rasdaemon: Can't open trace_clock

[Martins3]

What's happening

sometimes, systemd failed to bootstrap rasdasmon, the log looks like this:

rasdaemon: Can't open trace_clock
rasdaemon: Can't select a timestamp for tracing

Root Cause

systemd will start two rasdaemon process with different arguments, both of them share a common code path

1. firstly, create /sys/kernel/debug/tracing/instances/rasdaemon
   

   rasdaemon/ras-events.c

   Line 139 in c225517

   rc = mkdir(ras->tracing, S_IRWXU);

2. then, open /sys/kernel/debug/tracing/instances/rasdaemon/trace_clock
   

   rasdaemon/ras-events.c

   Line 629 in c225517

   fd = open_trace(ras, "trace_clock", O_RDONLY);

If the first process is creating the /sys/kernel/debug/tracing/instances/rasdaemon, then the second process's mkdir will fail at the kernel code path vfs_mkdir, then the second one try to open trace_clock, but the first one doesn't create it yet.

mkdir and populating the contents in the directory is the not atomic.

Possible solution

1. use file lock to guard the mkdir operation
2. change kernel code