unexpected slowdown and cancellation point

[Mathis Ahrens]

Hi,

I am trying to debug a multithreaded program with gdb but get unexpected 
side effects.

Test case code is attached.
Basically, main() is an endless loop that
	- creates two threads t1 & t2
	- joins t2
	- increments an iteration counter i.
t2 shall wait until t1 exists, then cancel and join t1.
t1 does nothing, but has a cleanup handler that increments a counter c.

When running without gdb, the output is as expected:
i=1, c=1, dt=0.000848
i=1001, c=1001, dt=0.095550
i=2001, c=2001, dt=0.081302
i=3001, c=3001, dt=0.089316
i=4001, c=4001, dt=0.088792
i=5001, c=5001, dt=0.086609
i=6001, c=6001, dt=0.086388
i=7001, c=7001, dt=0.090560
...

[ i: iteration, c: cancellation handler calls, dt: time ]

When running inside gdb (4.5 and master), I get:
i=1, c=1, dt=0.005421
i=1001, c=1000, dt=2.675437
i=2001, c=2000, dt=5.822094
i=3001, c=3000, dt=9.502770
i=4001, c=4000, dt=15.998418
i=5001, c=4999, dt=30.724420
i=6001, c=5999, dt=57.917127
i=7001, c=6999, dt=109.575370
i=8001, c=7999, dt=162.559419
i=9001, c=8999, dt=187.046422
...


First, c is not incremented for every iteration.
Maybe gdb inserts a cancellation point before the cleanup handler gets 
pushed ? Is this expected behavior ?

Second, the time per 1000 iterations increases rapidly.
I cannot figure a reason for that. Do you see this too ?

I am running this on ubuntu 12.04 64bit with eglibc-2.15 and gcc-4.6.3.

Thanks,
Mathis

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