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Subjectxchg and GCC's optimisation:-(
Hi!

I recently posted a patch against 2.2.19 which eliminates most
printk()s by (in short words):

#define printk(format, arg...) do {} while(0)

However, I got trouble using the floppy driver, because
./kernel/dma.c:free_dma() seems to get miscompiled:

void free_dma(unsigned int dmanr)
{
if (dmanr >= MAX_DMA_CHANNELS) {
printk("Trying to free DMA%d\n", dmanr);
return;
}

if (xchg(&dma_chan_busy[dmanr].lock, 0) == 0) {
/* ERROR */ printk("Trying to free free DMA%d\n", dmanr);
return;
}

} /* free_dma */

Including a real_printk() at the line marked with ERROR will
result in:
00000088 <free_dma>:
88: 83 ec 0c sub $0xc,%esp
8b: 8b 54 24 10 mov 0x10(%esp,1),%edx
8f: 83 fa 07 cmp $0x7,%edx
92: 77 1e ja b2 <free_dma+0x2a>
94: 31 c0 xor %eax,%eax
96: 87 04 d5 00 00 00 00 xchg %eax,0x0(,%edx,8)
9d: 85 c0 test %eax,%eax
9f: 75 11 jne b2 <free_dma+0x2a>
a1: 83 c4 f8 add $0xfffffff8,%esp
a4: 52 push %edx
a5: 68 11 00 00 00 push $0x11
aa: e8 fc ff ff ff call ab <free_dma+0x23>
af: 83 c4 10 add $0x10,%esp
b2: 83 c4 0c add $0xc,%esp
b5: c3 ret
b6: 8d 76 00 lea 0x0(%esi),%esi
b9: 8d bc 27 00 00 00 00 lea 0x0(%edi,1),%edi

...which is fine and contains the needed xchg call. However,
substituting the printk() with "do {} while (0)" above,
the "if" path seems to be completely removed by the optimizer:

00000088 <free_dma>:
88: c3 ret
89: 8d b4 26 00 00 00 00 lea 0x0(%esi,1),%esi


I've looked at ./include/asm-i386/system.h which does some black
magic with it, and I don't really understand that. However, the
result is that the xchg gets optimized away, rendering at least
the floppy module unuseable:-(

MfG, JBG

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