[TRIVIAL] Re: UP went into unexpected trashing

From: Rusty Trivial Russell (rusty@rustcorp.com.au)
Date: Fri Nov 08 2002 - 03:33:21 EST

[ find_first_bit on x86 returns 32 even if you said to stop at (say) 5
  bits. Uncovered by the larger cpu mask patch. ]

From: dipankar@in.ibm.com

  Well, my earlier find_first_bit() implementation was completely bogus.
  My sanity has now returned and I coded this patch below that fixes
  find_find_bit() to return "size" if all bits are zero. I have tested it
  extensively in userspace and it boots 2.5.44-mm5 which crashed with the earlier
  version of the bitops_fix patch. I have coded the assembly routine
  as optimal as I could think of and without introducing any new
  branches or memory loads.
  Along with this patch, I applied the larger_cpu_mask patch to -mm5
  and sanity tested both UP and SMP kernels for dcache leaks in a 4CPU P3 box.
  An ls -lR and subsequent unmounting of that filesystems showed that
  the dentries were correctly getting returned the dcache slab and
  that indicates that the larger_cpu_mask patch no longer breaks RCU.
  I will do some more testing with this combination later with
  rcu_stats applied on this tree (just to be sure), but so far it looks good.
  Dipankar Sarma <dipankar@in.ibm.com> http://lse.sourceforge.net
  Linux Technology Center, IBM Software Lab, Bangalore, India.

--- trivial-2.5-bk/include/asm-i386/bitops.h.orig 2002-11-08 18:47:20.000000000 +1100
+++ trivial-2.5-bk/include/asm-i386/bitops.h 2002-11-08 18:47:20.000000000 +1100
@@ -311,12 +311,13 @@
                 "repe; scasl\n\t"
                 "jz 1f\n\t"
                 "leal -4(%%edi),%%edi\n\t"
- "bsfl (%%edi),%%eax\n"
- "1:\tsubl %%ebx,%%edi\n\t"
+ "bsfl (%%edi),%%edx\n"
+ "subl %%ebx,%%edi\n\t"
                 "shll $3,%%edi\n\t"
- "addl %%edi,%%eax"
+ "addl %%edi,%%edx\n\t"
+ "1:\tmovl %%edx,%%eax\n\t"
                 :"=a" (res), "=&c" (d0), "=&D" (d1)
- :"1" ((size + 31) >> 5), "2" (addr), "b" (addr));
+ :"1" ((size + 31) >> 5), "2" (addr), "b" (addr), "d" (size));
         return res;

  Don't blame me: the Monkey is driving
  File: dipankar@in.ibm.com: Re: [long]2.5.44-mm3 UP went into unexpected trashing
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This archive was generated by hypermail 2b29 : Fri Nov 15 2002 - 22:00:15 EST