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net-sysfs: try not to restart the syscall if it will fail eventually

Antoine Tenart requested to merge atenart/centos-stream-9:cs9/bz2030634/sysfs-rtnl into main

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2030634
Upstream Status: linux.git
Tested: test script in bz

commit 146e5e733310379f51924111068f08a3af0db830
Author: Antoine Tenart atenart@kernel.org
Date: Thu Oct 7 16:00:51 2021 +0200

net-sysfs: try not to restart the syscall if it will fail eventually  

Due to deadlocks in the networking subsystem spotted 12 years ago[1],  
a workaround was put in place[2] to avoid taking the rtnl lock when it  
was not available and restarting the syscall (back to VFS, letting  
userspace spin). The following construction is found a lot in the net  
sysfs and sysctl code:  

  if (!rtnl_trylock())  
          return restart_syscall();  

This can be problematic when multiple userspace threads use such  
interfaces in a short period, making them to spin a lot. This happens  
for example when adding and moving virtual interfaces: userspace  
programs listening on events, such as systemd-udevd and NetworkManager,  
do trigger actions reading files in sysfs. It gets worse when a lot of  
virtual interfaces are created concurrently, say when creating  
containers at boot time.  

Returning early without hitting the above pattern when the syscall will  
fail eventually does make things better. While it is not a fix for the  
issue, it does ease things.  

[1] https://lore.kernel.org/netdev/49A4D5D5.5090602@trash.net/  
    https://lore.kernel.org/netdev/m14oyhis31.fsf@fess.ebiederm.org/  
    and https://lore.kernel.org/netdev/20090226084924.16cb3e08@nehalam/  
[2] Rightfully, those deadlocks are *hard* to solve.  

Signed-off-by: Antoine Tenart <atenart@kernel.org>  
Reviewed-by: Paolo Abeni <pabeni@redhat.com>  
Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: Antoine Tenart atenart@redhat.com

Edited by Antoine Tenart

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