Re: [PATCH v2 00/12] x86: WARN() hackery

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 10, 2025 at 08:18:02AM -0800, Linus Torvalds wrote:
> On Mon, 10 Nov 2025 at 03:58, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > We should probably just merge these, as they improve the code-gen for WARN()
> > significantly on x86
> 
> Yes..  And I'd actually like to see some example code generation in
> the commit messages just to make that "improve the code-gen" more
> explicit.

Damn, I actually thought to include some and then I totally forgot.

So lets see:

        if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
                      "corrupted preempt_count: %s/%d/0x%x\n",
                      current->comm, current->pid, preempt_count()))

#define WARN_ONCE(condition, format...)                         \
        DO_ONCE_LITE_IF(condition, WARN, 1, format)

#define WARN(condition, format...) ({                                   \
        int __ret_warn_on = !!(condition);                              \
        if (unlikely(__ret_warn_on))                                    \
                __WARN_printf(TAINT_WARN, format);                      \
        unlikely(__ret_warn_on);                                        \
})

#define __WARN_printf(taint, arg...) do {                               \
                instrumentation_begin();                                \
                __warn_printk(arg);                                     \
                __WARN_FLAGS("", BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
                instrumentation_end();                                  \
        } while (0)

Turns into this majestic pile:


# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
        cmpl    $2, %ecx        #, _7
        jne     .L1472  #,

...

.L1472:
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
        cmpb    $0, __already_done.11(%rip)     #, __already_done
        je      .L1513  #,

...

.L1513:
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
        movb    $1, __already_done.11(%rip)     #, __already_done
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,


# 0 "" 2
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
#NO_APP
        movl    1424(%r14), %edx        # _15->pid, _15->pid
        leaq    1912(%r14), %rsi        #, _17
        movq    $.LC43, %rdi    #,
        call    __warn_printk   #
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,

# 0 "" 2
# 5093 "../kernel/sched/core.c" 1
        1:       .byte 0x0f, 0x0b ;
.pushsection __bug_table,"aw"
912:
        .pushsection .discard.annotate_data,"M", @progbits, 8
        .long 912b - .
        .long 1
        .popsection
        2:      .long 1b - .    # bug_entry::bug_addr
        .long .LC1 - .  # bug_entry::file       #
        .word 5093      # bug_entry::line       #
        .word 2313      # bug_entry::flags      #
        .org 2b + 12    #
.popsection
.pushsection .discard.annotate_insn,"M", @progbits, 8
        .long 1b - .
        .long 8		# ANNOTYPE_REACHABLE
        .popsection


While afterwards it looks like so:


# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
        cmpl    $2, %ecx        #, _7
        jne     .L1442  #,

...

.L1442:
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
#APP
# 5093 "../kernel/sched/core.c" 1
        lea (2f)(%rip), %rdi    # bug
1:
.pushsection __bug_table,"aw"
        912:
        .pushsection .discard.annotate_data,"M", @progbits, 8
        .long 912b - .
        .long 1
        .popsection
        2:
        .long 1b - .    # bug_entry::bug_addr
        .long .LC43 - . # bug_entry::format     #
        .long .LC1 - .  # bug_entry::file       #
        .word 5093      # bug_entry::line       #
        .word 2323      # bug_entry::flags      #
        .org 2b + 16    #
.popsection
        
# 0 "" 2
#NO_APP  
        movl    1424(%r14), %edx        # pretmp_19->pid, pretmp_19->pid
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
        leaq    1912(%r14), %rsi        #, _13
# ../kernel/sched/core.c:5093:  if (WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
        call    __SCT__WARN_trap        #

Where at runtime we patch that "call __SCT__WARN_trap" to read like:

	ud1 (%ecx), %rdi


The whole DO_ONCE_LITE_IF/__already_done thing is gone (replaced with
BUGFLAG_ONCE) and the external __warn_prink() call is also gone, now
inside the exception handler through the pt_regs->va_list magic.

I'll see if I can clean that up somewhat and stick in the changelog.