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    Subject[PATCH 3.2 096/106] MIPS: Fix IRQ tracing & lockdep when rescheduling
    3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Paul Burton <paul.burton@imgtec.com>

    commit d8550860d910c6b7b70f830f59003b33daaa52c9 upstream.

    When the scheduler sets TIF_NEED_RESCHED & we call into the scheduler
    from arch/mips/kernel/entry.S we disable interrupts. This is true
    regardless of whether we reach work_resched from syscall_exit_work,
    resume_userspace or by looping after calling schedule(). Although we
    disable interrupts in these paths we don't call trace_hardirqs_off()
    before calling into C code which may acquire locks, and we therefore
    leave lockdep with an inconsistent view of whether interrupts are
    disabled or not when CONFIG_PROVE_LOCKING & CONFIG_DEBUG_LOCKDEP are
    both enabled.

    Without tracing this interrupt state lockdep will print warnings such
    as the following once a task returns from a syscall via
    syscall_exit_partial with TIF_NEED_RESCHED set:

    [ 49.927678] ------------[ cut here ]------------
    [ 49.934445] WARNING: CPU: 0 PID: 1 at kernel/locking/lockdep.c:3687 check_flags.part.41+0x1dc/0x1e8
    [ 49.946031] DEBUG_LOCKS_WARN_ON(current->hardirqs_enabled)
    [ 49.946355] CPU: 0 PID: 1 Comm: init Not tainted 4.10.0-00439-gc9fd5d362289-dirty #197
    [ 49.963505] Stack : 0000000000000000 ffffffff81bb5d6a 0000000000000006 ffffffff801ce9c4
    [ 49.974431] 0000000000000000 0000000000000000 0000000000000000 000000000000004a
    [ 49.985300] ffffffff80b7e487 ffffffff80a24498 a8000000ff160000 ffffffff80ede8b8
    [ 49.996194] 0000000000000001 0000000000000000 0000000000000000 0000000077c8030c
    [ 50.007063] 000000007fd8a510 ffffffff801cd45c 0000000000000000 a8000000ff127c88
    [ 50.017945] 0000000000000000 ffffffff801cf928 0000000000000001 ffffffff80a24498
    [ 50.028827] 0000000000000000 0000000000000001 0000000000000000 0000000000000000
    [ 50.039688] 0000000000000000 a8000000ff127bd0 0000000000000000 ffffffff805509bc
    [ 50.050575] 00000000140084e0 0000000000000000 0000000000000000 0000000000040a00
    [ 50.061448] 0000000000000000 ffffffff8010e1b0 0000000000000000 ffffffff805509bc
    [ 50.072327] ...
    [ 50.076087] Call Trace:
    [ 50.079869] [<ffffffff8010e1b0>] show_stack+0x80/0xa8
    [ 50.086577] [<ffffffff805509bc>] dump_stack+0x10c/0x190
    [ 50.093498] [<ffffffff8015dde0>] __warn+0xf0/0x108
    [ 50.099889] [<ffffffff8015de34>] warn_slowpath_fmt+0x3c/0x48
    [ 50.107241] [<ffffffff801c15b4>] check_flags.part.41+0x1dc/0x1e8
    [ 50.114961] [<ffffffff801c239c>] lock_is_held_type+0x8c/0xb0
    [ 50.122291] [<ffffffff809461b8>] __schedule+0x8c0/0x10f8
    [ 50.129221] [<ffffffff80946a60>] schedule+0x30/0x98
    [ 50.135659] [<ffffffff80106278>] work_resched+0x8/0x34
    [ 50.142397] ---[ end trace 0cb4f6ef5b99fe21 ]---
    [ 50.148405] possible reason: unannotated irqs-off.
    [ 50.154600] irq event stamp: 400463
    [ 50.159566] hardirqs last enabled at (400463): [<ffffffff8094edc8>] _raw_spin_unlock_irqrestore+0x40/0xa8
    [ 50.171981] hardirqs last disabled at (400462): [<ffffffff8094eb98>] _raw_spin_lock_irqsave+0x30/0xb0
    [ 50.183897] softirqs last enabled at (400450): [<ffffffff8016580c>] __do_softirq+0x4ac/0x6a8
    [ 50.195015] softirqs last disabled at (400425): [<ffffffff80165e78>] irq_exit+0x110/0x128

    Fix this by using the TRACE_IRQS_OFF macro to call trace_hardirqs_off()
    when CONFIG_TRACE_IRQFLAGS is enabled. This is done before invoking
    schedule() following the work_resched label because:

    1) Interrupts are disabled regardless of the path we take to reach
    work_resched() & schedule().

    2) Performing the tracing here avoids the need to do it in paths which
    disable interrupts but don't call out to C code before hitting a
    path which uses the RESTORE_SOME macro that will call
    trace_hardirqs_on() or trace_hardirqs_off() as appropriate.

    We call trace_hardirqs_on() using the TRACE_IRQS_ON macro before calling
    syscall_trace_leave() for similar reasons, ensuring that lockdep has a
    consistent view of state after we re-enable interrupts.

    Signed-off-by: Paul Burton <paul.burton@imgtec.com>
    Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
    Cc: linux-mips@linux-mips.org
    Patchwork: https://patchwork.linux-mips.org/patch/15385/
    Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
    [bwh: Backported to 3.2: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/mips/kernel/entry.S | 3 +++
    1 file changed, 3 insertions(+)

    --- a/arch/mips/kernel/entry.S
    +++ b/arch/mips/kernel/entry.S
    @@ -10,6 +10,7 @@

    #include <asm/asm.h>
    #include <asm/asmmacro.h>
    +#include <asm/irqflags.h>
    #include <asm/regdef.h>
    #include <asm/mipsregs.h>
    #include <asm/stackframe.h>
    @@ -145,6 +146,7 @@ work_pending:
    andi t0, a2, _TIF_NEED_RESCHED # a2 is preloaded with TI_FLAGS
    beqz t0, work_notifysig
    work_resched:
    + TRACE_IRQS_OFF
    jal schedule

    local_irq_disable # make sure need_resched and
    @@ -172,6 +174,7 @@ syscall_exit_work:
    beqz t0, work_pending # trace bit set?
    local_irq_enable # could let syscall_trace_leave()
    # call schedule() instead
    + TRACE_IRQS_ON
    move a0, sp
    jal syscall_trace_leave
    b resume_userspace
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