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    SubjectRCU: non-atomic assignment to long/pointer variables in gcc
    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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