[PATCH?] Separate pthread patches, #2 take 2.
Dave Korn
dave.korn.cygwin@googlemail.com
Wed Jun 3 23:13:00 GMT 2009
The attached patch implements ilockexch and ilockcmpexch, using the inline
asm definition from __arch_compare_and_exchange_val_32_acq in
glibc-2.10.1/sysdeps/i386/i486/bits/atomic.h, trivially expanded inline rather
than in its original preprocessor macro form.
It generates incorrect code. The sequence discussed before,
126 do
127 node->next = head;
128 while (InterlockedCompareExchangePointer (&head, node, node->next) !=
node->next);
is now compiled down to this loop:
L186:
.loc 3 127 0
movl __ZN13pthread_mutex7mutexesE+8, %edx # mutexes.head, D.28599
.loc 2 58 0
movl %edx, %eax # D.28599, tmp68
/APP
# 58 "/gnu/winsup/src/winsup/cygwin/winbase.h" 1
lock cmpxchgl %ebx, __ZN13pthread_mutex7mutexesE+8 # this,
# 0 "" 2
/NO_APP
movl %eax, -12(%ebp) # tmp68, ret
.loc 2 59 0
movl -12(%ebp), %eax # ret, D.28596
.loc 3 126 0
cmpl %eax, %edx # D.28596, D.28599
jne L186 #,
movl %edx, 36(%ebx) # D.28599, <variable>.next
... which is in fact the equivalent of
126 do
127 ;
128 while (InterlockedCompareExchangePointer (&head, node, node->next) !=
node->next);
(126) node->next = head;
As it caches the values of head in %edx during the spin loop, and only
stores it to node->next after having overwritten *head with node, there is a
short window after the new node is linked to the front of the chain during
which its chain pointer is incorrect.
Not OK for head?
cheers,
DaveK
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