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SubjectRe: [PATCH] ftrace: fix race between ftrace and insmod
On Wed, 16 Dec 2015 18:28:35 +0800
"Zhang, Yanmin" <yanmin_zhang@linux.intel.com> wrote:

> > + /*
> > + * If the tracing is enabled, go ahead and enable the record.
> > + *
> > + * The reason not to enable the record immediatelly is the
> > + * inherent check of ftrace_make_nop/ftrace_make_call for
> > + * correct previous instructions. Making first the NOP
> > + * conversion puts the module to the correct state, thus
> > + * passing the ftrace_make_call check.
> > + *
> > + * We also delay this to after the module code already set the
> > + * text to read-only, as we now need to set it back to read-write
> > + * so that we can modify the text.
> > + */
> > + if (ftrace_start_up)
> > + ftrace_arch_code_modify_prepare();
> > +
> > + do_for_each_ftrace_rec(pg, rec) {
> > + int cnt;
> > + /*
> > + * do_for_each_ftrace_rec() is a double loop.
> > + * module text shares the pg. If a record is
> > + * not part of this module, then skip this pg,
> > + * which the "break" will do.
> > + */
> > + if (!within_module_core(rec->ip, mod))
> > + break;
> > +
> > + cnt = 0;
> > +
> > + /*
> > + * When adding a module, we need to check if tracers are
> > + * currently enabled and if they are, and can trace this record,
> > + * we need to enable the module functions as well as update the
> > + * reference counts for those function records.
> > + */
> > + if (ftrace_start_up)
> > + cnt += referenced_filters(rec);
> > +
> > + /* This clears FTRACE_FL_DISABLED */
> > + rec->flags = cnt;
> > +
> > + if (ftrace_start_up && cnt) {
> > + int failed = __ftrace_replace_code(rec, 1);
>
> If we choose to call ftrace_module_enable when receiving module notification
> MODULE_STATE_COMING, TEXT section of the module is already changed to RO.

And that's why we call ftrace_arch_code_modify_prepare(). That should
change all text to RW.

-- Steve


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