Re: external systemtap meeting notes 20070816

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jkenisto wrote [untrimmed for forwarding to utrace-devel]:

> [...]
> Uprobes maintains one utrace engine per probed task.  When
> instrumentation modules A and B register uprobes at the same address,
> and that probepoint gets hit, uprobes's signal (SIGTRAP) callback gets
> called (once).  It calls both A's and B's handlers (a la aggregate
> kprobes), then eats the SIGTRAP.
> If A and B each got its own engine, then what would uprobes's callback
> do?  If the callback eats the SIGTRAP, then the 2nd handler never gets
> called.  If it doesn't eat the SIGTRAP, the probed process takes the
> SIGTRAP and dies.  We could work up some new way that A and B could
> coordinate this sort of thing, but then we're back to needing some sort
> of shared (among instrumentation modules) data structures.  And what we
> have now works just fine.

This is probably related to Roland's "engine interaction" note
(  It would be nice if concurrent engines
could nest transparently - or failing that, at least be aware of each
other enough to get this right.

- FChE

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