Messages in this thread | | | Date | Tue, 15 Jan 2013 14:30:32 +0400 | From | Konstantin Khlebnikov <> | Subject | RCU: non-atomic assignment to long/pointer variables in gcc |
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Documentation/atomic_ops.txt (182dd4b277177e8465ad11cd9f85f282946b5578) says that pointers, longs, ints, and chars are stored and loaded atomically.
But GCC actually may split assignment to 'long' variable into two instructions. see example in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981
GCC also splits assignments to 'volatile' variables and this is actually a bug in gcc.
volatile unsigned long y;
y = 0x100000001ul;
400728: c7 05 66 06 20 00 01 movl $0x1,0x200666(%rip) # 600d98 <y> 40072f: 00 00 00 400732: c7 05 60 06 20 00 01 movl $0x1,0x200660(%rip) # 600d9c <y+0x4> 400739: 00 00 00
fortunately for y = 0; it generates this:
40071d: 48 c7 05 70 06 20 00 movq $0x0,0x200670(%rip) # 600d98 <y> 400724: 00 00 00 00
Thus NULL is safe, but constant ERR_PTR may be dangerous.
Probably rcu_assign_pointer() should use ACCESS_ONCE() around lvalue, because splitting assignment for non-volatile variable seems like completely valid, but this may help only after fixing that bug in GCC.
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