URL:
<http://savannah.cern.ch/bugs/?99130>
Summary: atomicity of XrdSysAtomics
Project: XROOTD
Submitted by: gadde
Submitted on: 2012-11-28 14:59
Report Type: Bug
Priority: 5 - Normal
Severity: 3 - Normal
Status: None
Privacy: Public
Assigned to: None
Originator Email:
Open/Closed: Open
Discussion Lock: Any
Fixed by commit(s):
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Details:
Hello
in XrdSysAtomics.hh, some macros are defined when the compiler have atomics.
Among these macros are the two following ones:
#define AtomicDTZ(x) if (!(__sync_fetch_and_sub(&x, 1))) AtomicFAZ(x)
#define AtomicISM(x, y) if (AtomicGet(y) <= AtomicInc(x)) AtomicCAS(y, y,
x)
To my understanding, these macros are not atomic.
They can be interrupted by the scheduling of the threads and the values x or
y can change before the conditional instruction is executed.
for instance:
assume the intial state is x=1 y=1
a call to AtomicISM(x, y) by thread 1 can be interrupted this way by thread
2
thread 1 -> compute that AtomicGet(y) <= AtomicInc(x) is true
thread 2 -> set x to -1
thread 1 -> set y to -1 because of AtomicCAS(y, y, x)
final state x=-1 y=-1
if it was atomic, the expected final state would be
x=-1 y=2
There are of course, other ways to interefere with this macro and make it non
atomic.
Maybe, I miss a point.
Cheers
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