linux-kernel - Re: [RFC PATCH 08/13] x86/process/64: Clean up uintr task fork and exit paths

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Date:   Mon, 27 Sep 2021 18:23:11 -0700
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>, <x86@...nel.org>
CC:     Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Ingo Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "Gayatri Kammela" <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        <linux-api@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH 08/13] x86/process/64: Clean up uintr task fork and
 exit paths

On 9/23/2021 6:02 PM, Thomas Gleixner wrote:
> On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
>
>> The user interrupt MSRs and the user interrupt state is task specific.
>> During task fork and exit clear the task state, clear the MSRs and
>> dereference the shared resources.
>>
>> Some of the memory resources like the UPID are referenced in the file
>> descriptor and could be in use while the uintr_fd is still valid.
>> Instead of freeing up  the UPID just dereference it.
> Derefencing the UPID, i.e. accessing task->upid->foo helps in which way?
>
> You want to drop the reference count I assume. Then please write that
> so.


Ah! Not sure how I associated dereference to dropping the reference. 
Will update this.

>
>> @@ -260,6 +260,7 @@ int fpu_clone(struct task_struct *dst)
>>   {
>>   	struct fpu *src_fpu = &current->thread.fpu;
>>   	struct fpu *dst_fpu = &dst->thread.fpu;
>> +	struct uintr_state *uintr_state;
>>   
>>   	/* The new task's FPU state cannot be valid in the hardware. */
>>   	dst_fpu->last_cpu = -1;
>> @@ -284,6 +285,14 @@ int fpu_clone(struct task_struct *dst)
>>   
>>   	else
>>   		save_fpregs_to_fpstate(dst_fpu);
>> +
>> +	/* UINTR state is not expected to be inherited (in the current design). */
>> +	if (static_cpu_has(X86_FEATURE_UINTR)) {
>> +		uintr_state = get_xsave_addr(&dst_fpu->state.xsave, XFEATURE_UINTR);
>> +		if (uintr_state)
>> +			memset(uintr_state, 0, sizeof(*uintr_state));
>> +	}
> 1) If the FPU registers are up to date then this can be completely
>     avoided by excluding the UINTR component from XSAVES

You mentioned this in the other thread that the UINTR state must be 
invalidated during fpu_clone().

I am not sure if understand all the nuances here. Your suggestion seems 
valid to me. I'll have to think more about this.

> 2) If the task never used that muck then UINTR is in init state and
>     clearing that memory is a redunant exercise because it has been
>     cleared already

Yes. I'll add a check for that.

>> + * exit_thread() can happen in current context when the current thread is
>> + * exiting or it can happen for a new thread that is being created.
> A right that makes sense. If a new thread is created then it can call
> exit_thread(), right?


What I meant here is that exit_thread() can also be called during 
copy_process() if it runs into an issue.

bad_fork_cleanup_thread:

     exit_thread();

In this case is_uintr_receiver() will fail. I'll update the comments to 
reflect that.

>> + * For new threads is_uintr_receiver() should fail.
> Should fail?

Thanks,

Sohil