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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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31
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
function to avoid the compiler complaining about a defined, but unused
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("...");
}
```