Messages in this thread | | | Date | Wed, 16 Dec 2020 14:10:16 +0100 | From | Jessica Yu <> | Subject | Re: static_branch_enable() does not work from a __init function? |
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+++ Peter Zijlstra [16/12/20 13:47 +0100]: >On Wed, Dec 16, 2020 at 12:55:25PM +0100, Jessica Yu wrote: >> +++ Peter Zijlstra [16/12/20 10:26 +0100]: >> [snip] >> > > PS, I originally found: in arch/x86/kvm/vmx/vmx.c: vmx_init(), it looks >> > > like the line "static_branch_enable(&enable_evmcs);" does not take effect >> > > in a v5.4-based kernel, but does take effect in the v5.10 kernel in the >> > > same x86-64 virtual machine on Hyper-V, so I made the above test module >> > > to test static_branch_enable(), and found that static_branch_enable() in >> > > the test module does not work with both v5.10 and my v5.4 kernel, if the >> > > __init marker is used. >> >> Because the jump label code currently does not allow you to update if >> the entry resides in an init section. By marking the module init >> section __init you place it in the .init.text section. >> jump_label_add_module() detects this (by calling within_module_init()) >> and marks the entry by calling jump_entry_set_init(). Then you have >> the following sequence of calls (roughly): >> >> static_branch_enable >> static_key_enable >> static_key_enable_cpuslocked >> jump_label_update >> jump_label_can_update >> jump_entry_is_init returns true, so bail out >> >> Judging from the comment in jump_label_can_update(), this seems to be >> intentional behavior: >> >> static bool jump_label_can_update(struct jump_entry *entry, bool init) >> { >> /* >> * Cannot update code that was in an init text area. >> */ >> if (!init && jump_entry_is_init(entry)) >> return false; >> > >Only because we're having .init=false, incorrectly. See the other email.
Ah yeah, you're right. I also misread the intention of the if conditional :/ If we're currently running an init function it's fine, but after that it will be unsafe.
Btw, your patch seems to work for me, using the test module provided by Dexuan.
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