linux-kernel - Re: [PATCH 1/3] powerpc/uaccess: Switch __put_user_size_allowed() to __put_user_asm

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Message-ID: <87imav9r64.fsf@igel.home>
Date:   Wed, 28 Oct 2020 00:37:23 +0100
From:   Andreas Schwab <schwab@...ux-m68k.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] powerpc/uaccess: Switch __put_user_size_allowed()
 to __put_user_asm_goto()

On Okt 28 2020, Andreas Schwab wrote:

> On Sep 04 2020, Christophe Leroy wrote:
>
>> __put_user_asm_goto() provides more flexibility to GCC and avoids using
>> a local variable to tell if the write succeeded or not.
>> GCC can then avoid implementing a cmp in the fast path.
>
> That breaks CLONE_CHILD_SETTID.  I'm getting an assertion failure in
> __libc_fork (THREAD_GETMEM (self, tid) != ppid).

This is what schedule_tail now looks like.  As you can see, put_user has
become a nop:

000000000000455c <.schedule_tail>:
    455c:       7c 08 02 a6     mflr    r0
    4560:       f8 01 00 10     std     r0,16(r1)
    4564:       f8 21 ff 91     stdu    r1,-112(r1)
    4568:       4b ff cd 4d     bl      12b4 <.finish_task_switch>
    456c:       4b ff c0 99     bl      604 <.balance_callback>
    4570:       e8 6d 01 88     ld      r3,392(r13)
    4574:       e9 23 06 b0     ld      r9,1712(r3)
    4578:       2f a9 00 00     cmpdi   cr7,r9,0
    457c:       41 9e 00 14     beq     cr7,4590 <.schedule_tail+0x34>
    4580:       38 80 00 00     li      r4,0
    4584:       38 a0 00 00     li      r5,0
    4588:       48 00 00 01     bl      4588 <.schedule_tail+0x2c>
                        4588: R_PPC64_REL24     .__task_pid_nr_ns
    458c:       60 00 00 00     nop
    4590:       48 00 00 01     bl      4590 <.schedule_tail+0x34>
                        4590: R_PPC64_REL24     .calculate_sigpending
    4594:       60 00 00 00     nop
    4598:       38 21 00 70     addi    r1,r1,112
    459c:       e8 01 00 10     ld      r0,16(r1)
    45a0:       7c 08 03 a6     mtlr    r0
    45a4:       4e 80 00 20     blr

This is schedule_tail in 5.9:

000000000000455c <.schedule_tail>:
    455c:       7c 08 02 a6     mflr    r0
    4560:       fb c1 ff f0     std     r30,-16(r1)
    4564:       fb e1 ff f8     std     r31,-8(r1)
    4568:       f8 01 00 10     std     r0,16(r1)
    456c:       f8 21 ff 81     stdu    r1,-128(r1)
    4570:       4b ff cd 45     bl      12b4 <.finish_task_switch>
    4574:       4b ff c0 91     bl      604 <.balance_callback>
    4578:       eb cd 01 88     ld      r30,392(r13)
    457c:       eb fe 06 b0     ld      r31,1712(r30)
    4580:       2f bf 00 00     cmpdi   cr7,r31,0
    4584:       41 9e 00 2c     beq     cr7,45b0 <.schedule_tail+0x54>
    4588:       7f c3 f3 78     mr      r3,r30
    458c:       38 80 00 00     li      r4,0
    4590:       38 a0 00 00     li      r5,0
    4594:       48 00 00 01     bl      4594 <.schedule_tail+0x38>
                        4594: R_PPC64_REL24     .__task_pid_nr_ns
    4598:       60 00 00 00     nop
    459c:       e9 3e 0a b8     ld      r9,2744(r30)
    45a0:       7f bf 48 40     cmpld   cr7,r31,r9
    45a4:       41 9d 00 0c     bgt     cr7,45b0 <.schedule_tail+0x54>
    45a8:       2b a9 00 03     cmpldi  cr7,r9,3
    45ac:       41 9d 00 14     bgt     cr7,45c0 <.schedule_tail+0x64>
    45b0:       48 00 00 01     bl      45b0 <.schedule_tail+0x54>
                        45b0: R_PPC64_REL24     .calculate_sigpending
    45b4:       60 00 00 00     nop
    45b8:       38 21 00 80     addi    r1,r1,128
    45bc:       48 00 00 00     b       45bc <.schedule_tail+0x60>
                        45bc: R_PPC64_REL24     _restgpr0_30
    45c0:       39 20 00 00     li      r9,0
    45c4:       90 7f 00 00     stw     r3,0(r31)
    45c8:       4b ff ff e8     b       45b0 <.schedule_tail+0x54>


Andreas.

-- 
Andreas Schwab, schwab@...ux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."