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f2effca1ef
The libdw callback API has two levels: dwfl_getthread_frames() first finds the requested thread using the next_thread() or get_thread() callback and then walks its stack. Since perf only has a snapshot of the stack of a single thread, it provides a stubbed-out API that always returns the pid the Dwfl was attached with (i.e. whatever was passed to dwfl_attach_state()), rather than the actual sampled thread's TID. Commit6b2658b3f3("perf unwind-libdw: Don't discard loaded ELF/DWARF after every unwind") changed libdw unwinding from recreating the Dwfl object for each sample to caching it in struct maps, which is shared by every thread in the process. It left next_thread() unchanged. Since the pid passed to dwfl_attach_state() is only set at creation, only the thread of the first sample is ever found. As a result, dwfl_getthread_frames() fails with ESRCH when asked to unwind a sample from another thread. Make next_thread() return the current sample's TID, provide get_thread() so libdw can find it directly, and pass the process PID expected by dwfl_attach_state(). This allows libdw to unwind samples from every thread in a multi-threaded process. Add a shell regression test that records a four-thread workload and verifies that libdw recovers the worker callchain for every worker TID. Fixes:6b2658b3f3("perf unwind-libdw: Don't discard loaded ELF/DWARF after every unwind") Assisted-by: Codex:gpt-5.6-sol Signed-off-by: Alessio Podda <aleph.pi.gh@gmail.com> Signed-off-by: Namhyung Kim <namhyung@kernel.org>