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doc: document workaround for printk_once() and friends 

I couldn't find a good place in the document for this, so I just added
an "Other issues" section at the end.

Fixes #738.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
This commit is contained in:
Josh Poimboeuf 2017-09-22 14:49:00 -05:00
parent 10b37a16a4
commit 8bc0b5cf39
1 changed files with 31 additions and 0 deletions
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doc/patch-author-guide.md
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@ -476,3 +476,34 @@ new function.
In some patching cases it might be necessary to completely remove the original In some patching cases it might be necessary to completely remove the original
function to avoid the compiler complaining about a defined, but unused function to avoid the compiler complaining about a defined, but unused
function. This will depend on symbol scope and kernel build options. function. This will depend on symbol scope and kernel build options.
Other issues
------------
When adding a call to `printk_once()`, `pr_warn_once()`, or any other "once"
variation of `printk()`, you'll get the following eror:
```
ERROR: vmx.o: 1 unsupported section change(s)
vmx.o: WARNING: unable to correlate static local variable __print_once.60588 used by vmx_update_pi_irte, assuming variable is new
vmx.o: changed function: vmx_update_pi_irte
vmx.o: data section .data..read_mostly selected for inclusion
/usr/lib/kpatch/create-diff-object: unreconcilable difference
```
This error occurs because the `printk_once()` adds a static local variable to
the `.data..read_mostly` section. kpatch-build strict disallows any changes to
that section, because in some cases a change to this section indicates a bug.
To work around this issue, you'll need to manually implement your own "once"
logic which doesn't store the static variable in the `.data..read_mostly`
section.
For example, a `pr_warn_once()` can be replaced with:
```
static bool print_once;
...
if (!print_once) {
print_once = true;
pr_warn("...");
}
```