also be useful for e.g. manual bisection.
The default value matches the current behavior.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=211877
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

---
Changes since v1:
 - use sysctl instead of a config
---

        },
+       {
+               .procname       = "netdev_unregister_timeout_secs",
+               .data           = &netdev_unregister_timeout_secs,
+               .maxlen         = sizeof(unsigned int),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = SYSCTL_ZERO,
+               .extra2         = &int_3600,
+       },
        { }
 };

If we allow the sysctl to be 0, then we risk a flood of pr_emerg()
(one per jiffy ?)

If you really want the zero value, you need to change pr_emerg() to
pr_emerg_ratelimited()

Also, please base your patch on net-next, to avoid future merge conflicts
with my prior patch add2d7363107 "net: set initial device refcount to 1".