linux-kernel - [PATCH 0/2] x86/tsc: Fix native_sched

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Message-ID: <20181011103825.023036082@infradead.org>
Date:   Thu, 11 Oct 2018 12:38:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, hpa@...or.com, edumazet@...gle.com,
        eric.dumazet@...il.com, bp@...en8.de, peterz@...radead.org
Subject: [PATCH 0/2] x86/tsc: Fix native_sched_clock()

Eric reported that native_sched_clock() gets miscompiled; we loose a seqcount
loop. These patches cure that.

After these patches:

native_sched_clock:
	pushq   %rbp    #
	movq    %rsp, %rbp      #,

	... jump label ...

	rdtsc
	salq    $32, %rdx       #, tmp110
	orq     %rax, %rdx      # low, tmp110
	movq    %rdx, %r10      # tmp110, _23
	movq    $cyc2ns, %r9    #, tmp136
.L235:
	movl %gs:cyc2ns+32(%rip),%eax   # cyc2ns.seq.sequence, pfo_ret__
	movl    %eax, %ecx      # pfo_ret__, idx
	andl    $1, %ecx        #, idx
	salq    $4, %rcx        #, tmp116
	addq    %r9, %rcx       # tmp136, tmp117
	movq %gs:8(%rcx),%rdi   # cyc2ns.data[idx_14].cyc2ns_offset, pfo_ret__
	movl %gs:(%rcx),%esi    # cyc2ns.data[idx_14].cyc2ns_mul, pfo_ret__
	movl %gs:4(%rcx),%ecx   # cyc2ns.data[idx_14].cyc2ns_shift, pfo_ret__
	movl %gs:cyc2ns+32(%rip),%r8d   # cyc2ns.seq.sequence, pfo_ret__
	cmpl    %r8d, %eax      # pfo_ret__, pfo_ret__
	jne     .L235   #,
	movl    %esi, %esi      # pfo_ret__, pfo_ret__
	movq    %rsi, %rax      # pfo_ret__, tmp133
	mulq    %r10    # _23
	shrdq   %rdx, %rax      # pfo_ret__,, tmp134
	shrq    %cl, %rdx       # pfo_ret__,
	testb   $64, %cl        #, pfo_ret__
	cmovne  %rdx, %rax      #,, tmp134
	addq    %rdi, %rax      # pfo_ret__, <retval>
	popq    %rbp    #
	ret