• Steven Rostedt (VMware)'s avatar
    ring-buffer: Have ring_buffer_alloc_read_page() return error on offline CPU · a7e52ad7
    Steven Rostedt (VMware) authored
    Chunyu Hu reported:
      "per_cpu trace directories and files are created for all possible cpus,
       but only the cpus which have ever been on-lined have their own per cpu
       ring buffer (allocated by cpuhp threads). While trace_buffers_open, the
       open handler for trace file 'trace_pipe_raw' is always trying to access
       field of ring_buffer_per_cpu, and would panic with the NULL pointer.
       Align the behavior of trace_pipe_raw with trace_pipe, that returns -NODEV
       when openning it if that cpu does not have trace ring buffer.
       cat /sys/kernel/debug/tracing/per_cpu/cpu31/trace_pipe_raw
       (cpu31 is never on-lined, this is a 16 cores x86_64 box)
       Tested with:
       1) boot with maxcpus=14, read trace_pipe_raw of cpu15.
          Got -NODEV.
       2) oneline cpu15, read trace_pipe_raw of cpu15.
          Get the raw trace data.
       Call trace:
       [ 5760.950995] RIP: 0010:ring_buffer_alloc_read_page+0x32/0xe0
       [ 5760.961678]  tracing_buffers_read+0x1f6/0x230
       [ 5760.962695]  __vfs_read+0x37/0x160
       [ 5760.963498]  ? __vfs_read+0x5/0x160
       [ 5760.964339]  ? security_file_permission+0x9d/0xc0
       [ 5760.965451]  ? __vfs_read+0x5/0x160
       [ 5760.966280]  vfs_read+0x8c/0x130
       [ 5760.967070]  SyS_read+0x55/0xc0
       [ 5760.967779]  do_syscall_64+0x67/0x150
       [ 5760.968687]  entry_SYSCALL64_slow_path+0x25/0x25"
    This was introduced by the addition of the feature to reuse reader pages
    instead of re-allocating them. The problem is that the allocation of a
    reader page (which is per cpu) does not check if the cpu is online and set
    up for the ring buffer.
    Link: http://lkml.kernel.org/r/1500880866-1177-1-git-send-email-chuhu@redhat.com
    Cc: stable@vger.kernel.org
    Fixes: 73a757e6 ("ring-buffer: Return reader page back into existing ring buffer")
    Reported-by: default avatarChunyu Hu <chuhu@redhat.com>
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>