• Steven Rostedt (Red Hat)'s avatar
    tracing: Fix trace_printk() to print when not using bprintk() · 3debb0a9
    Steven Rostedt (Red Hat) authored
    The trace_printk() code will allocate extra buffers if the compile detects
    that a trace_printk() is used. To do this, the format of the trace_printk()
    is saved to the __trace_printk_fmt section, and if that section is bigger
    than zero, the buffers are allocated (along with a message that this has
    happened).
    
    If trace_printk() uses a format that is not a constant, and thus something
    not guaranteed to be around when the print happens, the compiler optimizes
    the fmt out, as it is not used, and the __trace_printk_fmt section is not
    filled. This means the kernel will not allocate the special buffers needed
    for the trace_printk() and the trace_printk() will not write anything to the
    tracing buffer.
    
    Adding a "__used" to the variable in the __trace_printk_fmt section will
    keep it around, even though it is set to NULL. This will keep the string
    from being printed in the debugfs/tracing/printk_formats section as it is
    not needed.
    Reported-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Fixes: 07d777fe "tracing: Add percpu buffers for trace_printk()"
    Cc: stable@vger.kernel.org # v3.5+
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    3debb0a9
trace_printk.c 8.47 KB