• Michael Ellerman's avatar
    powerpc/32: Avoid miscompilation w/GCC 4.6.3 - don't inline copy_to/from_user() · d6bd8194
    Michael Ellerman authored
    Larry Finger reported that his Powerbook G4 was no longer booting with v4.12-rc,
    userspace was up but giving weird errors such as:
    
      udevd[64]: starting version 175
      udevd[64]: Unable to receive ctrl message: Bad address.
      modprobe: chdir(4.12-rc1): No such file or directory
    
    He bisected the problem to commit 3448890c ("powerpc: get rid of zeroing,
    switch to RAW_COPY_USER").
    
    Al identified that the problem is actually a miscompilation by GCC 4.6.3, which
    is exposed by the above commit.
    
    Al also pointed out that inlining copy_to/from_user() is probably of little or
    no benefit, which is correct. Using Anton's copy_to_user benchmark, with a
    pathological single byte copy, we see a small increase in performance
    by *removing* inlining:
    
      Before (inlined):
      # time ./copy_to_user -w -l 1 -i 10000000	( x 3 )
      real	0m22.063s
      real	0m22.059s
      real	0m22.076s
    
      After:
      # time ./copy_to_user -w -l 1 -i 10000000	( x 3 )
      real	0m21.325s
      real	0m21.299s
      real	0m21.364s
    
    So as a small performance improvement and to avoid the miscompilation, drop
    inlining copy_to/from_user() on 32-bit.
    
    Fixes: 3448890c ("powerpc: get rid of zeroing, switch to RAW_COPY_USER")
    Reported-by: default avatarLarry Finger <Larry.Finger@lwfinger.net>
    Suggested-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    d6bd8194
uaccess.h 9.66 KB