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i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert() 

Use cpu_relax() in the busy loops, as atomic_read() doesn't automatically
imply volatility for i386 and x86_64. x86_64 doesn't have this issue because
it open-codes the while loop in smpboot.c:smp_callin() itself that already
uses cpu_relax().

For i386, however, smpboot.c:smp_callin() calls wait_for_init_deassert()
which is buggy for mach-default and mach-es7000 cases.

[ I test-built a kernel -- smp_callin() itself got inlined in its only
  callsite, smpboot.c:start_secondary() -- and the relevant piece of
  code disassembles to the following:

0xc1019704 <start_secondary+12>:        mov    0xc144c4c8,%eax
0xc1019709 <start_secondary+17>:        test   %eax,%eax
0xc101970b <start_secondary+19>:        je     0xc1019709 <start_secondary+17>

  init_deasserted (at 0xc144c4c8) gets fetched into %eax only once and
  then we loop over the test of the stale value in the register only,
  so these look like real bugs to me. With the fix below, this becomes:

0xc1019706 <start_secondary+14>:        pause
0xc1019708 <start_secondary+16>:        cmpl   $0x0,0xc144c4c8
0xc101970f <start_secondary+23>:        je     0xc1019706 <start_secondary+14>

  which looks nice and healthy. ]

Thanks to Heiko Carstens for noticing this.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Satyam Sharma 2007-08-16 06:09:25 +05:30 committed by Linus Torvalds
parent 06bfb7eb15
commit 62be90012c
2 changed files with 4 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/asm-i386/mach-default/mach_wakecpu.h
View File

@ -15,7 +15,8 @@
static inline void wait_for_init_deassert(atomic_t *deassert)
{
while (!atomic_read(deassert));
while (!atomic_read(deassert))
cpu_relax();
return;
}

3
include/asm-i386/mach-es7000/mach_wakecpu.h
View File

@ -31,7 +31,8 @@ wakeup_secondary_cpu(int phys_apicid, unsigned long start_eip)
static inline void wait_for_init_deassert(atomic_t *deassert)
{
#ifdef WAKE_SECONDARY_VIA_INIT
while (!atomic_read(deassert));
while (!atomic_read(deassert))
cpu_relax();
#endif
return;
}