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